Question

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 this logic and explain why the truck does not move.

Answer 1

Hint : To solve this question, we have to know that, Newton’s third law of motion states that “When one body exerts a force on the other body, the first body experiences a force which is equal in magnitude in the opposite direction of the force which is exerted”. The above statement means that in every interaction, there is a pair of forces acting on the interacting objects.

Complete Step By Step Answer:
We know that, When we push a truck parked along the roadside, then the road applies a static friction force in the opposite direction of force applied on the truck to oppose the motion of the truck. The static friction is adjustable in nature, i.e., the road offers only the required amount of friction force to keep the truck at rest. If we apply more force, the road applies more friction force (up to the highest value, known as limiting value of static friction. Hence the friction force cancels out the force applied.
Here the student's justification is that the two opposite and equal forces cancel each other and hence the truck does not move. However, the action and reaction forces do not cancel each other as they act on the different bodies whereas the truck did not move due to zero net force which is balanced by the static friction of the truck.

Note :
We can say, when one body exerts a force on the other body, the first body experiences a force which is equal in magnitude in the opposite direction of the force which is exerted.