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 two opposite and equal forces cancel each other. Comment on this logic and explain why the truck does not move.

Answer 1

Hint According to Newton’s third law of motion states for every action there is an equal and opposite reaction. That is if we push any object the object will push us back with the same force. We have to apply this logic to this question as well as the concept of static friction between the surface of the road and the truck.

Step by step solution
For a truck parked along the roadside, there will be a force acting on the truck by the surface of the road. This force is known as static friction. Static friction can be defined as the frictional force that exists between two surfaces which are in contact as long as the bodies are at rest. When the truck is not moving, the static friction between the road and the truck tyres will keep the truck from moving.
The truck is a massive body. Therefore the force applied by the road on the truck will also be very high. In order to move the truck, we have to apply a force that is greater than the force applied by the road on the truck. But the frictional force is flexible and it can adjust to a particular level. So the more force we apply on the truck, the frictional force will also increase to keep the truck at rest. Thus the force we apply on the truck will be canceled by an equal and opposite frictional force.

Therefore the justification of the student is correct.

Note
It is known that there is an equal and opposite reaction for every action. But the action and reaction forces will be acting on different bodies. Hence they will not cancel each other. For example, if we push the truck, that is action and the truck is exerting a force back on us. These forces will not cancel each other because the action is acting on the truck and the reaction is acting on the truck. Hence this question cannot be justified using Newton's third law. But since the student justified by telling equal and opposite forces to cancel each other, his justification is correct because both forces act on the truck and they cancel each other.