O-LevelPhysicsForces5054/22 · Oct/Nov 2019 · Q26 Marks

A student uses a pump to inflate a bicycle tyre. [Figure 2.1] shows the pump and the tyre.

(a)(i)The pressure of the trapped air in the pump is 3.8 × 10⁵ Pa and the cross-sectional area of the piston is 6.1 × 10⁻⁴m². Calculate the force exerted on the piston by the trapped air in the pump.[2]
(a)(ii)The student pushes the handle to the right and the piston forces the trapped air into the tyre. The force exerted by the student is less than the value in (a)(i). Suggest one reason why.[1]
(b)The temperature of the air in the pump remains constant as the handle moves to the right. Explain, in terms of its molecules, why the pressure of a gas increases when its volume is decreased at constant temperature.[3]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.

[C1] (F=) pA or 3.8 x 10⁵ x 6.1 x 10⁻⁴
[A1] 230 N
[B1] force on other side of piston due to air / atmospheric pressure or pressure of trapped air decreases as valve opens (and air enters the tyre)
[B1] molecules collide with walls / piston / pump / surface
[B1] number of molecules per unit volume increases (as volume decreases)
[B1] more collisions per unit area or more frequent collisions / collide more often

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