A ball is held at a height H above a floor. It is then released and falls to the floor. If air resistance can be ignored, which of the five graphs below correctly gives the mechanical energy E of the Earth-ball system as a function of the altitude y of the ball?

Straight line graph attached showing mechanical energy on x-axis and altitude y on y-axis

Explanation:

As mechanical energy is the sum of kinetic energy and potential energy, the ball at a height H possesses potential energy= m.g.H

and Kinetic energy= 0.

But the sum of both energies is equal to m.g.H.

When the ball starts moving, the height starts to decrease and potential energy also decreases with decreasing y. On the other hand kinetic energy starts increasing with decreasing Y. But their sum remains constant i.e equal to mechanical energy. It remains same until the ball touches the ground. The graph attached shows height H or altitude y on x-axis and Mechanical Energy on y-axis. It shows a straight horizental line showing that mechanical energy remains same as y decreases.  

the force required to move the cart is divided by three or 33.33 n.