4.1. Turning effects of forces
A subsection of Physics, 9702, through 4. Forces, density and pressure
Listing 10 of 50 questions
State what is meant by the centre of gravity of a body. A uniform square sign with sides of length 0.68 m is fixed at its corner points A and B to a wall. The sign is also supported by a wire CD, as shown in . wire 54 N 0.68 m 0.68 m 35° D C sign wall B A E W (not to scale) The sign has weight W and centre of gravity at point E. The sign is held in a vertical plane with side BC horizontal. The wire is at an angle of 35° to side BC. The tension in the wire is 54 N. The force exerted on the sign at B is only in the vertical direction. Calculate the vertical component of the tension in the wire. vertical component of tension = N Explain why the force on the sign at B does not have a moment about point A. By taking moments about point A, show that the weight W of the sign is 150 N. Calculate the total vertical force exerted by the wall on the sign at points A and B. total vertical force = N The sign in is held together by nuts and bolts. One of the nuts falls vertically from rest through a distance of 4.8 m to the pavement below. The nut lands on the pavement with a speed of 9.2 m s−1. Determine, for the nut falling from the sign to the pavement, the ratio change in gravitational potential energy final kinetic energy . ratio =
9702_s19_qp_22
THEORY
2019
Paper 2, Variant 2
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State what is meant by the centre of gravity of an object. A non-uniform rod XY is pivoted at point P, as shown in . F P wire sphere Y X 44.0 N 1.10 m 1.70 m 4.00 m 3.0 N (not to scale) The rod has length 4.00 m and weight 44.0 N. The centre of gravity of the rod is 1.70 m from end X of the rod. Point P is 1.10 m from end X. A sphere hangs by a wire from end Y of the rod. The weight of the sphere is 3.0 N. The weight of the wire is negligible. A force F is applied vertically downwards at end X so that the horizontal rod is in equilibrium. By taking moments about P, calculate F. F = N Calculate the force exerted on the rod by the pivot. force = N The sphere in is now immersed in a liquid in a container, as shown in . F P liquid, density 1100 kg m–3 container 44.0 N The density of the liquid is 1100 kg m–3. The upthrust acting on the sphere due to the liquid is 2.5 N. The magnitude of F is unchanged so that the horizontal rod is not in equilibrium. Use Archimedes’ principle to determine the radius r of the sphere. r = m Calculate the magnitude and direction of the resultant moment of the forces on the rod about P. magnitude of resultant moment = N m direction of resultant moment
9702_s22_qp_21
THEORY
2022
Paper 2, Variant 1
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Distinguish between the moment of a force and the torque of a couple. moment of a force torque of a couple One type of weighing machine, known as a steelyard, is illustrated in . hook metal rod 4.8 cm pivot 12 N sliding weight 2.5 N sliding weight The two sliding weights can be moved independently along the rod. With no load on the hook and the sliding weights at the zero mark on the metal rod, the metal rod is horizontal. The hook is 4.8 cm from the pivot. A sack of flour is suspended from the hook. In order to return the metal rod to the horizontal position, the 12 N sliding weight is moved 84 cm along the rod and the 2.5 N weight is moved 72 cm. Calculate the weight of the sack of flour. weight = …………………………N Suggest why this steelyard would be imprecise when weighing objects with a weight of about 25 N.
9702_w08_qp_2
THEORY
2008
Paper 2, Variant 0
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State what is meant by the centre of gravity of a body. A uniform rectangular sheet of card of weight W is suspended from a wooden rod. The card is held to one side, as shown in . rod card On , mark, and label with the letter C, the position of the centre of gravity of the card, mark with an arrow labelled W the weight of the card. The card in is released. The card swings on the rod and eventually comes to rest. List the two forces, other than its weight and air resistance, that act on the card during the time that it is swinging. State where the forces act. 1. 2. By reference to the completed diagram of , state the position in which the card comes to rest. Explain why the card comes to rest in this position.
9702_w10_qp_22
THEORY
2010
Paper 2, Variant 2
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Questions Discovered
50