A-LevelPhysicsWork, energy and power9702/22 · Feb/Mar 2021 · Q210 Marks

(b) A beach ball is released from a balcony at the top of a tall building. The ball falls vertically from rest and reaches a constant (terminal) velocity. The gravitational potential energy of the ball decreases by 60 J as it falls from the balcony to the ground. The ball hits the ground with speed 16 ms⁻¹ and kinetic energy 23 J.

(a)State what is meant by work done.[1]
(b)(i)Show that the mass of the ball is 0.18kg.[2]
(b)(ii)Calculate the height of the balcony above the ground. height = ..................................................... m[2]
(b)(iii)Determine the average resistive force acting on the ball as it falls from the balcony to the ground. average resistive force = ..................................................... N[2]
(c)State and explain the variation, if any, in the magnitude of the acceleration of the ball in (b) during the time interval when the ball is moving downwards before it reaches constant (terminal) velocity.[3]

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2021). The mark or mark type is shown in brackets.

[B1] (a) force × displacement in the direction of the force
[C1] (b)(i) E = 1/2mv 2
[A1] (b)(i) (m =) 23 × 2/162 = 0.18 (kg)
[C1] (b)(ii) (Δ)Ε = mg(△)h 60 = 0.18 x 9.81 × h
[A1] (b)(ii) h = 34 m
[C1] (b)(iii) (work done =) 60 – 23 = 37 (J)
[A1] (b)(iii) average resistive force = 37/34 = 1.1 N
[B1] (c) air resistance (acting on ball) increases
[B1] (c) resultant force (on ball) decreases or weight constant and air resistance increases
[B1] (c) acceleration decreases

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