Question

When an apple falls from a tree what happens to its gravitational potential energy just as it reaches the ground? After it strikes the ground?

Answer 1

Concepts and reason

The concept of gravitational potential energy is used in this problem. First, analyze the two situations and then look at the height of apple from the ground in two situations. Then answer the questions using the concept of gravitational potential energy.

Fundamentals

Gravitational Potential Energy:

When there is a gravitational force between the objects, there is a gravitational field. When an object placed in a gravitational field, it possesses gravitational potential energy.

$U = mgh$

Here, $m$ is the mass of apple, $g$ is acceleration due to gravity and $h$ is the height of the apple from the ground.

The gravitational potential energy is directly proportional to the distance of an apple from the ground.

$U = mgh$

As an apple falls from the tree, it starts to move towards the ground due to the force of gravity. The distance is decreasing as the apple comes closer to the ground and so the gravitational potential energy.

When the apple strikes the ground, there is no distance between the apple and the ground.

$h \approx 0$

The distance has decreased to almost zero. So the gravitational potential energy will decrease and become almost zero.

When object starts to move, then its potential starts converting into kinetic energy. Hence, the potential energy decreases.

Ans:

The gravitational potential energy decreases when apple reaches the ground.

The gravitational potential energy decreases and becomes zero when apple strikes the ground.