PULLEYS
Pulleys are devices that change the direction of the tension force in the cords that
slide over them. For the purposes of this text and the SAT Physics Subject Test,
we’ll consider each pulley to be frictionless and massless, which means their
masses are so much smaller than the objects attached to the ends of them, that they
can be ignored.
In the case of two single masses m 1 and m 2 that are attached to a pulley and cord,
the downward forces are due to the weight (mass and gravity exerted on it) of the
masses. The upward forces are due to the tension (T) in the cord.