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The Solar System

The Orbit of the Planets (continued).

If the forces acting on the Earth were balanced,
then the Earth would move through space in a straight line
at a constant velocity (see forces and motion).

The Earth is trying to move past the Sun.
The Sun's gravity is pulling the Earth sideways.

Gravity makes the Earth Orbit in a Circle

There are no other forces acting on the Earth,
no friction because the Earth is moving through empty space.
The Earth moves in a direction between the two arrows.

The attraction of gravity between the two masses
(the Sun and the Earth) stays the same
as long as the distance between them stays the same.
The Earth must move in a circle
to remain the same distance from the Sun.

The force that keeps an object moving in a circle
is called the centripetal force (continued on the next page).

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