A ladder standing on a rough floor leans against
a smooth wall. The ladder is at an angle alpha
to the horizontal. The forces on the ladder
are the normal reaction force at the wall,
NA, the normal reaction force at the floor,
NB, the ladder's weight, mg, acting at its
centre, a force P which is being applied a
quarter of the way up, and the frictional
force at the floor, Fr. The ladder has length
l. We can find NA without having to worry
about the normal reaction and the frictional
force at point B, by taking moments about point B.
As the ladder is in equilibrium, the total
of the clockwise moments around this point
must equal the total of the anticlockwise
moments about this point. Be careful when
calculating the moment due to each force,
to make sure that you use the correct angle.
