Momentum (Edexcel International A Level Physics)

Revision Note

Test Yourself
Lindsay Gilmour



Defining Momentum

  • Linear momentum (p) is defined as the product of mass and velocity

Linear momentum equation, downloadable AS & A Level Physics revision notes

Momentum is the product of mass and velocity

  • Momentum is a vector quantity - it has both a magnitude and a direction
  • This means it can have a negative or positive value
    • If an object travelling to the right has positive momentum, an object travelling to the left (in the opposite direction) has a negative momentum

  • The SI unit for momentum is kg m s−1


Negative momentum, downloadable AS & A Level Physics revision notes

When the ball is travelling in the opposite direction, its velocity is negative. Since momentum = mass × velocity, its momentum is also negative

Worked example

Which object has the most momentum?WE - Momentum comparison question image, downloadable AS & A Level Physics revision notes

WE - Momentum comparison answer image, downloadable AS & A Level Physics revision notes

  • Both the tennis ball and the brick have the same momentum
  • Even though the brick is much heavier than the ball, the ball is travelling much faster than the brick
  • This means that on impact, they would both exert a similar force (depending on the time it takes for each to come to rest)

Exam Tip

Since momentum is in kg m s−1:

  • If the mass is given in grams, make sure to convert to kg by dividing the value by 1000.
  • If the velocity is given in km s−1, make sure to convert to m s−1 by multiplying the value by 1000

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

Sketching a diagram which includes the signs on positive and negative values will help you avoid mistakes when calculating

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Lindsay Gilmour

Author: Lindsay Gilmour

Lindsay graduated with First Class Honours from the University of Greenwich and earned her Science Communication MSc at Imperial College London. Now with many years’ experience as a Head of Physics and Examiner for A Level and IGCSE Physics (and Biology!), her love of communicating, educating and Physics has brought her to Save My Exams where she hopes to help as many students as possible on their next steps.

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