Pulleys can also be used to decrease the force necessary to lift an object. For
example, consider a pulley system like the one above, except that you are pulling on
one side while there is a weight on the other side.
Pulling down with a force of magnitude FT creates a tension force FT throughout the
entire string. As a result, there are two tension forces, each of magnitude FT, pulling
up to lift the block. By using this setup, the force required to normally lift the block
would be halved.
In general, pulley systems multiply our force by the number of strings pulling on the
object.
- In the figure below, how much force would we need to exert on
the free end of the cord in order to lift the plank (mass m = 300 kg)
with constant velocity?
Here’s How to Crack It
There are a total of six tension forces pulling upward on the plank. In order to lift