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Momentum | |
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Momentum of a pool cue ball is transferred to the racked balls after collision. | |

Common symbols | p, p |

SI unit | kilogram meter per second kg⋅m/s |

Other units | slug⋅ft/s |

Conserved? | Yes |

Dimension | MLT^{−1} |

Part of a series of articles about |

Classical mechanics |
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Core topics |

In Newtonian mechanics, **linear momentum**, **translational momentum**, or simply **momentum** (pl. momenta) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction in three-dimensional space. If *m* is an object's mass and **v** is the velocity (also a vector), then the momentum is

The **plural**, in many languages, is one of the values of the grammatical category of number. Plural of nouns typically denote a quantity other than the default quantity represented by a noun, which is generally one. Most commonly, therefore, plurals are used to denote two or more of something, although they may also denote more than fractional, zero or negative amounts. An example of a plural is the English word *cats*, which corresponds to the singular *cat*.

**Mass** is both a property of a physical body and a measure of its resistance to acceleration when a net force is applied. An object's mass also determines the strength of its gravitational attraction to other bodies.

The **velocity** of an object is the rate of change of its position with respect to a frame of reference, and is a function of time. Velocity is equivalent to a specification of an object's speed and direction of motion. Velocity is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of bodies.

- Newtonian
- Single particle
- Many particles
- Relation to force
- Conservation
- Dependence on reference frame
- Application to collisions
- Multiple dimensions
- Objects of variable mass
- Relativistic
- Lorentz invariance
- Four-vector formulation
- Generalized
- Lagrangian mechanics
- Hamiltonian mechanics
- Symmetry and conservation
- Electromagnetic
- Particle in a field
- Conservation 2
- Quantum mechanical
- In deformable bodies and fluids
- Conservation in a continuum
- Acoustic waves
- History of the concept
- See also
- References
- Bibliography
- External links

In SI units, it is measured in kilogram meters per second (kg⋅m/s). Newton's second law of motion states that a body's rate of change in momentum is equal to the net force acting on it.

The **International System of Units** is the modern form of the metric system, and is the most widely used system of measurement. It comprises a coherent system of units of measurement built on seven base units, which are the second, metre, kilogram, ampere, kelvin, mole, candela, and a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units. The system also specifies names for 22 derived units, such as lumen and watt, for other common physical quantities.

The **kilogram**, also **kilogramme**, is the base unit of mass in the metric system, formally the International System of Units (SI), having the unit symbol **kg**. It is a widely used measure in science, engineering, and commerce worldwide, and is often called a **kilo**. The kilogram is almost exactly the mass of one litre of water.

Momentum depends on the frame of reference, but in any inertial frame it is a *conserved* quantity, meaning that if a closed system is not affected by external forces, its total linear momentum does not change. Momentum is also conserved in special relativity (with a modified formula) and, in a modified form, in electrodynamics, quantum mechanics, quantum field theory, and general relativity. It is an expression of one of the fundamental symmetries of space and time: translational symmetry.

In physics, a **frame of reference** consists of an abstract coordinate system and the set of physical reference points that uniquely fix the coordinate system and standardize measurements.

A **closed system** is a physical system that does not allow certain types of transfers in or out of the system. The specification of what types of transfers are excluded varies in the closed systems of physics, chemistry or engineering.

In physics, **special relativity** is the generally accepted and experimentally well-confirmed physical theory regarding the relationship between space and time. In Albert Einstein's original pedagogical treatment, it is based on two postulates:

- the laws of physics are invariant in all inertial systems ; and
- the speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.

Advanced formulations of classical mechanics, Lagrangian and Hamiltonian mechanics, allow one to choose coordinate systems that incorporate symmetries and constraints. In these systems the conserved quantity is *generalized momentum*, and in general this is different from the *kinetic* momentum defined above. The concept of generalized momentum is carried over into quantum mechanics, where it becomes an operator on a wave function. The momentum and position operators are related by the Heisenberg uncertainty principle.

**Lagrangian mechanics** is a reformulation of classical mechanics, introduced by the Italian-French mathematician and astronomer Joseph-Louis Lagrange in 1788.

**Hamiltonian mechanics** is a theory developed as a reformulation of classical mechanics and predicts the same outcomes as non-Hamiltonian classical mechanics. It uses a different mathematical formalism, providing a more abstract understanding of the theory. Historically, it was an important reformulation of classical mechanics, which later contributed to the formulation of statistical mechanics and quantum mechanics.

A **wave function** in quantum physics is a mathematical description of the quantum state of an isolated quantum system. The wave function is a complex-valued probability amplitude, and the probabilities for the possible results of measurements made on the system can be derived from it. The most common symbols for a wave function are the Greek letters *ψ* or Ψ.

In continuous systems such as electromagnetic fields, fluids and deformable bodies, a momentum density can be defined, and a continuum version of the conservation of momentum leads to equations such as the Navier–Stokes equations for fluids or the Cauchy momentum equation for deformable solids or fluids.

In physics, the **Navier–Stokes equations**, named after Claude-Louis Navier and George Gabriel Stokes, describe the motion of viscous fluid substances.

The **Cauchy momentum equation** is a vector partial differential equation put forth by Cauchy that describes the non-relativistic momentum transport in any continuum. In convective form it is written:

Momentum is a vector quantity: it has both magnitude and direction. Since momentum has a direction, it can be used to predict the resulting direction and speed of motion of objects after they collide. Below, the basic properties of momentum are described in one dimension. The vector equations are almost identical to the scalar equations (see multiple dimensions).

The momentum of a particle is conventionally represented by the letter *p*. It is the product of two quantities, the particle's mass (represented by the letter *m*) and its velocity (*v*):^{ [1] }

The unit of momentum is the product of the units of mass and velocity. In SI units, if the mass is in kilograms and the velocity is in meters per second then the momentum is in kilogram meters per second (kg⋅m/s). In cgs units, if the mass is in grams and the velocity in centimeters per second, then the momentum is in gram centimeters per second (g⋅cm/s).

Being a vector, momentum has magnitude and direction. For example, a 1 kg model airplane, traveling due north at 1 m/s in straight and level flight, has a momentum of 1 kg⋅m/s due north measured with reference to the ground.

The momentum of a system of particles is the vector sum of their momenta. If two particles have respective masses *m*_{1} and *m*_{2}, and velocities *v*_{1} and *v*_{2}, the total momentum is

The momenta of more than two particles can be added more generally with the following:

A system of particles has a center of mass, a point determined by the weighted sum of their positions:

If one or more of the particles is moving, the center of mass of the system will generally be moving as well (unless the system is in pure rotation around it). If the total mass of the particles is , and the center of mass is moving at velocity *v*_{cm}, the momentum of the system is:

This is known as Euler's first law.^{ [2] }^{ [3] }

If the net force *F* applied to a particle is constant, and is applied for a time interval Δ*t*, the momentum of the particle changes by an amount

In differential form, this is Newton's second law; the rate of change of the momentum of a particle is equal to the instantaneous force *F* acting on it,^{ [1] }

If the net force experienced by a particle changes as a function of time, *F(t)*, the change in momentum (or impulse *J*) between times *t*_{1} and *t*_{2} is

Impulse is measured in the derived units of the newton second (1 N⋅s = 1 kg⋅m/s) or dyne second (1 dyne⋅s = 1 g⋅cm/s)

Under the assumption of constant mass *m*, it is equivalent to write

hence the net force is equal to the mass of the particle times its acceleration.^{ [1] }

*Example*: A model airplane of mass 1 kg accelerates from rest to a velocity of 6 m/s due north in 2 s. The net force required to produce this acceleration is 3 newtons due north. The change in momentum is 6 kg⋅m/s due north. The rate of change of momentum is 3 (kg⋅m/s)/s due north which is numerically equivalent to 3 newtons.

In a closed system (one that does not exchange any matter with its surroundings and is not acted on by external forces) the total momentum is constant. This fact, known as the *law of conservation of momentum*, is implied by Newton's laws of motion.^{ [4] }^{ [5] } Suppose, for example, that two particles interact. Because of the third law, the forces between them are equal and opposite. If the particles are numbered 1 and 2, the second law states that *F*_{1} = *dp*_{1}/*dt* and *F*_{2} = *dp*_{2}/*dt*. Therefore,

with the negative sign indicating that the forces oppose. Equivalently,

If the velocities of the particles are *u*_{1} and *u*_{2} before the interaction, and afterwards they are *v*_{1} and *v*_{2}, then

This law holds no matter how complicated the force is between particles. Similarly, if there are several particles, the momentum exchanged between each pair of particles adds up to zero, so the total change in momentum is zero. This conservation law applies to all interactions, including collisions and separations caused by explosive forces.^{ [4] } It can also be generalized to situations where Newton's laws do not hold, for example in the theory of relativity and in electrodynamics.^{ [6] }

Momentum is a measurable quantity, and the measurement depends on the motion of the observer. For example: if an apple is sitting in a glass elevator that is descending, an outside observer, looking into the elevator, sees the apple moving, so, to that observer, the apple has a non-zero momentum. To someone inside the elevator, the apple does not move, so, it has zero momentum. The two observers each have a frame of reference, in which, they observe motions, and, if the elevator is descending steadily, they will see behavior that is consistent with those same physical laws.

Suppose a particle has position *x* in a stationary frame of reference. From the point of view of another frame of reference, moving at a uniform speed *u*, the position (represented by a primed coordinate) changes with time as

This is called a Galilean transformation. If the particle is moving at speed *dx*/*dt* = *v* in the first frame of reference, in the second, it is moving at speed

Since *u* does not change, the accelerations are the same:

Thus, momentum is conserved in both reference frames. Moreover, as long as the force has the same form, in both frames, Newton's second law is unchanged. Forces such as Newtonian gravity, which depend only on the scalar distance between objects, satisfy this criterion. This independence of reference frame is called Newtonian relativity or Galilean invariance.^{ [7] }

A change of reference frame, can, often, simplify calculations of motion. For example, in a collision of two particles, a reference frame can be chosen, where, one particle begins at rest. Another, commonly used reference frame, is the center of mass frame – one that is moving with the center of mass. In this frame, the total momentum is zero.

By itself, the law of conservation of momentum is not enough to determine the motion of particles after a collision. Another property of the motion, kinetic energy, must be known. This is not necessarily conserved. If it is conserved, the collision is called an * elastic collision *; if not, it is an * inelastic collision *.

An elastic collision is one in which no kinetic energy is absorbed in the collision. Perfectly elastic "collisions" can occur when the objects do not touch each other, as for example in atomic or nuclear scattering where electric repulsion keeps them apart. A slingshot maneuver of a satellite around a planet can also be viewed as a perfectly elastic collision. A collision between two pool balls is a good example of an *almost* totally elastic collision, due to their high rigidity, but when bodies come in contact there is always some dissipation.^{ [8] }

A head-on elastic collision between two bodies can be represented by velocities in one dimension, along a line passing through the bodies. If the velocities are *u*_{1} and *u*_{2} before the collision and *v*_{1} and *v*_{2} after, the equations expressing conservation of momentum and kinetic energy are:

A change of reference frame can simplify analysis of a collision. For example, suppose there are two bodies of equal mass *m*, one stationary and one approaching the other at a speed *v* (as in the figure). The center of mass is moving at speed *v*/2 and both bodies are moving towards it at speed *v*/2. Because of the symmetry, after the collision both must be moving away from the center of mass at the same speed. Adding the speed of the center of mass to both, we find that the body that was moving is now stopped and the other is moving away at speed *v*. The bodies have exchanged their velocities. Regardless of the velocities of the bodies, a switch to the center of mass frame leads us to the same conclusion. Therefore, the final velocities are given by^{ [4] }

In general, when the initial velocities are known, the final velocities are given by^{ [9] }

If one body has much greater mass than the other, its velocity will be little affected by a collision while the other body will experience a large change.

In an inelastic collision, some of the kinetic energy of the colliding bodies is converted into other forms of energy (such as heat or sound). Examples include traffic collisions,^{ [10] } in which the effect of loss of kinetic energy can be seen in the damage to the vehicles; electrons losing some of their energy to atoms (as in the Franck–Hertz experiment);^{ [11] } and particle accelerators in which the kinetic energy is converted into mass in the form of new particles.

In a perfectly inelastic collision (such as a bug hitting a windshield), both bodies have the same motion afterwards. If one body is motionless to begin with, the equation for conservation of momentum is

so

In a frame of reference moving at the speed *v*), the objects are brought to rest by the collision and 100% of the kinetic energy is converted to other forms of energy.

One measure of the inelasticity of the collision is the coefficient of restitution *C*_{R}, defined as the ratio of relative velocity of separation to relative velocity of approach. In applying this measure to a ball bouncing from a solid surface, this can be easily measured using the following formula:^{ [12] }

The momentum and energy equations also apply to the motions of objects that begin together and then move apart. For example, an explosion is the result of a chain reaction that transforms potential energy stored in chemical, mechanical, or nuclear form into kinetic energy, acoustic energy, and electromagnetic radiation. Rockets also make use of conservation of momentum: propellant is thrust outward, gaining momentum, and an equal and opposite momentum is imparted to the rocket.^{ [13] }

Real motion has both direction and velocity and must be represented by a vector. In a coordinate system with *x*, *y*, *z* axes, velocity has components *v*_{x} in the *x*-direction, *v*_{y} in the *y*-direction, *v*_{z} in the *z*-direction. The vector is represented by a boldface symbol:^{ [14] }

Similarly, the momentum is a vector quantity and is represented by a boldface symbol:

The equations in the previous sections, work in vector form if the scalars *p* and *v* are replaced by vectors **p** and **v**. Each vector equation represents three scalar equations. For example,

represents three equations:^{ [14] }

The kinetic energy equations are exceptions to the above replacement rule. The equations are still one-dimensional, but each scalar represents the magnitude of the vector, for example,

Each vector equation represents three scalar equations. Often coordinates can be chosen so that only two components are needed, as in the figure. Each component can be obtained separately and the results combined to produce a vector result.^{ [14] }

A simple construction involving the center of mass frame can be used to show that if a stationary elastic sphere is struck by a moving sphere, the two will head off at right angles after the collision (as in the figure).^{ [15] }

The concept of momentum plays a fundamental role in explaining the behavior of variable-mass objects such as a rocket ejecting fuel or a star accreting gas. In analyzing such an object, one treats the object's mass as a function that varies with time: *m*(*t*). The momentum of the object at time *t* is therefore *p*(*t*) = *m*(*t*)*v*(*t*). One might then try to invoke Newton's second law of motion by saying that the external force *F* on the object is related to its momentum *p*(*t*) by *F* = *dp*/*dt*, but this is incorrect, as is the related expression found by applying the product rule to *d*(*mv*)/*dt*:^{ [16] }

- (incorrect)
^{[ why? ]}

This equation does not correctly describe the motion of variable-mass objects. The correct equation is

where *u* is the velocity of the ejected/accreted mass *as seen in the object's rest frame*.^{ [16] } This is distinct from *v*, which is the velocity of the object itself as seen in an inertial frame.

This equation is derived by keeping track of both the momentum of the object as well as the momentum of the ejected/accreted mass (*dm*). When considered together, the object and the mass (*dm*) constitute a closed system in which total momentum is conserved.

Newtonian physics assumes that absolute time and space exist outside of any observer; this gives rise to Galilean invariance. It also results in a prediction that the speed of light can vary from one reference frame to another. This is contrary to observation. In the special theory of relativity, Einstein keeps the postulate that the equations of motion do not depend on the reference frame, but assumes that the speed of light *c* is invariant. As a result, position and time in two reference frames are related by the Lorentz transformation instead of the Galilean transformation.^{ [17] }

Consider, for example, a reference frame moving relative to another at velocity *v* in the *x* direction. The Galilean transformation gives the coordinates of the moving frame as

while the Lorentz transformation gives^{ [18] }

where *γ* is the Lorentz factor:

Newton's second law, with mass fixed, is not invariant under a Lorentz transformation. However, it can be made invariant by making the *inertial mass**m* of an object a function of velocity:

*m*_{0} is the object's invariant mass.^{ [19] }

The modified momentum,

obeys Newton's second law:

Within the domain of classical mechanics, relativistic momentum closely approximates Newtonian momentum: at low velocity, *γm*_{0}**v** is approximately equal to *m*_{0}**v**, the Newtonian expression for momentum.

In the theory of special relativity, physical quantities are expressed in terms of four-vectors that include time as a fourth coordinate along with the three space coordinates. These vectors are generally represented by capital letters, for example **R** for position. The expression for the *four-momentum* depends on how the coordinates are expressed. Time may be given in its normal units or multiplied by the speed of light so that all the components of the four-vector have dimensions of length. If the latter scaling is used, an interval of proper time, *τ*, defined by^{ [20] }

is invariant under Lorentz transformations (in this expression and in what follows the (+ − − −) metric signature has been used, different authors use different conventions). Mathematically this invariance can be ensured in one of two ways: by treating the four-vectors as Euclidean vectors and multiplying time by √−1; or by keeping time a real quantity and embedding the vectors in a Minkowski space.^{ [21] } In a Minkowski space, the scalar product of two four-vectors **U** = (*U*_{0},*U*_{1},*U*_{2},*U*_{3}) and **V** = (*V*_{0},*V*_{1},*V*_{2},*V*_{3}) is defined as

In all the coordinate systems, the (contravariant) relativistic four-velocity is defined by

and the (contravariant) four-momentum is

where *m*_{0} is the invariant mass. If **R** = (*ct,x,y,z*) (in Minkowski space), then

Using Einstein's mass-energy equivalence, *E* = *mc*^{2}, this can be rewritten as

Thus, conservation of four-momentum is Lorentz-invariant and implies conservation of both mass and energy.

The magnitude of the momentum four-vector is equal to *m*_{0}*c*:

and is invariant across all reference frames.

The relativistic energy–momentum relationship holds even for massless particles such as photons; by setting *m*_{0} = 0 it follows that

In a game of relativistic "billiards", if a stationary particle is hit by a moving particle in an elastic collision, the paths formed by the two afterwards will form an acute angle. This is unlike the non-relativistic case where they travel at right angles.^{ [22] }

The four-momentum of a planar wave can be related to a wave four-vector^{ [23] }

For a particle, the relationship between temporal components, *E* = *ħ* ω, is the Planck–Einstein relation, and the relation between spatial components, **p**= *ħ***k**, describes a de Broglie matter wave.

Newton's laws can be difficult to apply to many kinds of motion because the motion is limited by *constraints*. For example, a bead on an abacus is constrained to move along its wire and a pendulum bob is constrained to swing at a fixed distance from the pivot. Many such constraints can be incorporated by changing the normal Cartesian coordinates to a set of * generalized coordinates * that may be fewer in number.^{ [24] } Refined mathematical methods have been developed for solving mechanics problems in generalized coordinates. They introduce a *generalized momentum*, also known as the *canonical* or *conjugate momentum*, that extends the concepts of both linear momentum and angular momentum. To distinguish it from generalized momentum, the product of mass and velocity is also referred to as *mechanical*, *kinetic* or *kinematic momentum*.^{ [6] }^{ [25] }^{ [26] } The two main methods are described below.

In Lagrangian mechanics, a Lagrangian is defined as the difference between the kinetic energy *T* and the potential energy *V*:

If the generalized coordinates are represented as a vector **q** = (*q*_{1}, *q*_{2}, ... , *q*_{N}) and time differentiation is represented by a dot over the variable, then the equations of motion (known as the Lagrange or Euler–Lagrange equations) are a set of *N* equations:^{ [27] }

If a coordinate *q*_{i} is not a Cartesian coordinate, the associated generalized momentum component *p*_{i} does not necessarily have the dimensions of linear momentum. Even if *q*_{i} is a Cartesian coordinate, *p*_{i} will not be the same as the mechanical momentum if the potential depends on velocity.^{ [6] } Some sources represent the kinematic momentum by the symbol **Π**.^{ [28] }

In this mathematical framework, a generalized momentum is associated with the generalized coordinates. Its components are defined as

Each component *p*_{j} is said to be the *conjugate momentum* for the coordinate *q*_{j}.

Now if a given coordinate *q*_{i} does not appear in the Lagrangian (although its time derivative might appear), then

This is the generalization of the conservation of momentum.^{ [6] }

Even if the generalized coordinates are just the ordinary spatial coordinates, the conjugate momenta are not necessarily the ordinary momentum coordinates. An example is found in the section on electromagnetism.

In Hamiltonian mechanics, the Lagrangian (a function of generalized coordinates and their derivatives) is replaced by a Hamiltonian that is a function of generalized coordinates and momentum. The Hamiltonian is defined as

where the momentum is obtained by differentiating the Lagrangian as above. The Hamiltonian equations of motion are^{ [29] }

As in Lagrangian mechanics, if a generalized coordinate does not appear in the Hamiltonian, its conjugate momentum component is conserved.^{ [30] }

Conservation of momentum is a mathematical consequence of the homogeneity (shift symmetry) of space (position in space is the canonical conjugate quantity to momentum). That is, conservation of momentum is a consequence of the fact that the laws of physics do not depend on position; this is a special case of Noether's theorem.^{ [31] }

In Maxwell's equations, the forces between particles are mediated by electric and magnetic fields. The electromagnetic force (* Lorentz force *) on a particle with charge *q* due to a combination of electric field **E** and magnetic field **B** is

(in SI units).^{ [32] }^{:2} It has an electric potential *φ*(**r**, *t*) and magnetic vector potential **A**(**r**, *t*).^{ [28] } In the non-relativistic regime, its generalized momentum is

while in relativistic mechanics this becomes

In Newtonian mechanics, the law of conservation of momentum can be derived from the law of action and reaction, which states that every force has a reciprocating equal and opposite force. Under some circumstances, moving charged particles can exert forces on each other in non-opposite directions.^{ [33] } Nevertheless, the combined momentum of the particles and the electromagnetic field is conserved.

The Lorentz force imparts a momentum to the particle, so by Newton's second law the particle must impart a momentum to the electromagnetic fields.^{ [34] }

In a vacuum, the momentum per unit volume is

where *μ*_{0} is the vacuum permeability and *c* is the speed of light. The momentum density is proportional to the Poynting vector **S** which gives the directional rate of energy transfer per unit area:^{ [34] }^{ [35] }

If momentum is to be conserved over the volume *V* over a region *Q*, changes in the momentum of matter through the Lorentz force must be balanced by changes in the momentum of the electromagnetic field and outflow of momentum. If **P**_{mech} is the momentum of all the particles in *Q*, and the particles are treated as a continuum, then Newton's second law gives

The electromagnetic momentum is

and the equation for conservation of each component *i* of the momentum is

The term on the right is an integral over the surface area *Σ* of the surface *σ* representing momentum flow into and out of the volume, and *n*_{j} is a component of the surface normal of *S*. The quantity *T*_{ij} is called the Maxwell stress tensor, defined as

^{ [34] }

The above results are for the *microscopic* Maxwell equations, applicable to electromagnetic forces in a vacuum (or on a very small scale in media). It is more difficult to define momentum density in media because the division into electromagnetic and mechanical is arbitrary. The definition of electromagnetic momentum density is modified to

where the H-field **H** is related to the B-field and the magnetization **M** by

The electromagnetic stress tensor depends on the properties of the media.^{ [34] }

In quantum mechanics, momentum is defined as a self-adjoint operator on the wave function. The Heisenberg uncertainty principle defines limits on how accurately the momentum and position of a single observable system can be known at once. In quantum mechanics, position and momentum are conjugate variables.

For a single particle described in the position basis the momentum operator can be written as

where ∇ is the gradient operator, *ħ* is the reduced Planck constant, and *i* is the imaginary unit. This is a commonly encountered form of the momentum operator, though the momentum operator in other bases can take other forms. For example, in momentum space the momentum operator is represented as

where the operator **p** acting on a wave function *ψ*(*p*) yields that wave function multiplied by the value *p*, in an analogous fashion to the way that the position operator acting on a wave function *ψ*(*x*) yields that wave function multiplied by the value *x*.

For both massive and massless objects, relativistic momentum is related to the phase constant by^{ [36] }

Electromagnetic radiation (including visible light, ultraviolet light, and radio waves) is carried by photons. Even though photons (the particle aspect of light) have no mass, they still carry momentum. This leads to applications such as the solar sail. The calculation of the momentum of light within dielectric media is somewhat controversial (see Abraham–Minkowski controversy).^{ [37] }^{ [38] }

In fields such as fluid dynamics and solid mechanics, it is not feasible to follow the motion of individual atoms or molecules. Instead, the materials must be approximated by a continuum in which there is a particle or fluid parcel at each point that is assigned the average of the properties of atoms in a small region nearby. In particular, it has a density *ρ* and velocity **v** that depend on time *t* and position **r**. The momentum per unit volume is *ρ***v**.^{ [39] }

Consider a column of water in hydrostatic equilibrium. All the forces on the water are in balance and the water is motionless. On any given drop of water, two forces are balanced. The first is gravity, which acts directly on each atom and molecule inside. The gravitational force per unit volume is *ρ***g**, where **g** is the gravitational acceleration. The second force is the sum of all the forces exerted on its surface by the surrounding water. The force from below is greater than the force from above by just the amount needed to balance gravity. The normal force per unit area is the pressure *p*. The average force per unit volume inside the droplet is the gradient of the pressure, so the force balance equation is^{ [40] }

If the forces are not balanced, the droplet accelerates. This acceleration is not simply the partial derivative *∂***v**/*∂t* because the fluid in a given volume changes with time. Instead, the material derivative is needed:^{ [41] }

Applied to any physical quantity, the material derivative includes the rate of change at a point and the changes due to advection as fluid is carried past the point. Per unit volume, the rate of change in momentum is equal to *ρ**D***v**/*Dt*. This is equal to the net force on the droplet.

Forces that can change the momentum of a droplet include the gradient of the pressure and gravity, as above. In addition, surface forces can deform the droplet. In the simplest case, a shear stress *τ*, exerted by a force parallel to the surface of the droplet, is proportional to the rate of deformation or strain rate. Such a shear stress occurs if the fluid has a velocity gradient because the fluid is moving faster on one side than another. If the speed in the *x* direction varies with *z*, the tangential force in direction *x* per unit area normal to the *z* direction is

where *μ* is the viscosity. This is also a flux, or flow per unit area, of x-momentum through the surface.^{ [42] }

Including the effect of viscosity, the momentum balance equations for the incompressible flow of a Newtonian fluid are

These are known as the Navier–Stokes equations.^{ [43] }

The momentum balance equations can be extended to more general materials, including solids. For each surface with normal in direction *i* and force in direction *j*, there is a stress component *σ*_{ij}. The nine components make up the Cauchy stress tensor **σ**, which includes both pressure and shear. The local conservation of momentum is expressed by the Cauchy momentum equation:

where **f** is the body force.^{ [44] }

The Cauchy momentum equation is broadly applicable to deformations of solids and liquids. The relationship between the stresses and the strain rate depends on the properties of the material (see Types of viscosity).

A disturbance in a medium gives rise to oscillations, or waves, that propagate away from their source. In a fluid, small changes in pressure *p* can often be described by the acoustic wave equation:

where *c* is the speed of sound. In a solid, similar equations can be obtained for propagation of pressure (P-waves) and shear (S-waves).^{ [45] }

The flux, or transport per unit area, of a momentum component *ρv _{j}* by a velocity

In about 530 AD, working in Alexandria, Byzantine philosopher John Philoponus developed a concept of momentum in his commentary to Aristotle's *Physics*. Aristotle claimed that everything that is moving must be kept moving by something. For example, a thrown ball must be kept moving by motions of the air. Most writers continued to accept Aristotle's theory until the time of Galileo, but a few were skeptical. Philoponus pointed out the absurdity in Aristotle's claim that motion of an object is promoted by the same air that is resisting its passage. He proposed instead that an impetus was imparted to the object in the act of throwing it.^{ [48] } Ibn Sīnā (also known by his Latinized name Avicenna) read Philoponus and published his own theory of motion in *The Book of Healing* in 1020. He agreed that an impetus is imparted to a projectile by the thrower; but unlike Philoponus, who believed that it was a temporary virtue that would decline even in a vacuum, he viewed it as a persistent, requiring external forces such as air resistance to dissipate it.^{ [49] }^{ [50] }^{ [51] } The work of Philoponus, and possibly that of Ibn Sīnā,^{ [51] } was read and refined by the European philosophers Peter Olivi and Jean Buridan. Buridan, who in about 1350 was made rector of the University of Paris, referred to impetus being proportional to the weight times the speed. Moreover, Buridan's theory was different from his predecessor's in that he did not consider impetus to be self-dissipating, asserting that a body would be arrested by the forces of air resistance and gravity which might be opposing its impetus.^{ [52] }^{ [53] }

René Descartes believed that the total "quantity of motion" (Latin : *quantitas motus*) in the universe is conserved,^{ [54] } where the quantity of motion is understood as the product of size and speed. This should not be read as a statement of the modern law of momentum, since he had no concept of mass as distinct from weight and size, and more importantly he believed that it is speed rather than velocity that is conserved. So for Descartes if a moving object were to bounce off a surface, changing its direction but not its speed, there would be no change in its quantity of motion.^{ [55] }^{ [56] } Galileo, in his * Two New Sciences *, used the Italian word *impeto* to similarly describe Descartes' quantity of motion.

Leibniz, in his "Discourse on Metaphysics", gave an argument against Descartes' construction of the conservation of the "quantity of motion" using an example of dropping blocks of different sizes different distances. He points out that force is conserved but quantity of motion, construed as the product of size and speed of an object, is not conserved.^{ [57] }

The first correct statement of the law of conservation of momentum was by English mathematician John Wallis in his 1670 work, *Mechanica sive De Motu, Tractatus Geometricus*: "the initial state of the body, either of rest or of motion, will persist" and "If the force is greater than the resistance, motion will result".^{ [58] } Wallis uses *momentum* and *vis* for force. Newton's * Philosophiæ Naturalis Principia Mathematica *, when it was first published in 1687, showed a similar casting around for words to use for the mathematical momentum. His Definition II defines *quantitas motus*, "quantity of motion", as "arising from the velocity and quantity of matter conjointly", which identifies it as momentum.^{ [59] } Thus when in Law II he refers to *mutatio motus*, "change of motion", being proportional to the force impressed, he is generally taken to mean momentum and not motion.^{ [60] } It remained only to assign a standard term to the quantity of motion. The first use of "momentum" in its proper mathematical sense is not clear but by the time of Jenning's *Miscellanea* in 1721, five years before the final edition of Newton's *Principia Mathematica*, momentum M or "quantity of motion" was being defined for students as "a rectangle", the product of Q and V, where Q is "quantity of material" and V is "velocity", *s*/*t*.^{ [61] }

In physics, **acceleration** is the rate of change of velocity of an object with respect to time. An object's acceleration is the net result of all forces acting on the object, as described by Newton's Second Law. The SI unit for acceleration is metre per second squared (m⋅s^{−2}). Accelerations are vector quantities and add according to the parallelogram law. The vector of the net force acting on a body has the same direction as the vector of the body's acceleration, and its magnitude is proportional to the magnitude of the acceleration, with the object's mass as proportionality constant.

In physics, **angular momentum** is the rotational equivalent of linear momentum. It is an important quantity in physics because it is a conserved quantity—the total angular momentum of a closed system remains constant.

**Continuum mechanics** is a branch of mechanics that deals with the mechanical behavior of materials modeled as a continuous mass rather than as discrete particles. The French mathematician Augustin-Louis Cauchy was the first to formulate such models in the 19th century.

In physics the **Lorentz force** is the combination of electric and magnetic force on a point charge due to electromagnetic fields. A particle of charge *q* moving with a velocity *v* in an electric field **E** and a magnetic field **B** experiences a force of

In physics, **equations of motion** are equations that describe the behavior of a physical system in terms of its motion as a function of time. More specifically, the equations of motion describe the behaviour of a physical system as a set of mathematical functions in terms of dynamic variables: normally spatial coordinates and time are used, but others are also possible, such as momentum components and time. The most general choice are generalized coordinates which can be any convenient variables characteristic of the physical system. The functions are defined in a Euclidean space in classical mechanics, but are replaced by curved spaces in relativity. If the dynamics of a system is known, the equations are the solutions for the differential equations describing the motion of the dynamics.

**Work** is the product of force and distance. In physics, a force is said to do work if, when acting, there is a movement of the point of application in the direction of the force.

In special relativity, **four-momentum** is the generalization of the classical three-dimensional momentum to four-dimensional spacetime. Momentum is a vector in three dimensions; similarly four-momentum is a four-vector in spacetime. The contravariant four-momentum of a particle with relativistic energy *E* and three-momentum **p** = = *γm***v**, where **v** is the particle's three-velocity and γ the Lorentz factor, is

A **continuity equation** in physics is an equation that describes the transport of some quantity. It is particularly simple and powerful when applied to a conserved quantity, but it can be generalized to apply to any extensive quantity. Since mass, energy, momentum, electric charge and other natural quantities are conserved under their respective appropriate conditions, a variety of physical phenomena may be described using continuity equations.

In theoretical physics and mathematical physics, **analytical mechanics**, or **theoretical mechanics** is a collection of closely related alternative formulations of classical mechanics. It was developed by many scientists and mathematicians during the 18th century and onward, after Newtonian mechanics. Since Newtonian mechanics considers vector quantities of motion, particularly accelerations, momenta, forces, of the constituents of the system, an alternative name for the mechanics governed by Newton's laws and Euler's laws is *vectorial mechanics*.

In analytical mechanics, specifically the study of the rigid body dynamics of multibody systems, the term **generalized coordinates** refers to the parameters that describe the configuration of the system relative to some reference configuration. These parameters must uniquely define the configuration of the system relative to the reference configuration. This is done assuming that this can be done with a single chart. The **generalized velocities** are the time derivatives of the generalized coordinates of the system.

**Rigid-body dynamics** studies the movement of systems of interconnected bodies under the action of external forces. The assumption that the bodies are rigid, which means that they do not deform under the action of applied forces, simplifies the analysis by reducing the parameters that describe the configuration of the system to the translation and rotation of reference frames attached to each body. This excludes bodies that display fluid, highly elastic, and plastic behavior.

The **Boltzmann equation** or **Boltzmann transport equation** (**BTE**) describes the statistical behaviour of a thermodynamic system not in a state of equilibrium, devised by Ludwig Boltzmann in 1872. The classic example of such a system is a fluid with temperature gradients in space causing heat to flow from hotter regions to colder ones, by the random but biased transport of the particles making up that fluid. In the modern literature the term Boltzmann equation is often used in a more general sense, referring to any kinetic equation that describes the change of a macroscopic quantity in a thermodynamic system, such as energy, charge or particle number.

In a compressible sound transmission medium - mainly air - air particles get an accelerated motion: the **particle acceleration** or sound acceleration with the symbol a in metre/second^{2}. In acoustics or physics, **acceleration** is defined as the rate of change of velocity. It is thus a vector quantity with dimension length/time^{2}. In SI units, this is m/s^{2}.

In physics, **relativistic mechanics** refers to mechanics compatible with special relativity (SR) and general relativity (GR). It provides a non-quantum mechanical description of a system of particles, or of a fluid, in cases where the velocities of moving objects are comparable to the speed of light *c*. As a result, classical mechanics is extended correctly to particles traveling at high velocities and energies, and provides a consistent inclusion of electromagnetism with the mechanics of particles. This was not possible in Galilean relativity, where it would be permitted for particles and light to travel at *any* speed, including faster than light. The foundations of relativistic mechanics are the postulates of special relativity and general relativity. The unification of SR with quantum mechanics is relativistic quantum mechanics, while attempts for that of GR is quantum gravity, an unsolved problem in physics.

In classical mechanics, **Euler's laws of motion** are equations of motion which extend Newton's laws of motion for point particle to rigid body motion. They were formulated by Leonhard Euler about 50 years after Isaac Newton formulated his laws.

In classical mechanics, the **central-force problem** is to determine the motion of a particle under the influence of a single central force. A central force is a force that points from the particle directly towards a fixed point in space, the center, and whose magnitude only depends on the distance of the object to the center. In many important cases, the problem can be solved analytically, i.e., in terms of well-studied functions such as trigonometric functions.

In mechanics, a **variable-mass system** is a collection of matter whose mass varies with time. It can be confusing to try to apply Newton's second law of motion directly to such a system. Instead, the time dependence of the mass *m* can be calculated by rearranging Newton's second law and adding a term to account for the momentum carried by mass entering or leaving the system. The general equation of variable-mass motion is written as

In theoretical physics, **relativistic Lagrangian mechanics** is Lagrangian mechanics applied in the context of special relativity and general relativity.

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