Chapter 4, Problem 8TE

You push a heavy car by hand. The car, in turn, pushes back with an opposite but equal force on you. Doesn’t this mean the forces cancel one another, making acceleration impossible? Why or why not?

To determine

The reason behind forces not being cancelled, when a person pushes heavy car by hand and resulting in no acceleration.

Answer to Problem 8TE

The reason behind forces not being cancelled when a person pushes heavy car by hand and resulting in no acceleration is that the forces must act on same body, having equal magnitude and opposite in sense of direction.

Explanation of Solution

In the case, when a person pushes a car by hands, it exerts a force on the car. Simultaneously, the car will also exert equal, opposite force on the hand, according to Newton’s third law. But, the force applied by the car is on a person’s hand, not on a car. Thus, they cannot be cancelled out. For the cancellation of force, forces should have equal magnitude but reverse direction, which should be acting on the same object.

When the hands push the car, the car accelerates in the forward direction, but the person accelerates in the backward direction. The acceleration depends on the mass of the object, and also the frictional force acting over the object.

Conclusion:

Therefore, the reason behind forces not being cancelled when a person pushes heavy car by hand and resulting in no acceleration is that the forces must act on same body, having equal magnitude and opposite in sense of direction.