The pulleys used in this type of question are made up a grooved wheel and a block which holds it. A rope runs in the groove around the wheel and one end will usually be attached to either: a weight, a fixed object like the ceiling or to another pulley. For the purposes of these questions you can ignore the effect of friction.
Single Pulley
3. Which weight requires the least force to move?
Answer
3. B – Weight B requires a force equal to 5 Kg whereas A requires a force equal to 10 Kg.
Single pulley questions are relatively straightforward. If the pulley is fixed, then the force required is equal to the weight. If the pulley moves with the weight then the force is equal to half of the weight. Another way of thinking about this is to divide the weight by the number of sections of rope supporting it to obtain the force needed to lift it. In A there is only one section of rope supporting the weight, so 10/1 = 10 Kg required to lift the weight. In B there are two sections of rope supporting the weight, so 10/2 = 5 Kg required to lift it.
Double Pulleys
There are two possible ways that two pulleys can be used. Either one pulley can be attached to the weight or neither of them can be.
4. Which weight requires the least force to move?
· A
· B
· Both require the same force
Answer
4. A – Weight A requires a force equal to 5 Kg whereas weight B requires a force equal to 10 Kg. Remember to divide the weight by the number of sections of rope supporting it to get the force needed to lift the weight.
Using More Than Two Pulleys
5. How much force is required to move the weight?
· 100kg
· 150kg
· 50kg
· 60kg
Answer
5. C – The weight is 300 Kg and there are 6 sections of rope supporting it. Divide 300 by 6 to get 50 Kg. In all cases, just divide the weight by the number of sections of rope supporting it to get the force needed to lift the weight.
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