Question

When an apple falls freely from a tree towards the ground, its total mechanical energy throughout the fall ____.

• A. increases
• B. decreases
• C. first increases and then decreases
• D. remains constant

Answer 1

The correct option is D remains constant

Before falling, the apple only has potential energy. Once the apple starts falling, the amount of potential energy decreases and the kinetic energy increases simultaneously, thereby, keeping the total energy constant. In other words, the potential energy of the apple converts into its kinetic energy when it falls.
Hence,the total energy of an isolated system will always remain constant.

It should be noted that here the apple here is in free fall. However, if there is air resistance, some mechanical energy will be converted to heat.