# Slight confusion regarding Normal Force

So please correct me if im wrong but... isnt normal force the perpendicular force which stops objects from passing through each other?

Also when two objects put forces on each other, there is of course an action force and a reaction force which occurs all at the same time... but at that time isnt there also a normal force which stops the objects from passing through each other? Does normal force always occur when two objects apply forces on one another?

You phrase "stops objects from passing through each other" is very similar to the one I use in the classroom. I use "resists" rather than "stops" because things can break or be forced through pores, or, or, or ...

It's worth emphasizing that it doesn't matter whether the objects are being slowing pushed together by some external force or simply move free into each other (collide). As long as they would have needed to occupy the same space, the normal force comes into play.

Now, you seem to be a bit confused on Newton's 3rd law and its relationship to normal forces.

Newton's law says that

When body A exerts at force (any force) on body B there is at the same time an equal and opposite force on body A from body B.

That's true for any1 force, including the nromal force.

When applied to normal forces Newton's law just tells you that both bodies experience the normal force at the same time, with the same magnitude and in opposite directions (which is good for preventing them from occupying the same space, they are pushed away from one the interface between them).

1 The form of the rule I have written here isn't actually true for all forces, but there are other version that are true for absolutely every force. Don't worry about the exception to this version yet; just keep them in mind for later on.

• Thanks a lot, I appreciate your help.
– user243275
Oct 2, 2019 at 9:46

A normal force is a contact force, and as you pointed out, it is an action reaction pair, but then again Newton’s third law applies to all forces not just normal forces.

As far as such forces preventing objects from "passing through each other" is concerned, it is probably better to think in terms of resisting deformation.

In statics we treat bodies as being perfectly rigid and our concern for the effect of forces is primarily for determining compliance with static equilibrium requirements, namely, that the sum of the forces and sum of the moments on a rigid body are zero. So when we consider normal forces it is not with respect to the deformation of objects.

However, in mechanics of deformable solids, we do deal with the deformation of objects due to forces. Normal forces cause normal or axial stress and deformation. Forces parallel to the surface cause shear stress and deformation. Finally forces causing torque are associated with torsional stress and deformation.

Hope this helps

• Thanks a lot for the help!
– user243275
Oct 2, 2019 at 9:47