Momentum and Collisions

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This article is a topic within the subject Higher Physics 1A.



[1]Momentum is the product of an object's mass and velocity.

p = mv

Momentum is a vector, with the same direction as the velocity.


[2]The impulse on a particle is equal to the change in momentum of a particle.

Δp = I

The impulse (I) is due to a net force (ΣF) acting on the particle. Impulse may be calculated by integrating the net force with respect to time:

I = ʃΣFdt (the limits of integration are the time over which the force acts)

If the force is constant, then the integration simplifies to:

I = ΣFΔt (Δt is the time over which the force acts).[3])

Conservation of Momentum

If no external forces act in a direction, momentum is conserved in that direction. If momentum is conserved, then it remains constant. Don't confuse this with conservation of mechanical energy, which is different (although mechanical energy and momentum may both be conserved in some situations[4]).

Centre of Mass

[5]Most objects are not single particles, but rather are finite bodies with their mass distributed across the body, or systems of particles in a container. The motion of these objects may be described using their centre of mass. When a net force acts on an object, the object undergoes translational motion as though its entire mass is concentrated at its centre of mass. This is not affected other types of motion (such as rotation or vibration) or by deformation of the object.

rcm = Σ(miri)/Σmi

The displacement of the centre of mass is given by "rcm". To find this point, each individual mass (mi) is multiplied by its displacement (ri) and the sum of these values is taken. This is then divided by the total mass (or the sum of individual masses, Σmi). Note that this equation will have to be applied in both x and y directions, to find both coordinates of the centre of mass.

For a continuous body, integration may be used:

rcm = (1/Σmi)ʃrdm

Multi-particle systems

[6]A multi-particle system is one in which there are multiple particles (multiple masses). If a system has constant mass M, then it will respond to external forces as though it were a single particle of mass M, located at the system's centre of mass. The total momentum of the system (ptot)is equal to the sum of individual momenta (pi) in the system:

ptot = Σpi

The total momentum may be described as the product of the system's mass (M) and the velocity of the centre of mass (vcm):

ptot = Mvcm

Furthermore, all internal forces appear in pairs and cancel out (Newton's third law) so only external forces will affect the system. In other words, the change in momentum of the system (Δptot) is equal to the total impulse (I) imparted to the system by external forces:

Δptot = I

Note that if the impulse is zero in any direction, then no external forces act in that direction and so momentum is conserved in that direction. This means that any interactions between particles in the system will conserve momentum.


[7]Collisions occur when two objects interact by means of forces. Most often this represents two object physically striking one another, but another example is the slingshot effect, using gravity. Any external forces are assumed to be negligible compared to the internal forces of the interaction, so momentum is conserved.

Collisions in one dimension

[8]Momentum is conserved during a collision. The momentum of the system before (i) and afterwards (f) will be the same. In equation form, for a collision between two objects (1 and 2):

p1,i + p2,i = p1,f + p2,f

(m1v1)i + (m2v2)i = (m1v1)f + (m2v2)f

These equations are useful for determining the motion of objects before and after collisions. Depending on whether an equation is elastic or not, other equations are produced which are also useful in determining colliding the objects' behaviour.

Elastic and inelastic collisions

[9] Whether a collision is elastic or inelastic indicates whether or not kinetic energy is conserved during the collision. The kinetic energy of the system's centre of mass is constant throughout the collision, but the kinetic energy of the individual particles relative to the centre of mass may change. This means the total kinetic energy of the system may or may not be conserved (note that momentum is conserved in collisions regardless of kinetic energy).

  • In an elastic collision, all kinetic energy is conserved (non conservative forces do no work).
    • The particles do not stick together at all in the collision.
    • In reality, some kinetic energy is always lost as heat or sound, so no collision is truly elastic.
    • The constant kinetic energy of elastic collisions may be represented using the equation:

(½m1v12)i + (½m2v22)i = (½m1v12)f + (½m2v22)f , since K = ½mv2

  • In an inelastic collision, kinetic energy is transferred to other forms (not conserved).
    • The objects will partly stick to one another.
    • Most collisions are inelastic.
    • A perfectly inelastic collision is one where all kinetic energy is lost, so the objects will not separate after the collision.
    • A perfectly inelastic collision may be described using the following equation (since all the mass is together after the collision):

(m1v1)i + (m2v2)i = (m1 + m2)vf

Collisions in two dimensions

[10]Collisions occur in two dimensions when particles are no longer constrained to move in a single direction. One common example is a glancing collision, where one object strikes another off-center, so the objects both change direction and momentum.

Two-dimensional collisions are best dealt with by splitting the collision up into x and y components. Momentum is conserved in the x and y directions, and so the equations describing conservation of momentum may be used for each direction. The problem then becomes one of simultaneous equations.

Example 1 - elastic collision in two dimensions

Elastic collision 2 dimensions.jpg

 In the x direction: m1v1i = m1v1fcosθ + m2v2fcosφ
 In the y direction: 0 = m1v1fsinθ + m2v2fsinφ

Example 2 - perfectly inelastic collision in two dimensions

Inelastic collision 2 dimensions.jpg

 In the x direction: m1v1i = (m1 + m2)vfcosθ
 In the y direction: m2v2i = (m1 + m2)vfsinθ


This is the end of this topic. Click here to go back to the main subject page for Higher Physics 1A.


Textbook refers to Serway & Jewett, Physics for Scientists and Engineers (Brooks/Cole , 8th ed, 2010)
(Slides) refers to those distributed by Wolfe, J (2012) on his First Year Physics site

  1. Textbook, pp235-236
  2. Textbook, pp240-241
  3. (Slides), Mechanics for systems of particles and extended bodies, p11
  4. (Slides), Mechanics for systems of particles and extended bodies, p12
  5. Textbook, pp253-255
  6. Textbook, pp258-259
  7. Textbook, pp242-243
  8. Textbook, pp242-243
  9. Textbook, pp243-244
  10. Textbook, pp250-251
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