Linear Momentum (AQA 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.

[2]

Answer:

Step 1: Calculate the momentum of the tennis ball

$p=mv$

$p=0.06\times 75$

$p=4.5\mathrm{kg}\mathrm{m}{\mathrm{s}}^{-1}$

Step 2: Calculate the momentum of the brick

$p=mv$

$p=3\times 1.5$

$p=4.5\mathrm{kg}\mathrm{m}{\mathrm{s}}^{-1}$ [1 mark]

Step 3: Compare the momentum of each object

• Both the tennis ball and the brick have the same momentum [1 mark]

• 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 as the signs of the numbers (positive or negative) are consistent with this throughout the question