WJEC Physics for A2: Study and Rev Guide

2. A washing machine has a drum (a metal cylinder open at one end and closed at the other) of diameter 0.51m , mounted with its axis horizontal. On the spin-dry phase the cylinder rotates at 1500 revolutions per minute . (a) Calculate the drum’s angular velocity (in SI units). (b) Calculate the centripetal force on a wet sock of mass 0.085kg clinging to the inside wall of the drum. (c) Hence explain why gravity has almost no effect on the action of the dryer. (d) There are small holes in the curved surface of the drum. Explain why water droplets from the socks escape through these holes rather than staying in the sock. 3. A pendulum consists of a bob of mass m suspended from a xed point by a string of length l . The string is held taut and horizontal, then the bob is released (Fig. 3.1.7). l m Fig. 3.1.7 Pendulum (a) Use the principle of conservation of energy to show that the bob’s speed at its lowest point is given by v = 2gl (b) Hence express the centripetal force on the bob at its lowest point in terms of m and g . (c) Calculate the tension in the string, for the bob at its lowest point, remembering that the centripetal force is the resultant of two forces on the bob. 12 A Level Physics: Study and Revision Guide