According to the third law of motion when we push on an object, the object pushes back on us with an equal and opposite force. If the object is a massive truck parked along the roadside, it will probably not move. A student justifies this by answering that the two opposite and equal forces cancel each other. Comment on this logic and explain why the truck does not move.



The logic of the student is wrong .


According to third law of motion,

For every action, there is an equal and opposite reaction.

But, they act on two different bodies.


Action and reaction do not cancel each other as they are on two different bodies.

Not the same body. Hence, action and reaction can never cancel each other.


In case of truck, when we push the truck

The logic that the two forces cancel each other is not correct since action and reaction force act on two different bodies and hence can never cancel each other.

The reason why the truck does not move even though a force is applied on it is because, due to the large mass of the truck the force required to produce acceleration would be large. (F = mxa)

The force applied on the truck must have not been enough to overcome the frictional force between the wheels of the truck and the ground, and hence the truck does not move.

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Davneet Singh
Davneet Singh is a graduate from Indian Institute of Technology, Kanpur. He has been teaching from the past 9 years. He provides courses for Maths and Science at Teachoo.