Gravity, Planets and Moons

Why objects orbit

Gravity• That attractive force

between objects• That attractive force

that increases with mass and decreases with distance

• That attractive force that keeps planets orbiting the sun and moons orbiting their planets

How Does This Happen?• According to Newton’s first

law, if you throw a ball what will happen?

• It will continue in a straight line at a constant speed (inertia)

UNLESS• A force is applied.• What is that force?

Let’s Use Our Imagination• Imagine the earth is the ball. It

wants to continue in a straight line.

• The sun is pulling the earth towards its center (gravity)

• We have two things affecting the motion of the earth!

• Earth’s Inertia and the Sun’s Gravity!

Two Opposing Actions• If the Sun weren't there, the earth

would travel in a straight line (Newton’s first law – the law of inertia)

• But the gravity of the Sun pulls on the earth and changes the direction the earth travels (law of gravity)

• What is the end result?• Study the diagram on the next slide,

paying close attention to the arrows

Earth is Falling!

Earth is Falling

• Because the sun is curved, the earth keeps on going, constantly "falling," toward the sun.

• The earth never is able to hit the sun because the sun curves away before it can.

• Cool, huh?

Moon is Falling Also!• In just the same way the

earth orbits the sun, the moon orbits the earth.

RememberThe motion of all objects is

affected by forces!What force and what

property keep the planets orbiting the sun and the moons orbiting their planets?

Questions?