The linear momentum is defined as:

Impulse is defined as an average force

If an impulse acts on a particle of mass

where

When two objects collide, momentum is conserved. The figure below shows a collision between two objects (before and after).

Where:

The vector equation for conservation of linear momentum can be expressed as:

For all three directions in x, y, z this becomes:

For an elastic collision, kinetic energy is conserved. Thus,

For elastic collisions in one-dimension (head-on collision):

The angular momentum

Where:

If no net external torque acts on the body,

Thus,

where the subscripts 1 and 2 denote "initial" and "final".

For example, a figure skater pulling his arms in during rotation will spin faster as a result. This is because there is no net external torque acting on his body. So as he pulls in his arms he is decreasing his

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