# Newton's Third Law
We have learned from Newton's first law what happens when no force is applied. We have learned from Newton's second law what happens when a force is applied. We will learn from Newton's third law what type of reaction takes place between the two objects when an object applies force on another object.
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Newton's Third Law: When an object applies a force on another object, then that object also applies a force of equal magnitude on the first object but in the opposite direction.
In physics, generally, Newton's third law is written as, "Every action has an equal and opposite reaction", although we have not written it in this form. By this time since we have a little idea about force, confusion may occur if force is termed as "action" or "reaction" "suddenly". Moreover, those who are the new learners of physics, their first question will be if every reaction (a force) has an opposite reaction (another force) then why action and reaction are not canceling each other, producing zero? For this reason, the third law will have to be written very precisely. The third law states that if there are two objects A and B, then when A exerts a force on B, then B also exerts a force on A. It is worth mentioning that two opposite forces act on different objects, never on a single object. If the two forces were applied to a single object, only then one could cancel the other. Here there is no scope of cancellation.
The matter will be clarified with the help of some examples. Let us consider that a mass m is allowed to fall from a certain height (figure 3.12). We know that due to the earth's gravitational force, the mass m experiences a force F towards the center of the earth:
F=GmMR2
Already we have seen that this force is written as mg.
From Newton's third law we know that the mass m also attracts the giant earth towards itself. That force is also F but in the opposite direction. We do not bother about this force, the reason is that the acceleration of the earth due to this force can be determined:
F=Ma
Here, M is the mass of the earth and a is the acceleration,
Therefore,
a=FM=mgM=mMg
If the mass of the earth is M= 5.98 x 1024 kg, then if an object of mass 1 kg is released from above then the acceleration of the earth will be,
a = 1.6 x 10-24ms2
This is so small that nobody is bothered about it! Next time when you will jump anywhere, remember that you attracted the whole earth towards you when you were falling downwards. (As small as the acceleration of the earth may be, you pulled the whole earth towards you, you may be a little proud of it!) The easiest way of understanding Newton's third law is to understand the mechanism of our walking. We can all walk but nobody knows the physics hidden behind this.
Since you have started to learn physics a simple question can be put to you. Since you have walked from a stationary position, so you have an acceleration, that means force has been applied to you. But all of us know that nobody is applying a force on us. We walk by ourselves. How is that possible? Unless we know Newton's third law we never can explain the mechanism of walking. When we walk we exert a force on the ground (i.e. apply force). Then
Since you have started to learn physics a simple question can be put to you. Since you have walked from a stationary position, so you have an acceleration, that means force has been applied to you. But all of us know that nobody is applying a force on us. We walk by ourselves. How is that possible? Unless we know Newton's third law we never can explain the mechanism of walking. When we walk we exert a force on the ground (i.e. apply force). Then
According to Newton's third law, the ground also applies an equal and opposite force on our body (Figure 3.13). This equal and opposite force creates the acceleration and we walk. The persons who have a little problem understanding this, they may be remind-end that it is easier walking on hard ground compared to the sandy ground. The reason is that it is not possible to apply force on the sand, sand is displaced. Therefore the reaction force in Newton's third law cannot be achieved properly.
The matter can be clarified more if one is allowed to walk on a very smooth surface lubricated with soap-water or oil! Their friction is hardly present so we cannot apply a backward force at all and due to this, we will not get any force on us as the reaction force. So we cannot walk at all (You may try if you don't believe it). If force is applied, equal and opposite force is found, but if we cannot apply force at all, how can we get a reaction force? Then how can we walk?
The matter can be clarified more if one is allowed to walk on a very smooth surface lubricated with soap-water or oil! Their friction is hardly present so we cannot apply a backward force at all and due to this, we will not get any force on us as the reaction force. So we cannot walk at all (You may try if you don't believe it). If force is applied, equal and opposite force is found, but if we cannot apply force at all, how can we get a reaction force? Then how can we walk?
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