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According to Newton's third law of motion every action and reaction have opposite direction and are equal in magnitude, so for example, if I apply force on a block then every time applied force should be equal to frictional force and the box shouldn't move, then why the block finally moves? why my force applied overcomes friction?

asd.123
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rakshit rawat
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2 Answers2

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  • The important part of the third law is that the two forces are not applied to the same object: you apply a force on a block, whereas the block applies the force on you. The frictional force acting on the block is produced by the surface on which it lays, and the block acts on the surface with an equal and opposite force.
  • There is no reason why the frictional force should be equal to your force, since they come from different sources and are not related by the third law.
Roger V.
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  • yes i am applying the force with my hands to the block so the block will apply the force on my hand BUT IT WILL NOT CANCEL THE FORCES BRO , if u will make FBD of block then we will consider my applied force and then frictional force should be equal to it always but that's not the case – rakshit rawat Sep 30 '20 at 07:36
  • No. The floor-block system form a pair of forces. Your arm applies an additional force which moves the block. – joseph h Sep 30 '20 at 07:49
  • @Drjh so that additional force should always bd equal to frictional force of surface according to the law then – rakshit rawat Sep 30 '20 at 07:53
  • @rakshitrawat No, the friction force and your force do not form a pair related by the third law - this is what I stress in my answer. – Roger V. Sep 30 '20 at 08:00
  • @Vadim then frictional force is opposing which force and is Pair of which forces? – rakshit rawat Sep 30 '20 at 08:06
  • As I said in my answer: "The frictional force acting on the block is produced by the surface on which it lays, and the block acts on the surface with an equal and opposite force." In any case no two forces applied to the same body are related by the third law. – Roger V. Sep 30 '20 at 08:16
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    @Vadim thanks a lot , i understood, sorry for sounding dumb – rakshit rawat Sep 30 '20 at 08:25
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Look you are being confused with Newton's third law and action - reaction pairs.

Suppose a block is kept on a table. Now you apply a force $F$ on that block , so Newton said that when you apply a force on an object then that object also applies a force of equal magnitude on you but in the opposite direction ( the two forces are on different objects and not on the same ).

What is a friction force ?

Friction is a force that opposes the relative motion between two surfaces which are in contact (not exactly or literally) or very close to each other. )

Now because of your force on the block , the block will move or tend to move in the direction of applied force i.e. it will have or tend to have some relative velocity with respect to the surface of the table below the object. So the table will oppose this relative motion of the block and apply a force $f_r$ in opposite direction of $F$ . So , here again from Newton's third law, the block will also apply the same force $f_r$ on the table in the opposite direction.

Now you can notice that there are two forces on the block , $F$ and $f_r$ in opposite directions. They may be equal in magnitude and cancel each other out and in that case the block will not move. But since they are applied by two different sources they need not to be same all the time and when $F$ is greater than $f_r$ (since $f_r$ can not exceed once it reaches the maximum value obtained by this relation $F_{max} = \mu mg$ ) , the block will start moving in the direction of $F$.

In you question there are two action - reaction pairs :

1 : force between you and the block

2 : force between the block and the table.

So the main thing to notice here is that your force $F$ on the block does not form action - reaction pair with the force $f_r$ applied by the table on the block. And so there is nothing to stop the block from moving after certain values of force.

Note In response of the comment :

So the table will oppose this relative motion of the block and apply a force $f_r$ in opposite direction of $F$ . So , here again from Newton's third law, the block will also apply the same force $f_r$ on the table in the opposite direction.

Now we have one force $f_r$ on the table . So the table would also tend to slip over the surface of the floor below it (Since the table was not floating and was in contact of the floor) , so from the definition of friction force the floor will also oppose the relative motion of the table by applying a force $f'_r$ on the surface of the table in contact in the opposite direction and depending upon the friction coefficients ($\mu$, a parameter which determines the magnitude of frictional force) between the surfaces, the table can be at rest or can move in the direction of $f_r$ . And in return , the table also applies the same force $f'_r$ on the floor in the opposite direction.

Hope it helps ☺️.

Ankit
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