A-LevelPhysicsWork, energy and power9702/22 · May/June 2020 · Q39 Marks

Explain what is meant by work done.

(a)Explain what is meant by work done.[1]
(b)(i)1.the decrease in gravitational potential energy[2]
(b)(i)2.the increase in kinetic energy.[2]
(b)(ii)Use your answers in (b)(i) to determine the average resistive force acting on the ball as it falls from the top of the building to the ground.[2]
(c)The ball in (b) is dropped at time t = 0 and hits the ground at time t = T. The acceleration of free fall is g. On Fig. 3.1, sketch a line to show the variation of the acceleration a of the ball with time t from time t = 0 to t = T. [Figure 3.1][2]

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Mark scheme

Marking points from the official mark scheme (May/June 2020). The mark or mark type is shown in brackets.

[B1] (work done =) force × displacement in direction of the force
[C1] (Δ)E = mg(△)h
[A1] = 0.42 x 9.81 × 78 = 320 J
[C1] E = 1/2mv²
[A1] (Δ)E = 1/2 × 0.42 × 23² = 110 J
[C1] work done = 320 – 110 (= 210 N)
[A1] average resistive force = 210/78 = 2.7 N
[M1] downward sloping line from (0, g) to a non-zero value on the time axis
[A1] line is curved with a gradient that becomes less negative and the line meets t-axis at time t < T

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