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GCSE Questions: Momentum

Q2.

(a) An adult of mass 80 kg has more inertia than a child of mass 40 kg.

What is inertia?

Inertia is the 'tendency of an object to continue in its state of rest or motion'. It indicates how difficult it is to change the velocity of an object.

[1 mark]

(b) A teacher demonstrated the idea of a 'safety surface'. She dropped a raw egg into a box filled with pieces of soft foam. The egg did not break.

The diagram shows the demonstration.

Explain why the egg is less likely to break when dropped onto soft foam rather than onto a concrete floor.

When an object hits a surface the impact it experiences changes the momentum of the object to zero.

The force it experiences is equal to the rate of change of momentum.

Ft = Δp

F = Δp/t

The soft foam increases the time taken for the egg to stop and change its momentum. This decreases its rate of change of momentum and therefore reduces the force the egg experiences - making it less likely to break.

[4 marks]

(c) The diagram shows a child on a playground swing. The playground has a rubber safety surface.

A child of mass 32 kg jumped from the swing.

When the child reached the ground she took 180 milliseconds to slow down and stop.

During this time an average force of 800 N was exerted on her by the ground.

Calculate the velocity of the child when she first touched the ground.

t = 180 ms = 180 x 10-3s

F = 800 N

m = 32 kg

Ft = Δp

Ft = Δ (mv)

Δ v = Ft/m

Δ v = 800 x 180 x 10-3 /32

Δ v = 4.5 m/s

Her final velocity is 0 m/s

So her velocity on hitting the floor is 4.5 m/s

[4 marks]

[9 Marks TOTAL]