Syllabus Edition

First teaching 2023

First exams 2025

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Force & Momentum (SL IB Physics)

Revision Note

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Force & Momentum

  • The resultant force on a body is the rate of change of momentum
  • The change in momentum is defined as:

increment p space equals space p subscript f space minus space p subscript i

  • Where:
    • Δp = change in momentum (kg m s–1)
    • pf = final momentum (kg m s–1)
    • pi = initial momentum (kg m s–1)
  • These can be expressed as follows:

F space equals space fraction numerator increment p space over denominator increment t end fraction

  • Where:
    • F = resultant force (N)
    • Δ= change in time (s)
  • This equation can be used in situations where the mass of the body is not constant

F space equals space m a

  • This equation can only be used when the mass is constant
  • The force and momentum equation can be derived from Newton's second law and the definition of acceleration

Direction of Forces

  • Force and momentum are vector quantities with both magnitude and direction 
  • The force that is equal to the rate of change of momentum is still the resultant force
  • The positive direction is taken to be the direction of the initial motion; therefore:
    • a force on an object will be negative if the force opposes its initial velocity
    • the opposing force is exerted by the object it has collided with 
    • the forces will be of equal magnitude and opposite in direction, in accordance with Newton's Third Law

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

The car exerts a force on the wall of  300 N, and due to Newton's third law, the wall exerts a force of -300N on the car 

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 s1. The car is in contact with the wall for 3.0 seconds.

Calculate the average force experienced by the car.

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

Worked example-force on a car (2), downloadable AS & A Level Physics revision notes

Exam Tip

The direction you consider positive is your choice, as long the signs of the numbers (positive or negative) are consistent with this throughout the question.

In an exam question, carefully consider what forces are exerted on what objects. Look out for words such as ‘from’, ‘acting on’ or 'exerted on' to determine this, and sketch a quick free body force diagram if you need to.

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Ashika

Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.