Normal Force: Mass on a Surface

[ShayanJ]

When a mass is on a surface,It applies a force to it because of its weight.According Newton's 3rd law,the surface applies a force to the mass.Can we call this force,Normal force?

 

[Filip Larsen]

In general you would have that the normal force is the projection of the reaction force onto the surface normal.

In your example, if the surface is horizontal, then the reaction force (the force Newtons 3rd law say is opposite the weight in a static situation) is already normal to the surface, so the normal force is in this case the same as the reaction force. If the surface was tilting (but still flat, to keep it simple), then the projection of the reaction force onto the surface normal would give a normal force of smaller magnitude than the reaction force.

See http://en.wikipedia.org/wiki/Normal_force for a few diagrams.