A LevelPhysicsOCRRevision Notes3. Forces & Motion3.8 Newton's Laws of Motion & Momentum3.8.2 Linear Momentum

Linear Momentum (OCR A Level Physics)

Revision Note

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Katie M

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Linear Momentum

  • When an object with mass is in motion and therefore has a velocity, the object also has momentum
  • Linear momentum is the momentum of an object that is moving in only one dimension
  • The linear momentum of an object remains constant unless the system is acted upon by an external resultant force

  • Momentum is defined as the product of mass and velocity

p space equals space m v

  • Where:
    • p = momentum, measured in kg m s−1 
    • m - mass, measured in kg
    • v = velocity, measured in m s−1 

  • Momentum is a vector quantity with both magnitude and direction
    • The initial direction of motion is usually assigned the positive direction

 

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

A tennis ball of mass 60 g travels to the right with a speed of 75 m s−1.

A brick of mass 3 kg is thrown to the right at a speed of 1.5 m s−1.   

Determine which object has the greatest 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)

Examiner Tip

Since the SI units for momentum are kg m s−1:

  • If the mass is given in grams, you need to convert to kg by dividing the value by 1000
  • If the velocity is given in km s−1, you need 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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      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.