Why are reaction forces normal to the surface?

Question

I've been studying mechanics for a while now, but I still don't know a very basic point: why is it that when a body is in contact with a plane that the reaction/contact force acts perpendicular to the plane at the point of contact?

Thanks a lot.

Answer 1

The normal force is a feature of solids. When two solids come into contact, they resist occupying the same space, interpenetrating each other.

If a force brings two solids towards each other and they are prevented from moving into each other's volume, there must be a force present - a resistance of solids to occupy the same space.

The normal force is that force which resists an attempt to cause two solids to interpenetrate.

Hence, the force acts perpendicularly to the surface of contact because a force along the surface of contact would not cause interpenetration, rather that would be involving the frictional force and it's behaviours/rules are different.

Answer 2

Simply because they are defined that way. When a body is in contact with a plane there will be some contact force. This can be split into a component normal to the plane and one parallel to the plane. The component normal to the plane is called reaction force and the component parallel to the plane is called friction force.