The Normal Force

The Normal Force

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When an object is placed on a surface, for example think of the case of putting a book on a table, there are a number of forces acting. Firstly, if the table were not there the book would fall to the floor. The force that causes this is gravity. The table stops the book falling to the floor. The only way this can happen is for the table to exert a force on the book. The force that the table exerts on the book must balance out the force of gravity. This tells us a few things immediately! Gravity is a force pulling the book down, it is a vector. The force that the table exerts must balance this out and it can only do this if it has the same magnitude and acts in the opposite direction.

This occurs often, gravity pulls a person towards the earth but when you are standing on the ground something must be balancing it, if you put a heavy box on the ground the gravitational force is balanced. If you put a brick on water it will sink because nothing balances the gravitational force. We give the force that a surface (any surface) exerts to balance the forces on an object in contact with that surface the normal force.

The normal force is a force that acts on the object as a result of the interaction with the surface and is perpendicular to the surface. This last part might be seem unexpected (counter-intuitive) because if we tilt the table slightly the direction of the gravitational force hasn’t changed but the direction of the normal force has a little (the normal is not always directly opposite gravity). Don’t panic, this will all make sense before the end of this chapter. Remember: the normal force is always perpendicular (at a right angle) to the surface.

Definition: Normal force

The normal force, \(\vec{N}\), is the force exerted by a surface on an object in contact with it.

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