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Key Words

Direction  - a line along which something moves.

Directly proportional - if a dependent variable doubles when the independent variable doubles, this is directly proportional.

Force - push, pull, or twist.

Kinetic Energy - energy of movement.

Mass - the amount of material in an object.

Momentum - the product between mass and velocity.

Time - period in which something happens (or doesn't).

Transferred - moved from one place to another.

Velocity - speed in a certain direction

Test Yourself

Homework
Physics GCSE
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Grade E

The Law of Conservation of Momentum tells us that momentum is conserved when no outside force is applied.  When a force is applied, momentum changes:

  • If it speeds up or slows down, the velocity changes.  Therefore the momentum changes.
  • If the direction changes, the velocity must change too.  So the momentum changes.

Remember that direction is very important in momentum.

The change in momentum not only depends on the force, but also on the time:

If we apply a small force for a long time, we get a large change in momentum.  We would also get the same change in momentum by applying a really big force for a short time.

Think about this:

  1. A car travelling at 30 m/s brakes and slows down to zero in 10 s. 
  2. The same car travelling at the same speed hits a tree.  It slows down in 0.1 s. 

The change in momentum is the same.  The force in (2) is 100 times bigger, because it causes the same change in momentum, but in 1/100th the time.  The implication is that in case (1), there is no risk of injury.  In case 2, the people inside will be killed.

 

Grade C

The crumple zone in a car is designed to increase the time taken for the car to slow down.  In the old days, cars were designed to be very strong and solid.  The car could come away from a fatal accident perfectly drivable.  But that would be no good to its late owner.

A modern car is more easily damaged.  However, it's much easier to repair a car or get another one than to replace its owner.

The same applies to padded mats or sandpits that athletes jump into.

This picture shows the athlete Chris Tomlinson landing in a sandpit.  If he did this onto tarmac, he would not be jumping for long.

Grade A

We can link momentum with acceleration:

force (N) = mass (kg) × acceleration (m/s2)

We know that:

acceleration (m/s2) = change in velocity (m/s) ÷ time (s)

We can combine these to give:

force (N) = mass (kg) × (change in velocity (m/s) / time (s))

Since momentum is mass × velocity, it is reasonable to say that:

 change in momentum = mass × change in velocity

So we can write:

force (N) = change in momentum (kg m/s) ÷ time (s)

We can rearrange this to give:

change in momentum (kg m/s) = force (N) × time (s)

 

You may have spotted that the units for change in momentum could also be newton seconds (NOT newton per second).  1 kg m/s = 1 Ns.

The change in momentum is often called impulse.