# The concept of force normal

1. May 8, 2011

### PieOperator

Heres a simple situation I can't seem to adequately explain to students. A box experiences a force exerted on it by the earth. The third law of newton states that an object that feels a force exerts an equal and opposite force, which is the force that the box exerts onto the earth. If the box and the earth are now in contact, is the force exerted by the box onto the earth the normal force? I'm thinking that it's a very unique force that pertains to two masses glued together. Can somebody share their ideas on normal force? Thanks!

2. May 9, 2011

### Andrew Mason

The sum of all forces on the box must be equal to its mass x acceleration. There are only two forces here: the force of gravity between the box and earth and the mechanical force exerted by the earth on the box due to contact. These are, respectively: gravity and the normal force.

Since the box is not accelerating, the two forces must sum to 0. Since the force of gravity is toward the centre of the earth, the normal force must be equal in magnitude and opposite in direction:

$$\vec{F_g} = m\vec g = - \vec{F_N}$$

So the normal force is the upward mechanical force exerted by the earth on the box that balances the downward force of gravity on the box.

AM