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The study of Physics Topics can help us understand and solve real-world problems, from climate change to medical imaging technology.
Explaination of Newton’s Third Law of Motion
If a boy wearing roller skates stands facing a wall and pushes the wall with his hands, the boy finds himself moving backwards, away from the wall (see Figure). It appears as if the wall also pushes the boy away. Actually, when the boy exerts a force on the wall by pushing it with his hands, then the wall exerts an equal force on the boy in the opposite direction. Since the boy is wearing roller skates, the opposite
force exerted by the wall makes him move backwards. In Figure, the boy on roller skates is exerting force on the wall towards left side. The wall exerts an equal force on the boy towards right side. Due to this, the boy moves backwards to the right side [see Figure], From this discussion we conclude that when a boy exerts a force on the wall, the wall exerts an equal and opposite force on the boy. This is just an illustration of Newton’s third law of motion.
When one body influences another body by applying force, we say that the first body is interacting with the second body. In any interaction between two bodies, there are always two forces that come into play. And Newton’s third law of motion describes the relationship between the forces that come into play when the two bodies interact with one another.
According to Newton’s third law of motion : Whenever one body exerts a force on another body, the second body exerts an equal and opposite force on the first body. The force exerted by the first body on the second body is known as “action” and the force exerted by the second body on the first body is known as “reaction”. It should be noted that “action” and “reaction” are just forces. We can now write another definition of Newton’s third law of motion : To every action there is an equal and opposite reaction. Action (force) and reaction (force) act on two different bodies, but they act simultaneously. We will now describe a simple experiment to prove the Newton’s third law of motion, that is, to prove that action (force) and reaction (force) are always equal and opposite.
We take two similar spring balances A and B and join them hook to hook as shown in Figure. The other end of spring balance B is attached to a hook H fixed in a wall. Let us pull the free end of the spring
balance A to the right side by our hand. We find that both the spring balances show the same reading. For example. in Figure, both the spring balances show the same force of 4 N. This can be explained as follows.
When we pull the balance A, it exerts a force of 4 N on the balance B. The balance B pulls the
balance A with an equal force of 4 N, but in the opposite direction. In other words, when balance A exerts a force of action on balance B, then balance B exerts an equal and opposite force of reaction on balance A. Since both the spring balances show the same reading (of 4 N), we conclude that the action and reaction forces are equal in magnitude. In Figure we find that the action force is acting towards east and the reaction force is acting towards west. Thus, action and reaction forces act in opposite directions.
Action and Reaction Act on Two Different Bodies
Suppose a box is resting on the ground (Figure). The box is exerting a downward force of its weight on the ground. The downward weight of the box is balanced by an equal, upward force supplied by the ground. Now, the force exerted by the weight of the box is “action” and it acts on the ground whereas the force exerted by the ground on the box is “reaction” and it acts on the box. Since the box is in equilibrium under the action of two forces, it neither goes up nor goes down, the “action” of the box must be equal and opposite to the “reaction” of the ground. It is obvious that the “action” of the box acts on the ground and “reaction” of the ground acts on the box. Thus, action and reaction act on two different bodies.
Some Examples to Illustrate Newton’s Third Law of Motion
We will now give some examples from our everyday life which will illustrate Newton’s third law of motion.
1. How do We Walk
When we walk on the ground, then our foot pushes the ground backward and, in return, the ground pushes our foot forward (Figure). The forward reaction exerted by the ground on our foot makes us
walk forward. If, however, the ground is slippery or if there is all ice, it becomes very difficult to walk. This is due to the fact that on the slippery ground or ice, the friction is much less, and we cannot exert a backward action force on slippery ground or ice which would produce a forward reaction force on us.
Let us discuss the case of a swimmer now. A swimmer pushes the water backwards (or applies force on the water backwards) with his hands and feet to move in the forward direction in water. It is the equal and opposite reaction to this force which pushes the swimmer forward.
Please note that though action and reaction forces are equal in magnitude but they do not produce equal acceleration in the two bodies on which they act. This is because the two bodies on which action and reaction forces act usually have different masses. So, the acceleration produced will be more in the body having less mass whereas the acceleration produced will be less in the body having more mass. This point will become more clear from the following example of the recoil of a gun on firing. ‘Recoil’ of gun means ‘sudden backward movement’ or ‘jerk’ of gun when a bullet is fired from it.
2. Why the Gun Recoils
When a bullet is fired from a gun, the force sending the bullet forward is equal to the force sending the gun backward (Figure). But due to high mass of the gun, it moves only a little distance backward and
3. The Flying of Jet Aeroplanes and Rockets
Jet aeroplanes utilise the principle of action and reaction. In the modern jet aircraft, the hot gases obtained by the rapid burning of fuel rush out of a jet (a nozzle) at the rear end (back end) of the aircraft at a great speed. The equal and opposite reaction of the backward going gases pushes the aircraft forward at a great speed.
We can demonstrate the principle of working of a jet engine by using a balloon filled with compressed air as follows : If a balloon filled with compressed air and its mouth untied is released with its mouth on the left side, the balloon moves very fast towards the right side (see Figure). This means that the balloon flies off in the opposite direction to that of the escaping air. Here the compressed air present in balloon rushes to the left side with a high speed. The equal and opposite reaction of the left going air pushes the balloon to the right side.
Please note that if the inflated balloon is released with its mouth in the downward direction, then it will move upwards (like a rocket) (see Figure). In this case, the air rushes out of balloon in the downward direction. The equal and opposite reaction of downward going air pushes the balloon upwards. We will now discuss the case of rockets. The rockets also work on the principle of action and reaction. In a rocket, the hot gases produced by the rapid burning of fuel rush out of a jet at the bottom of the rocket at a very high speed (Figure). The equal and opposite reaction force of the downward going gases pushes the rocket upward with a great speed. Please note that a rocket can propel itself even in vacuum (or outer space) because it does not require air for obtaining uplift or for burning its fuel. This is not so in the case of a jet aircraft. A jet aircraft cannot fly in outer space (where there is no air) because it needs air to provide an uplift and also to burn its fuel.
4. The Case of a Boat and ‘the Ship
During the rowing of a boat, the boatman pushes the water backwards with the oars. The water exerts an equal and opposite push on the boat which makes the boat move forward. In fact, harder the boatman pushes back the water with oars, greater is the reaction force exerted by water and faster the boat moves forward.
It is a common experience that when a man jumps out of a boat to the bank of the river (or lake), the boat moves backwards, away from him. This is due to the fact that to step out of the boat, the man presses the boat with his foot in the backward direction (Figure 39). The push of the man on the boat is the action (force). The boat exerts an equal force on the man in the forward direction which enables him to move forward. This force exerted by the boat on the man is reaction (force). Since the boat is floating on water and not fixed, it moves backward due to the action force exerted by man.
Another point to be noted is that when a boatman wants to take the boat away from the bank of the river, he sits in the boat and pushes the river bank with his oar. When the boatman exerts a force of action on the bank (with his oar), the bank exerts an equal and opposite force of reaction on the boat. So, the boat moves away from the bank.
The propellers of a ship are at its back end. When these propellers work, they exert a backward force on water in the sea. The equal and opposite reaction (force) exerted by water on the ship, moves the ship forward.
5. The Case of Hose Pipe
When firemen are directing a powerful stream of water on fire from a hose pipe, they have to hold the hose pipe strongly because of its tendency to go backward. The backward movement of the hose pipe is due to the backward reaction of water rushing through it in the forward direction at a great speed.
6. The Case of Horse Pulling a Cart
Let us apply the third law of action and reaction to the situation of a horse pulling a cart. According to the third law of motion, the horse exerts some force on the cart, and the cart exerts an equal and opposite force on the horse. So, at first glance it seems that the forces being equal and opposite cancel out and hence the cart would not move. But it should be noted that it is only the force on the cart which determines whether the cart will move or not, and that the force exerted by the cart on the horse affects the horse alone. Thus, if the horse is able to apply enough force to overcome the frictional forces present, the cart will move. So, to make the cart move, the horse bends forward and pushes the ground with its feet. When the forward reaction to the backward push of the horse is greater than the opposing frictional forces of the wheels, the cart moves.
In all the above examples, the two interacting bodies are in direct contact with each other. It is not always necessary that two bodies can exert force on one another only when they are in contact. In some cases the two bodies can also exert force on each other even when they are not in contact with each other. That is, the interaction can also take place even when the two bodies are not in contact. In all such cases the forces of action and reaction are equal and opposite. For example, a magnet can interact with a piece of iron and exert a force on it even when they are separated by a distance. The magnet exerts a force on iron piece, and the iron piece exerts an equal and opposite force on the magnet.
The interaction between an electrically charged comb and a piece of paper also takes place from a distance. The electrically charged comb and the piece of paper exert equal and opposite forces on each other. The interaction between a falling stone and the earth also takes place though they are not in contact with each other and two forces come into play. While the earth pulls the stone downwards by the force of gravity, the stone also pulls the earth towards itself with an equal force. It should be clear by now that we cannot think of a single isolated force – for every force there, is an equal and opposite force. In other words, the forces always occur in pairs, and Newton’s third law of motion concerns these pairs of forces.