# Doubt in equilibrium of rigid body

Hello there! I am currently studying the equilibrium of rigid body and encountering a doubt. Now, in their explanation of reaction forces by a roller or a rocker or a smooth surface, they generalize it as in the preceding examples prevent the translation of a beam for example in ONE direction only whereas a smooth pin or hinge or a rough surface prevent translation of the beam in ANY direction, thus there are two components of the force. How is it so? I can understand that a roller cannot prevent the beam from moving upwards for instance but at least in can prevent the beam from penetrating the surface downwards in any direction.

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Simon Bridge
Homework Helper
If the body would accelerate in some direction but for an obstruction, then we deduce the obstruction supplies an equal and opposite force to oppose the acceleration.

Since forces can point every which way, it is often handy to represent them as components, usually at right angles to each other, which add up to the actual force. This is to make the maths easier.

A ladder leans against a wall - the feet of the ladder are on a rough floor.
It is easy to see that the floor prevents sliding away from the wall and also prevents the ladder falling directly downwards - so these are two components of the overall force provided from the floor. The total force from the floor is the sum of these two. It is a lot harder to find the total force in one go.

The total force does not act normal to the floor - so the total is sometimes called a reaction force.