WJEC Physics for AS Level: Revision Workbook

Ffion’s answers (a) The sum of the forces in any one direction on the object is zero. ✓ bod The sum of the clockwise moments about the pivot is equal to the sum of anticlockwise moments ✓ about the pivot. غ (b) (i) (I) 35.316 11.76 11.8 N 35.3 N ✓ ✓ (II) 1.3T A + 24.71 = 1.3T B T A + T B = 47.1 N ✓ Ą T A + 24.71 = 1.3 (47.1 - T A ) Ą T A = 46.52 Ą T A = 17.9 N and T B = 29.2 N ✓ غ ✓ (ii) 0.2 W = 1.3 × 11.8 غ ✓ Ą : 1 mass = . . 9 81 76 7 = 7.8 kg ✓ MARKER NOTE Ffion should have written 'the sum of the components of the forces in any direction is zero’. This is a subtle point and the examiner awarded the first mark bod. The third mark was not awarded because there does not have to be a pivot – moments can be taken about any point. 2 marks MARKER NOTE A perfect answer. 2 marks MARKER NOTE The first equation resulted from applying the PoM about the centre of the rod. Since the candidate did not tell us this, any slip in calculating the 24.7(1) N m moment of the spotlight would have made the method almost impossible to follow and most of the marks would have been lost – a high-risk tactic! Note also that it would have been neater to take moments about A or B because the ‘perpendicular distance’ of T A or T B would be zero so only one of the unknown tensions would appear in the equation. As it is, the candidate has solved a pair of simultaneous equations, making only an arithmetical mistake (46.52 instead of 36.52), and losing the third mark. 3 marks MARKER NOTE The first mark is lost because the candidate did not make any attempt to explain her reasoning, as required in the question. The analysis was spot on. 2 marks TOTAL 9 marks / 12 22 | WJEC Physics for AS Level: Revision Workbook | Unit 1: Motion, Energy and Matter