A rope tied to a body is pulled, causing the body to accelerate. But according to Newton’s third law, the body pulls back on the rope with a force of equal magnitude and opposite direction. Is the total work done then zero? If so, how can the body’s kinetic energy change? Explain.
Solution 3DQ Step 1: If work is done on a system, the kinetic energy of the system will change. In this case, a rope is tied to a body and it is pulled. So, the body will exert a same force opposite to the direction of pulling. But, if the pulling force is more, the body will move towards the direction of pulling force. Since it is getting accelerated, there will be a non zero work and change in kinetic energy. Newton’s 3rd law is applicable for a single system. We know that, the action reaction pair should be inside the system. But here, the pulling force is external. So, the work done will not be zero and the total force too a non zero value.