Chapter: 3.5

Newton's third law

The Newton’s third law or action equals reaction.

Newton's third law

When considering the Newton’s third law, a pair of forces between two objects is observed.  


The Newton’s third law or action equals reaction says: 

If one object acts on another one with a certain force, the other object acts back with an equal, but oppositely directed force.

Let’s take a look on the example with a vase. 

The vase on the table acts on the table with the force that equals to its gravitational force. At the same time, the table acts backwards with a force that equals in size, but acts in the opposite direction.

If these two forces are added, their sum is zero. 

From the figure below, the connection between Newton’s first and the third law is evident. 

This example could also be described by the Newton’s first law: 

There are two forces applied to the vase, the force of the support (table) and the force of gravity.

Test yourself

Computer is on the table. Choose the right pair of forces according to Newton’s third law.

b) Force of the computer on the table – force of the table on the computer

The horse tries to get out of the harness. It exerts a force of 500 N on the harness. How much force does the harness exert on the horse?