4. Forces, density and pressure
A section of Physics, 9702
Listing 10 of 283 questions
Explain what is meant by centre of gravity. Define moment of a force. A student is being weighed. The student, of weight W, stands 0.30 m from end A of a uniform plank AB, as shown in . 0.30 m 0.20 m 0.50 m 2.0 m 80 N 70 N A W B P (not to scale) The plank has weight 80 N and length 2.0 m. A pivot P supports the plank and is 0.50 m from end A. A weight of 70 N is moved to balance the weight of the student. The plank is in equilibrium when the weight is 0.20 m from end B. State the two conditions necessary for the plank to be in equilibrium. 1. 2. Determine the weight W of the student. W = N If only the 70 N weight is moved, there is a maximum weight of student that can be determined using the arrangement shown in . State and explain one change that can be made to increase this maximum weight.
9702_s11_qp_21
THEORY
2011
Paper 2, Variant 1
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Define centre of gravity. A uniform rod AB is attached to a vertical wall at A. The rod is held horizontally by a string attached at B and to point C, as shown in . 50° C A B string wall 1.2 m 8.5 N mass M O T The angle between the rod and the string at B is 50°. The rod has length 1.2 m and weight 8.5 N. An object O of mass M is hung from the rod at B. The tension T in the string is 30 N. Use the resolution of forces to calculate the vertical component of T. vertical component of T = N State the principle of moments. Use the principle of moments and take moments about A to show that the weight of the object O is 19 N. Hence determine the mass M of the object O. M = kg Use the concept of equilibrium to explain why a force must act on the rod at A.
9702_s13_qp_22
THEORY
2013
Paper 2, Variant 2
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A cylinder is made from a material of density 2.7 g cm–3. The cylinder has diameter 2.4 cm and length 5.0 cm. Show that the cylinder has weight 0.60 N. The cylinder in is hung from the end A of a non-uniform bar AB, as shown in . 12 cm bar cylinder B P A 20 cm 0.25 N 0.60 N 50 cm X The bar has length 50 cm and has weight 0.25 N. The centre of gravity of the bar is 20 cm from B. The bar is pivoted at P. The pivot is 12 cm from B. An object X is hung from end B. The weight of X is adjusted until the bar is horizontal and in equilibrium. Explain what is meant by centre of gravity. Calculate the weight of X. weight of X = N The cylinder is now immersed in water, as illustrated in . water X B P A 0.25 N An upthrust acts on the cylinder and the bar is not in equilibrium. Explain the origin of the upthrust. Explain why the weight of X must be reduced in order to obtain equilibrium for AB.
9702_s17_qp_21
THEORY
2017
Paper 2, Variant 1
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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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Questions Discovered
283