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momentum (mōmĕnˈtəm), in mechanics, the quantity of motion of a body, specifically the product of the mass of the body and its velocity. Momentum is a vector quantity; i.e., it has both a magnitude and a direction, the direction being the same as that of the velocity vector. When an external force acts upon a body or a system of bodies in motion, it causes a change in the momentum of the body. The impulse of a force acting on a body is the product of the force and the duration of time in which it acts and is equal to the change in momentum of the body. When no external force acts upon a body in motion or a system of bodies there is no change in the total momentum even though, as in the case of a system of bodies, there may be an internal disturbance of the system resulting in changes in the momenta of individual bodies. This conclusion is commonly known as the principle of the conservation of momentum (see conservation laws, in physics). The momentum of a body should not be confused with its kinetic energy. The distinction between them can be seen in the action of a pile driver. The distance to which the pile is driven depends upon its kinetic energy; the length of time required for the action to cease, upon its momentum. In addition to the momentum a body has because of its linear motion, the body may also have angular momentum because of rotation. The angular momentum of a particle rotating about a point is equal to the product of the mass of the particle, its angular velocity, and the square of its distance from the axis of rotation. More simply, the angular momentum is the product of the instantaneous linear momentum and the distance. Angular momentum is a vector quantity directed perpendicular to the plane of motion.
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Linear momentum is the product of the mass and the linear velocity of a body. It is defined by Eq. (1),

where m is the mass and v is the linear velocity. Since linear momentum is the product of a scalar and a vector quantity, it is a vector and hence has both magnitude and direction.

According to the general statement of Newton's second law, for a force F , a momentum P , and a time t, Eq. (2) holds.

Thus Newton's second law involves the time rate of change of momentum. Changes of momentum are important in collision processes. See Collision (physics)

When a group of bodies is subject only to forces that members of the group exert on one another, the total momentum of the group remains constant. See Angular momentum, Conservation of momentum, Impulse (mechanics)

McGraw-Hill Concise Encyclopedia of Physics. © 2002 by The McGraw-Hill Companies, Inc.


1. (linear momentum) Symbol: p. A property of a body moving in a straight line. It is equal in magnitude to the product of the mass and velocity of the body. It is a vector quantity, mv , having the same direction as the velocity. See also conservation of momentum; Newton's laws of motion.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



a measure of mechanical motion that for a mass point is equal to the product of the mass m of the point and its velocity v. Momentum, mv, is a vector quantity, with the same direction as the velocity of the point. Upon the action of a force, the momentum of the point changes, usually both numerically and in direction. This change is determined by the second (basic) law of dynamics.

The momentum Q of a mechanical system is equal to the geometric sum of the momentums of all its points or the product of the mass M of the entire system and the velocity vc of its center of mass: Q = Σ mkvk = Mvc. A change in the momentum of the system takes place only through the action of external forces—that is, forces acting on the system from bodies that are not part of the system. According to the theory of the change of momentum, Q1 – Q0 = ΣSke, where Q0 and Q1 are the momentums of the system at the beginning and end of a certain time interval and Ske are the momentums of external forces Fke over the time interval (this theorem is expressed in differential form by the equation dQ/dt = ΣFke). This theorem is used for the solution of many problems in dynamics, particularly in impact theory.

For a closed system (that is, for a system that does not experience external forces), or in the case in which the geometric sum of the external forces acting on the system equals zero, the law of conservation of momentum is valid. In such cases the momentums of the individual parts of the system (for example, during the action of internal forces) may change, but in such a way that the quantity Q = Σ mkvk remains constant. This law explains such phenomena as jet propulsion, the recoil upon firing a gun, and the operation of a screw propeller or oars. For example, if a gun and bullet are regarded as a single system, then the pressure of the powder gases upon firing will be an internal force for the system and cannot change the momentum of the system, which was equal to zero before the firing. Therefore, in imparting the momentum m1v1 to the bullet in the direction toward the muzzle, the powder gases simultaneously impart to the gun a momentum m2v2, which is numerically equal but opposite in direction, resulting in the recoil. If the velocity v1 of the bullet upon its exit from the barrel is known, the maximum recoil velocity v2 may be found from the equation m1v1 = m2v2 (Where v1 and v2 are scalar quantities of velocity).

At velocities close to the speed of light c, the momentum of a free particle is given by the formula Momentum, where β = v/c. When ν << c, this formula becomes the ordinary formula p = mv.

Momentum exists for physical fields (electromagnetic, gravitational, and so on). The momentum of a field is characterized by momentum density (the relationship of the momentum of a unit volume to the entire volume) and is expressed as field intensity or field potential.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


Also known as linear momentum; vector momentum.
For a single nonrelativistic particle, the product of the mass and the velocity of a particle.
For a single relativistic particle, m v/(1 -v 2/ c 2)1/2, where m is the rest-mass,vthe velocity, and c the speed of light.
For a system of particles, the vector sum of the momenta (as in the first or second definition) of the particles.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


Of a moving body, the product of the mass of the body and its velocity.
McGraw-Hill Dictionary of Architecture and Construction. Copyright © 2003 by McGraw-Hill Companies, Inc.


1. Physics the product of a body's mass and its velocity.
2. the impetus of a body resulting from its motion
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
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