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) A |
B) B |
C) 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?
|
A) 100kg |
B) 150kg |
C) 50kg |
D)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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