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This is a dumb question but I don't seem to understand Newton's third law of motion.If an object is on rest at table,it exerts a force of magnitude F=mg in downward direction.Similarly,table exerts equal and opposite force to the object.But what is the use of second pair of force in this process?What would happen if there were no force acting by the table to the object.I mean what is the usefulness of action reaction pair in this system.Please help,I'm a highschool student with very low understanding of physics.

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  • $\begingroup$ If the object exerted force on the table but the table did not exert force on the object, then the object would just sink through the table. $\endgroup$ – user1247 Dec 29 '16 at 5:30
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You have not understood Newton's third law.

The reaction to the action force on object due to gravitational attraction of the Earth is the force on the Earth due to gravitational attraction of b.

The reaction to the action force on object due to table is the force on table due to object.

It so happens that when the object is resting on a table the force on object due to gravitational attraction of the Earth is equal in magnitude but opposite in direction to the force on the object due to the table.
Note that these two forces both act on the object whereas a N3L pair of forces must act on different objects.
Further confirmation that they are not N3L pairs is imagine removing the table.
The gravitational attraction still acts but the force due to the table has vanished, so where is the N3L pair?

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In this example, I think it could be useful to consider a balance of forces on the center of mass of the object.

You have an object standing still on a table. You correctly wrote that on the object is acting the gravitational force: $$F=mg$$ with $m $ mass of the object and $g$ gravitational acceleration.

Suppose for a moment that this is the only force acting on the object. Therefore, there is an acceleration, in this case $g$, acting on the object; this is a consequence of Newton's second law. But an acceleration means a change in speed, therefore your object can't stay still on the table, and that is in contradiction with what we stated in our example.

Now, suppose that Newton's third law is right, and on the object is acting a force, exerted by the table, equal and opposite to the one that the object is exerting on it. This means that on the object are acting two forces, one directed downwards and the other upwards, both with the same module: $$F=mg$$ By adding them as vectors, you can see that the resulting net force on the body is equal to zero. This means, from Newton's second law, that acceleration is zero and therefore the object has always the same speed: if it was still at the beginning of the example, it will always be still.

The same reasoning can be applied to the table.

This how they usually explain Newton's third law in high school examples.

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  • $\begingroup$ But why doesn't this apply if the table is suddenly removed?Where does the balance reaction pair go? $\endgroup$ – Suraz Basnet Dec 29 '16 at 14:02
  • $\begingroup$ @SurazBasnet what do you mean? If you remove the table, you have just the gravitational force that is acting on the object, and it falls. $\endgroup$ – JackI Dec 29 '16 at 14:08
  • $\begingroup$ Thanks,now I think I understood it a little bit.But what when we push a stone across the road?According to this law,stone must also exert equal and opposite force to us when we do but we anyways push the stone.How does this happen? $\endgroup$ – Suraz Basnet Dec 29 '16 at 14:12
  • $\begingroup$ @SurazBasnet in this case, I think you can say that when you push the stone, you are experiencing a force pushing you, a sort of resistance by the stone to be pushed. $\endgroup$ – JackI Dec 29 '16 at 14:50
  • $\begingroup$ but shouldn't these two forces cancel each other out and stone still continue to be in the rest like the object in the table did? $\endgroup$ – Suraz Basnet Dec 29 '16 at 15:45

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