O-LevelPhysicsForces5054/21 · Oct/Nov 2017 · Q16 Marks

Fig. 1.1 shows a painter standing on a wooden plank, directly above the right-hand support. [Figure 1.1] The plank has length 3.6m, width 0.35m and thickness 0.025m. The gravitational field strength g is 10N/kg and the mass of the plank is 23 kg.

(a)Calculate the density of the wood from which the plank is made. density = [2]
(b)(i)The centre of mass of the plank is in the middle of the plank at a distance of 1.3m from each of the supports. Calculate the weight of the plank, weight = [1]
(b)(ii)the moment of the plank about the right-hand support. moment = [2]
(c)The painter moves further to the right along the plank and the plank rotates about the right-hand support. Explain why the plank rotates.[1]

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Mark scheme

Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.

[C1] (p = )m / V or 23 / (3.6 × 0.35 × 0.025) or 23 / 0.0315
[A1] 730 kg/m³
[B1] 230 N
[C1] (Γ= )Wx₁r or 230 × 1.3
[A1] 300 Nm
[B1] moment of painter / clockwise moment (about support) is greater than / not equal to / different from moment of plank/anticlockwise moment

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