Question

Describe the “energy story” for the situation below. Be sure to specify the system while including all the relevant energy transfers to/from the system, and transformations within the system. You should also

indicate whether you think the the process described in the situation is reversible or not (as described in lecture, a qualitative way to deter- mine whether a process is reversible would be to watch a recording of that process played backwards, and conclude whether that would normally occur in nature, e.g., seeing an egg unbreak would not be normal, which hints at the idea that the overall entropy of the uni- verse increases).

A box is resting on the floor. You give it a push, and it starts to move. Friction acts on the box and brings it to rest.

Answer 1

Here, the system is box + floor

The box was initially at rest. Therefore, it had zero kinetic energy and net force acting on it was zero means the weight of the box is perfectly balanced by the normal force of floor on the box. No other forces were present.

Now, we give a push to the box and introduce an unbalanced force. Due to this, the box starts to move and acquire kinetic energy. Note that here the total energy of the system remains constant always assuming no dissipation of energy is there.

Now, the ground (floor) starts exerting a resistive force on the block. This force will act in the direction opposite to motion of box. This force will try to balance the force applied in the beginning and will try to make net force zero. As more and more friction is acting on box, the box comes to rest and again the net force is zero. Now, again kinetic energy is zero.

Here, the kinetic energy is being transferred to the ground as heat or other form of energy to the box + floor system.