Third law of motion

Newton’s third law of motion states that:

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

It’s worded more like this:

At any instant of time, the net force on a body is equal to the body’s acceleration multiplied by its mass or, equivalently, the rate at which the body’s momentum is changing with time.

In simple terms

If object A puts a force to object B, object B will put a force

For example:

• If you push against a wall, the wall pushes back against you with the same amount of force, but in the opposite direction.
• If you jump off a boat, you push the boat backwards with the same amount of force as the boat pushes you forwards.
• If you hit a ball with a bat, the ball exerts an equal and opposite force on the bat.

What Newton’s third law isn’t

• Suppose we have a book on a table.
• You might think that the normal contact force from the table on the book is the ‘equal and opposite’ force to the weight of the book (the force of gravity pulling it down).
• But these two forces aren’t actually an action-reaction pair, because they act on the same object (the book).

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