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# How to label forces on moments diagram watch

1. Hi,

I don't really understand how to label forces on moments questions.
I came across these two moments questions and found out that sometimes the forces act perpendicular to the rod and sometimes perpendicular to the floor/ground.

For example:
a) A uniform ladder AB, of length 6 metres and mass 22 kg, rests with its foot, A, on rough horizontal ground. The ladder rests against the top of a smooth vertical wall at the point C, where the length AC is 5 metres. The vertical plane containing the ladder is perpendicular to the wall, and the angle between the ladder and the ground is 60. A man, of mass 88 kg, is standing on the ladder. The man may be modelled as a particle at the point D, where the length of AD is 4 metres. The ladder is on the point of slipping. (According to the MS, resultant force acts perpendicular to the floor at A, and perpendicular to the rod at C.) So why is it different? why not perpendicular to the rod/floor either way?

b) A uniform rod, PQ , of length 2a , rests with one end, P, on rough horizontal ground and a point T resting on a rough fixed prism of semicircular cross-section of radius a, as shown in the diagram. The rod is in a vertical plane which is parallel to the prism’s cross-section. The coefficient of friction at both P and T is m.
(MS: Resultant force acts perpendicular to the ground at P while resultant force acts perpendicular to the rod at T.)

It would be really nice of you if you could explain to me with easy words
Thanks for helping!
2. (Original post by liemluji)
It would be really nice of you if you could explain to me with easy words
Thanks for helping!
At the point of contact, at least one of the two bodies is a plane. The normal reaction is perpendicular to that plane.

If there is no friction, then the normal force is the only one acting at the point of contact.

If there is friction, you will get a force perpendicular to the normal one.

Try those rules against the examples you've given.

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Updated: January 18, 2017
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