# Equilibrium

##    BHS -> Staff -> Mr. Stanbrough -> Physics -> Mechanics -> Newton's Laws -> Newton's First Law -> this page

## Equilibrium - What is it?

If an object is not accelerating, it is in equilibrium. Equilibrium is often considered as two separate cases:

• Static Equilibrium - An object is at rest and not accelerating.
• Dynamic Equilibrium - An object is moving (relative to us) and not accelerating.

Of course, since velocities are relative we know that static equilibrium and dynamic equilibrium are really the same thing, depending on where you are standing when you observe the motion (or lack of it). Since the net force on this block is zero, it is in equilibrium - which means that it is not accelerating. the block is either: at rest, or moving with a constant velocity

Newton's First Law says that" if the net force on an object is zero, it will be in equilibrium - it won't accelerate". The converse of this statement is also true - "If an object is in equilibrium (is not accelerating), then the net force on it must be zero." This can be useful.

Suppose there is a book lying (at rest) on a table. The book is not accelerating - in other words, it is in equilibrium. Since it is in equilibrium, the net force on the book is zero. What forces are pushing or pulling on the book? Well, the Earth is pulling downward on the book with a force we call the book's weight. Suppose the book's weight is 10 Newtons. Is this the only force acting on the book? No! If this were the only force on the book, there would be a net force on the book, and the book would not be in equilibrium! The table must be exerting an upward force on the book of exactly 10 N in order to balance (or cancel) the weight. This force, exerted by the table on the book, is called a support force or normal force. If a book is in equilibrium, the net force on it must be zero.

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last update November 26, 2002 by JL Stanbrough