Gravity

•We know that planets travel around the Sun. Earth travels around the sun every 365 ¼ days.

•But what keeps the planets in this constant motion around the Sun?

•Answer: Gravity and Inertia!

What is Gravity?

Gravity is a force that attracts all objects toward each other.

• Both the Sun and the planets have gravity that pulls on one another.

Gravity

The strength of gravity between two objects depends on two factors:

•1. masses of the objects (If mass increases, force of gravity increases)

•2. distance between the objects (If distance increases, force of gravity decreases)

Remember Newton’s First Law of Motion?

•The law of INERTIA

•An object in motion stays in motion, with the same speed and in the same direction, unless acted upon by an unbalanced force.

Why do planets revolve around the Sun?

•Inertia and gravity combine to keep planets revolving around the sun.•If there were no gravity, inertia would cause the planets to move in a straight line. •If only gravity existed, the planets would be pulled into the sun.

Question:

What two factors act together to keep the Earth in orbit around the sun?

Question: What two factors act together to keep the Earth in orbit around the sun?

Answer: •Inertia and gravity

Question: what planet would have the fastest orbit around the Sun?

Question: what planet would have the fastest orbit around the Sun?

Answer: The planet closest to the Sun, Mercury

Question

•Describe what would happen to the planets if the sun’s gravity:•Increased•Decreased

Draw a diagram to support your answer.

If the Sun’s gravity increased, the planets would begin to move closer to the Sun.

If the Sun’s gravity decreased, the planets would begin to move farther away from the Sun.

Answer: