WJEC Physics for AS Level: Revision Workbook

Question and mock answer analysis Q & A 1 (a) An object is acted on by forces whose lines of action are all in one plane. State the conditions necessary for the object to be in equilibrium. [3] (b) In a theatre a spotlight of mass 3.60 kg hangs from a uniform rod of length 3.00 m and mass 1.20 kg. The rod is kept horizontal by vertical wires attached to points A and B as shown. T B T A 0.20 m 0.20 m 2.20 m A B 3.60 kg Rod of length 3.00 m and mass 1.20 kg (i) (I) Use labelled arrows to show the magnitude and direction of the two other main forces on the system. [2] (II) Determine the tensions T A and T B . [4] (ii) Explaining your reasoning, determine the least mass that can be hung from anywhere on the bar to make the bar tilt. The 3.60 kg spotlight has been removed. [3] What is being asked This question is about the equilibrium of objects when acted upon by a number of forces. It has several parts involving AO1, AO2 and AO3, with verbal and mathematical skills. It starts with a recall part (a), on the conditions for equilibrium of an object under the influence of a number of forces. This part is designed to point you in the right direction for the analysis and evaluation of part (b). Mark scheme Question part Description AOs Total Skills 1 2 3 M P (a) 1. The [vector] sum of the forces on the object is zero or equivalent. [1] 2. The sum of the clockwise moments about any point equals the sum of the anticlockwise moments about that point. Non italic: [1], italic: [1] 3 3 (b) (i) (I) Vertical downward arrow from centre of rod, labelled 11.8 N (unit needed)[1] Vertical downward arrow on the spotlight, labelled 35.3 N (unit needed) [1] 2 2 2 (II) Correct equation from applying Principle of moments, or by implication ecf on 11.8 N and 35.3 N [1] 2nd correct eqn, e.g. T A + T B = 11.8 + 35.3 or PoM about a 2nd point. [1] T A = 14.0 N (accept 14 N) [1] T B = 33.1 N (accept 33 N) [1] Ecf on second force to be calculated if wrong only because of mistake in first. 4 4 4 20 | WJEC Physics for AS Level: Revision Workbook | Unit 1: Motion, Energy and Matter