• Force and momentum
• Impulse = change in momentum
• Force and impulse
• Examples

Force and Momentum

Physicists often say “momentum is conserved”. However, this is not always the case. It is only true of there is no external force acting.

If there is an external force acting on an object, then the momentum of that object is not conserved, it is changing.

Impulse = Change in Momentum

The word impulse means the change in momentum. In Physics, we use the greek symbol Δ (“Delta”) to mean “change in”, so

Δp = “change in momentum” = impulse

Force and Impulse

The external force acting on an object is related to the impulse given to the object by:

where

• F = force (Newtons, N)
• Δp = impulse = change in momentum (kgm/s or Ns)
• t = time (seconds, s)

Worked Examples

1. A 2000kg car is travelling at 60mph (27m/s) crashes into a wall. The impact lasts 0.02s. What force does the wall exert on the car?
2. What force does a passenger experience if his mass is 70kg?
3. Suppose the car has a much better crumple zone, and the passenger wears his seatbelt, such that the impact time is reduced to 0.6s. What force would he experience then?