O-LevelPhysicsForces5054/21 · May/June 2022 · Q19 Marks

Fig. 1.1 shows a model of the human arm. The rubber band represents the muscle that moves part of the arm XY up. A mass is suspended from XY, as shown in Fig. 1.2. The weight of section XY is negligible and the model is at rest.[Figure 1.1][Figure 1.2 (not to scale)]

(a)(i)State two ways in which the dimensions of the rubber band change as the mass is added to section XY.[2]
(a)(ii)State the form of energy stored in the stretched rubber band.[1]
(b)(i)State the principle of moments.[2]
(b)(ii)Explain why the force that the rubber band exerts on section XY is larger than the weight of the mass.[1]
(b)(iii)The mass suspended from section XY in Fig. 1.2 has a weight of 4.0N. Calculate the force that the rubber band exerts on section XY.[2]
(b)(iv)Explain how your answer to (b)(iii) is different if the weight of section XY is not negligible.[1]

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Mark scheme

Marking points from the official mark scheme (May/June 2022). The mark or mark type is shown in brackets.

[1] (a)(i) longer
[1] (a)(i) thinner or smaller (cross-sectional) area
[1] (a)(ii) elastic potential or strain (energy)
[1] (b)(i) sum of / total / net / resultant moment is zero
[1] (b)(i) when (the object is) in equilibrium
[1] (b)(ii) distance of rubber band to pivot is smaller than the distance of the mass (to pivot)
[1] (b)(iii) 4 x 18 = F × 2
[1] (b)(iii) 36 N
[1] (b)(iv) greater and the weight (of XY) contributes to the (clockwise) moment/turning effect or the moment of the rubber band force is larger

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