Newton’s Third Law of Motion says that if two bodies are in contact with one another then the force exerted on body B by body A is equal in magnitude and opposite in direction to the force exerted on body A by body B.
You can see Newton’s Third Law of Motion in action by observing an object resting on the ground (Note that the upward force is that of the ground pushing upward on the object and the downward force is that of the object pushing downward on the ground).
As you can see, the downward force exerted on the ground by the object is the object’s mass m multiplied by gravity. It follows that, since the object doesn’t move, the ground exerts an upward reactive force on the object equal in magnitude.
Note that the same can also be said for accelerating objects. By way of example, consider a vehicle that is accelerating and pulling along a trailer.
The trailer has the same acceleration as the vehicle and is pulled forwards by the tension in the tow bar. Conversely, resistance from the trailer exerts an equal and opposite force on the vehicle.
If multiple objects are connected and are acting in the same straight line, they can be treated as a single object.
Newton’s Third Law in Connected Particles
In the following examples objects in contact with one another are treated as connected particles. In addition, we assume that an object is a particle with the same weight acting at a single point.
These examples involve a light inextensible string over a smooth pulley. This means that we assume that the string has negligible weight, it doesn’t stretch or have friction with the pulley.
See the Mechanics area of the Edexcel A-level specification – sections 6-9, pages 34-37 of this document.