Hanging over a pulley there is a
rope, with a weight at one end. At the other end hangs a monkey of
equal weight. What happens if the monkey starts to ascend the rope?
Assume that the mass of the rope and pulley are negligible, and the pulley
is frictionless.
The monkey is pulling down on the
rope hard enough to pull itself up. This increases the tension in
the rope just enough to cause the weight to rise at the same rate as the
monkey, since they are of equal mass.