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The Change in Momentum in a Car Crash.

What is the Size of the Force on a Car when it Crashes?

A moving car which comes to a stop has a change in momentum.

In example 1 on the previous
page a car is uses its

braking
force to bring it to a stop.

In example 2 the same car is brought to a stop

by crashing into a wall.

Example 2.

A car has a mass of 1000 kg and moves with a velocity of 18 m/s.

The car crashes
into a wall and stops
in 1 second.

What is the stopping force during the collision?

Change in
momentum
= Force x time.

or mv - mu
= F x t.

mv = final momentum (the one it ended up
with)

mu = initial momentum (the
one it started with)

t = time

Final momentum

m x v
= 1000
x 0 because the car stopped.

= 0 kgm/s
.

Initial momentum

m
x u = 1000 x 18

= 18000
kgm/s.

The change in momentum mv - mu

= 0 - 18000

= - 18000
kgm/s.

(The negative sign only shows that the car lost
momentum).

Change in
momentum
= Force x time.

18000 = F x 1

F = 18000
÷ 1

=
18000
N.

The car that crashes
experiences a much larger force

than the car that stopped
safely
by braking.

The force is 15 times larger
for the car that crashes

because the car stopped in ^{1}/15^{th}
of the time.

This has implications for driver
and passenger safety.

Links Forces and Motion Momentum Revision Questions

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