GCSE PHYSICS - When does Momentum Change? - What is the Equation for a Change in Momentum?

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When does Momentum Change?

The law of conservation of momentum only applies if there are
no external forces. An example of an external force is friction.
Friction is a resultant force that makes
objects slow down and stop if there is no force to balance it.

Newton's second law tells us that a resultant force will cause
the motion (and therefore the momentum) of an object to change.

What is the Equation for a Change in Momentum?

Force = change in momentum ÷ time taken for the change.

This equation is written as
F = (mv - mu) ÷ t

where F = Force
mv = final momentum (the one it ended up with)
mu = initial momentum (the one it started with)
t = time

This equation can be rearranged to give

Change in momentum = Force x time.
or mv - mu = F x t

The units of momentum can be therefore be written as
Ns called Newton seconds (Force x time) as well as kgm/s.

An example of a change in momentum is a moving car
being stopped by using its brakes or by crashing into a wall.

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