Physics/Newton/Third Law

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Newton's Third Law formally states "For every action, there exists an equal and opposite action".

However, when Newton was alive, the English used was a little different from today's English. So, this law is normally simplified by using the word "force" instead of "action".

To understand this law, think of a book resting on a table. If there isn't a table, the book will fall towards the ground due to Earth's gravitational force pulling on the book. In order to stop the book from falling, the table must be pushing the book upwards to work against that gravitational force. Imagining yourself as the book, you will feel the table pushing upwards on you. Now that we have identified this force of table on book, what Newton's Third Law says is that the table will be experiencing a force from the book that is same in strength, but pointing downwards. This is so as the force on the book from the table. By imagining yourself as the table, you will feel the book pushing you downwards as strongly as you would feel if you were the book.

Because there is a force of table on book, there must thus be a force of the book on the table that is of the same strength but opposite direction.

Newton realised that it must be necessary for both forces to exist together. This extends to all forces, and so he wrote this law. Even in the case of you punching a boy in class, your fist will receive the same force as his face, though you will probably feel less painful. This just means that it is useless to fight.

Newton's Third Law is also known as the Universal law because there is no known way of thinking about the world where this law does not hold.

When using this law, it must be remembered that, from any one point of view, only one of the pair of forces exist at any point in time. For example, the table will never "feel" the force of table on book. Later in a physicist's course, it is rather common to get confused when working on different parts of a complicated diagram.