Force & Momentum
- Force is defined as the rate of change of momentum on a body
Force is equal to the rate of change in momentum
- The change in momentum is defined as the final momentum minus the initial momentum:
pfinal − pinitial
- Force and momentum are vectors so they can be either positive or negative values
Worked example
A car of mass 1500 kg hits a wall at an initial velocity of 15 m s-1.
It then rebounds off the wall at 5 m s-1 and comes to rest after 3.0 s.
Calculate the average force experienced by the car.
Direction of Forces
- The force that is equal to the rate of change of momentum is still the resultant force
- A force on an object will be negative if it is directed in the opposite motion to its initial velocity. This means that the force is produced by the object it has collided with
Fcar = –Fwall
- The diagram shows a car colliding with a wall
- It is the wall that produces a force of -300N on the car
- Due to Newton’s Third Law (see “Newton’s Laws of Motion”), the car also produces a force of 300N back onto the wall
Maths tip
- ‘Rate of change’ describes how one variable changes with respect to another. In maths, how fast something changes with time is represented as dividing by Δt (e.g. acceleration is the rate of change in velocity)
- More specifically, Δt is used for finite and quantifiable changes such as the difference in time between two events
Worked example
A tennis ball hits a racket with a change in momentum of 0.5 kg m s-1.For the different contact times, which tennis racket experiences more force from the tennis ball?
Examiner Tip
In an exam question, carefully consider what produces the force(s) acting. Look out for words like ‘from’ and ‘acting on’ to determine this and don’t be afraid to draw a force diagram to figure out what is going on.
