You have probably heard of Newton's 3rd law in one of its many forms. Perhaps the most famous one is:


 "to every action, there is an equal and opposite reaction" 


which is essentially saying that when 2 objects interact, the forces that they exert on each other are equal and opposite.


Newton's third law can be quite difficult to grasp, because if the forces are always equal, how does anything ever go anywhere? The important thing to remember (and the tricky part) is that the 2 forces are acting on different objects. Let's take a look at an example:


A popular classroom activity is for two students to sit facing each other on chairs with castors fitted (castors are small wheels fitted to the base of some office chairs which allows the person using the chair to move around short distances in the office without getting up). If the students sit facing each other and then push against each other they will both go backwards, and the extent to which each student goes back will depend on the force being pushed with, and the individual mass of the student / chair combination. 


You could do the same standing on skateboards, roller skates or ice skates, the effects would be similar.


Consider the images below:





The top picture shows an example of Newton's third law. There is an equilibrium setup here because a man pushes against the wall and the wall is pushing back with an equal and opposite intensity, However in the case of the book resting on a table, this is not an example of Newton's third law because there are TWO forces acting on the book but they are DIFFERENT types. The book is pushing down on the table with a similar but opposite force that the table is exerting on the book, similarly the weight of the book is pushing down on the earth with an equal and opposite intensity to the force that the earth is imposing on the book. As a result of these forces being in balance, the book does not move.


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