Introduction to SAT II Physics - FreeExamPapers

We use pulleys to lift objects because they reduce the amount of force we need to exert. For
example, say that you are applying force F to the mass in the figure above. How does F compare
to the force you would have to exert in the absence of a pulley?
To lift mass m at a constant velocity without a pulley, you would have to apply a force equal to the
mass’s weight, or a force of mg upward. Using a pulley, the mass must still be lifted with a force
of mg upward, but this force is distributed between the tension of the rope attached to the ceiling,
T, and the tension of the rope gripped in your hand, F.
Because there are two ropes pulling the block, and hence the mass, upward, there are two equal
upward forces, F and T. We know that the sum of these forces is equal to the gravitational force
pulling the mass down, so F + T = 2F = mg or F = mg/2. Therefore, you need to pull with only
one half the force you would have to use to lift mass m if there were no pulley.
Standard Pulley Problem
The figure above represents a pulley system where masses m and M are connected by a rope over
a massless and frictionless pulley. Note that M > m and both masses are at the same height above
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