Linear Momentum (OCR A Level Physics) : Revision Note

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.

Answer:

• 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)

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