The Third Law

This is both the simplest in its statement and the most misunderstood of the three laws. The statement given here is not a direct translation of the original, but it helps to remove some of the misunderstanding.

Newton’s third law can be stated as:
If object A exerts a force on object B, then B exerts a force equal in size and opposite in direction on A.

According to the third law, forces do not exist individually but in pairs. However, it is important to remember that:

• the forces are of the same type, i.e. both gravitational or electrical
• the forces act on different objects
• the third law applies to all situations.

The laws were originally written in Latin. The third law is directly translated as ‘to every action there is an equal and opposite reaction’. This was widely misinterpreted as meaning that two equal and opposite forces act on the same object, resulting in zero acceleration.