question_answer

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

Answer:

                  The logic given by student is wrong; because the action and reaction forces do not act on same body; but act on different bodies. The reason of not moving the truck is as follows: The truck is massive, so it has very large inertia. Moreover, it is parked on the ground; there is a frictional force between the truck and ground. The force exerted by us on the truck is insufficient to overcome the force of friction; so these are balanced forces on the truck; (force exerted by us plus force of friction in opposite direction); that is net force is zero and hence, the truck does not move.