Lesson Objective Understand the modelling assumptions used when modelling pulleys BE able to sole modelling problems involving pulleys in equilibrium The blocks in the diagram below are in equilibrium. Assume that the pulley is smooth and that g = 10ms -2 4 kg 3 kg 6 kg a)What does the pulley actually do? b)Why is it important that the pulley is smooth? What would happen if it wasnt smooth? c) What is the friction force on the 4kg block? d) What is the tension in the string between the 6kg and 3kg block? e) What is the force exerted by the string on the pulley? Pulleys If two particles are connected by a string passing over a smooth pulley or peg, the heavier particle will move downwards and the lighter one upwards. Three modelling assumptions are important here (and often appear in examination questions). * As before, if they are connected by an inextensible string, they will have the same acceleration. * If the pulley or peg is smooth, the tension in the portions of string either side of the pulley or peg will be the same. Anyone who has made a guitar from an empty shoe box and an elastic band is relying on the friction between box and band to keep one part of the band more taut than the other. * If the string is light, it has no mass, and therefore the tension will be constant along its length. If the string had mass, the portion of the string higher up would be more tense as it would have more mass to support beneath it.