This statement of this law is:

Action and Reaction are Equal and Opposite

This means that is a body A exerts a force on a body B then B exerts an equal force in the opposite direction on A. This is true whether the two bodies are in contact or are some distance apart, whether they are moving or are stationary.

 

 

 

 

 

 

 

 

 

 

 

 

 

Consider, for example, a mass resting in a scale pan. The scale pan is exerting an upward force on the mass and the mass is exerting an equal downward on the scale pan.

 

Now consider two particles connected by a taut string. The objects are not in direct contact but exert equal and opposite forces on each other by means of the equal tensions in the string when act inwards  at each end.

 

 

 

 

 

 

 

 

 

 

 

This is true even if the string passes round a smooth body, such as a pulley, which changes the direction of the string.

 

 

 

 

 

 

 

 

 

 

 

The tensions in the two portions of the string are the same and each portion exerts an inward pull at each end. So in each portion the tension at one end acts on the particle and at the other end the tension acts on the pulley; all these tensions are equal.

However, if the string passes round a rough surface the tensions in the different portions of the string are not equal, but the study of this situation is beyond the scope of this web site.