GCSE SCIENCE PHYSICS HIGH SCHOOL - Momentum - Changing - Friction - Example

Momentum - Changing.

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 things slow down and stop.

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

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 car using its braking force to bring it to a stop.

See the next page for calculations.

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