O-LevelPhysicsThermal Physics5054/21 · Oct/Nov 2023 · Q19 Marks

Fig. 1.1 shows a toy water rocket. [Figure 1.1] The rocket is half-filled with water and connected by rubber tubing to a pump.

(a)(i)Explain, in terms of particles, why the air in the rocket exerts a pressure on the walls of the rocket.[3]
(a)(ii)The temperature of the air in the rocket remains constant as air is pumped into the rocket. Explain why increasing the number of air particles in the rocket increases the pressure of the air in the rocket.[1]
(b)When the air pressure in the rocket is high, the rubber tubing is removed from the rocket. The water in the rocket is expelled downwards. Explain, in terms of momentum, why the rocket begins to accelerate upwards.[3]
(c)As the water is expelled from the rocket, the upward acceleration of the rocket changes. Suggest two reasons for the change in the acceleration of the rocket. 1 2[2]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2023). The mark or mark type is shown in brackets.

[B1] particles move / collide / hit
[B1] particles collide with (inner) walls (of rocket)
[B1] force exerted by collisions over / on an area
[B1] more collisions per unit time
[B1] (expelled) water gains / has (downward) momentum
[B1] momentum conserved
[B1] rocket gains (upward) momentum (and accelerates)
[B2] any two from: mass (of water and rocket) decreases / changes; pressure (of air) decreases; (air) resistance (acts); gravitational field strength decreases (with height); less water expelled per second / water expelled at slower speed

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