Problem A12:
A ladder, weight W, leans up against a wall. There is no friction
between the wall and the ladder, but there is friction between the
ladder and the floor. The top of the ladder is a distance n_{2} from
the floor, and the bottom of the ladder is a distance n_{1} from the wall.
What is the magnitude of the frictional force f, directed towards
the wall, that the floor exerts on the ladder. If f = W/n_{3},
what is n_{3}? Note, n_{1} and n_{2} have
the same units of length. n_{3} is unitless.

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