Newton's Third Law: action and reaction

Introduction

Newton's Third Law states that for any action force, there is always an equal and opposite reaction force. Imagine a cannon. As an explosion in the cannon forces the cannonball out of the cannon, an equal force pushes the cannon backwards. (Cannons usually have more mass than cannonballs, so the cannon would not accelerate at the same rate as the cannonball would.)

Newton's Third Law in everyday life

Newton's Third Law is easy to see in daily life. If two people sit in chairs with wheels on them and push off each other, they will move in opposite directions. When a person shoots a gun, the gun recoils as the bullet is propelled forward. Pushing on a bed allows a person to propel her body off the bed so he/she can stand up. With some objects, the reaction force is not easy to see. If you throw a ball at the ground, for example, the Earth does not appear to move. The Earth does move, however, but it moves such a small amount that it can barely be detected. Even when simply sitting in a chair, action forces and reaction forces are at work. You exert force on the chair due to gravity (refer Chapter 5), but the chair exerts a reaction force on you.

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Jet propulsion

Newton's Third Law is the main principle that allows many forms of propulsion to work. Rockets operate by combustion. This combustion expels large quantities of gases from the back of the rocket at high speeds. By pushing the gas down and out of the back of the rocket, the reaction force pushes the rocket upwards.

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Another form of propulsion that uses Newton's Third Law is jet propulsion. Jet engines pull air into a chamber with a series of large fans, then expel it out of the back of the engine. This creates a reaction force that pushes the engine (and the aeroplane it is attached to) forward.

Newton's Third Law is also important in the way that aeroplanes fly. Aeroplanes have wings that are shaped in such a way that they direct air downwards when air passes by them at high speeds. These shapes are called airfoils. Since air is pushed downward, the reaction force pushes the aeroplane upwards.

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