Newton's Third Law of Motion

Newton's Third Law of Motion

Newton's third law of Motion deals with the interaction between pairs of objects. For example, if you hold a book up against a wall you are exerting a force on the book (to keep it there) and the book is exerting a force back at you (to keep you from falling through the book). This may sound strange, but if the book was not pushing back at you, your hand would push through the book! These two forces (the force of the hand on the book (\({F}_{1}\)) and the force of the book on the hand (\({F}_{2}\))) are called an action-reaction pair of forces. They have the same magnitude, but act in opposite directions and act on different objects (the one force is onto the book and the other is onto your hand).

There is another action-reaction pair of forces present in this situation. The book is pushing against the wall (action force) and the wall is pushing back at the book (reaction). The force of the book on the wall (\({F}_{3}\)) and the force of the wall on the book ( \({F}_{4}\)) are shown in the diagram.

Newton's action-reaction pairs.

Optional Video on Newton's Third Law

These action-reaction pairs have several properties:

• the same type of force acts on the objects,
• the forces have the same magnitude but opposite direction, and
• the forces act on different objects.

Newton's action-reaction pairs can be found everywhere in life where two objects interact with one another. The following worked example will illustrate this:

Example: Newton's Third Law: Seat Belt

Question

Dineo is seated in the passenger seat of a car with the seat belt on. The car suddenly stops and he moves forwards (Newton's first law - he continues in his state of motion) until the seat belt stops him. Draw a labelled force diagram identifying two action-reaction pairs in this situation.

Step 1: Draw a force diagram

Start by drawing the picture. You will be using arrows to indicate the forces so make your picture large enough so that detailed labels can also be added. The picture needs to be accurate, but not artistic! Use stick-men if you have to.

Step 2: Label the diagram

Take one pair at a time and label them carefully. If there is not enough space on the drawing, then use a key on the side.