Princess Sophie E: A uniform ladder of weight w rests on rough horizontal ground against a smooth vertical wall. The vertical plane containing the ladder is inclined at an angle x to the vertical. Prove that if the ladder is on the point of slipping and U is the coefficient of friction between it and the ground, then tanx=wU.
Hi Princess Sophie!
Where the ladder touches the ground it is pushed up by a normal force that is equal and opposite to the force of gravity w.
The friction with the ground is U times this normal force, so that is Uw.
These 2 forces form a right triangle that is the same as the right triangle that the ladder forms with the wall and the ground.
Since the tangent of x is the opposite side divided by the neighboring side, we have that tanx = (Uw) / w = U.
It would appear that what you are supposed to prove (tanx=wU) is not correct. It should be tanx=U.