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First teaching 2020

Last exams 2024

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Force & Momentum (CIE AS Physics)

Revision Note

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Katie M

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Force & Momentum

  • Force is defined as the rate of change of momentum on a body

 Force and momentum equation, downloadable AS & A Level Physics revision notes

Force is equal to the rate of change in momentum

 

  • The change in momentum is defined as the final momentum minus the initial momentum:

pfinalpinitial

  • 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.

WE - Force on a car answer image (1), downloadable AS & A Level Physics revision notesWorked example-force on a car (2), downloadable AS & A Level Physics revision notes

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

 

Direction of forces, downloadable AS & A Level Physics revision notes

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

 

WE - Tennis ball contact time content part, downloadable AS & A Level Physics revision notes

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?

WE - Tennis ball contact time answer image, downloadable AS & A Level Physics revision notes

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.

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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.