### Forces in Collisions

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• #### Message 1.

Posted by BitesizePhysicsTeacher (U15165682) on Saturday, 9th February 2013

In any collision there will be a change in momentum, Δmv, which involves an impulse, F t.

We know that the impulse is equal to the change in momentum -

F t = Δmv

The materials involved in collisions have a very large influence on the forces involved, because they change the time of duration of the impact.

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Example -

When a steel hammer hits an iron anvil, the surfaces are very hard and the objects bounce off each other very quickly, so the time of impact, t, is very short. This means that force, F, must be very large.

If a rubber hammer of the same mass were used, it would have the same change in momentum, and the same impulse. But because the rubber would change shape during the collision, making the time of impact, t, longer. Because the impulse must be the same, the Force, F, is reduced.

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Many safety devices use this effect to make impacts last longer, in order to reduce the forces acting in collisions -

Crash helmets - the foam lining increases the time of impact for collisions involving the skull, reducing the forces acting

Airbags - slows the passenger down over a longer period of time using an inflated bag, reducing the forces acting during impact

Crumple zones - deform the body of the vehicle to make it slow down over a longer period of time, reducing the forces acting on the occupants
[these also absorb energy in the collision as the forces acting over a distance do WORK in changing the shape of the body work -
i.e. F(w) = F d]

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