The Earth and the ball

The idea

Gravitational potential energy is the stored energy of the Earth–object system; because the Earth hardly moves, it is simply called the potential energy of the object.

Ball and Earth

You raise a ball above the ground. The Earth pulls on the ball, and the ball pulls on the Earth.

When the ball falls, which do you think moves noticeably, the ball or the Earth?

• The ball
• The Earth
• Both about the same

A system of two

Press Next step through the three pictures.

Why we say the ball's energy

The gravitational pull is between the Earth and the ball, so the stored energy belongs to the pair. But the Earth is much more massive and hardly moves, so we simply call it the potential energy of the ball.

Gravitational potential energy is the stored energy of the Earth-object system. Because the Earth hardly moves (it is much more massive), it is simply called the potential energy of the object.

Notes

Gravitational potential energy is the stored energy of the Earth-object system, called simply the potential energy of the object since the Earth hardly moves.

Check yourself

Gravitational potential energy is the stored energy of…

Why is it simply called the potential energy of the object?

In this chapter, 'potential energy' usually means…

• the object alone, with no connection to the Earth. It is stored in the system of the Earth and the object, because of the gravitational force between them.
• the Earth alone. It is the energy of the Earth-object system, though it is simply called the energy of the object.
• the Earth-object system — correct. Yes!
• The Earth is much more massive and hardly moves — correct. Yes!
• The Earth has no gravitational force. The Earth does pull on the object. It hardly moves because it is much more massive.
• The object is much more massive than the Earth. It is the other way round. The Earth is much more massive than the ball.
• the energy of a stretched band only. The book takes the simplest case, the gravitational potential energy of the Earth-ball system.
• gravitational potential energy — correct. Yes!
• the energy of a moving object. The energy of a moving object is kinetic energy.