AQA A Level Physics

Revision Notes

4.5.1 Linear Momentum

Linear 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 with this throughout the question

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