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GLE 6.2.D.b: Describe how the Planets’ gravitational pull keeps satellites and moons in orbit around them.

Gravity holds us to the Earth's surface. Everything on or anywhere near the Earth is pulled toward the Earth's center by gravitational force. The gravitational pull of planets keeps their moons in orbit around them.

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http://curious.astro.cornell.edu/question.php?number=211

GLE 6.2.D.c: Describe how the Sun’s gravitational pull holds the Earth and other planets in their orbit

The sun's gravitational pull holds the Earth and other planets in their orbit.

Newton's Law of Universal Gravitation

Mass = a quantity of matter.
The more mass an object has, the harder it is to move.
More force is needed to give the same acceleration to a more massive object. Twice the mass requires twice the force.

The Sun is much more massive than the planets.

Both feel the same force, but the planets have greater accelerations.

The force of gravity is proportional to the mass of the sun (M) times the mass of the planet (m).

The force diminishes with distance - The force is inversely proportional to the square of the planet's distance from the Sun (d). So as an object moves further away, the gravitational pull decreases.

Force of gravity "F" is proportional to 1/d2.

For a large spherical mass (like Sun, planet), the force can be considered as concentrated at their centers.

Mathematical expression for the force acting between planets and the Sun:

F = G (mass of Sun) (mass of planet) / d2 (distance between the 2 objects-squared)

= F = G / d 2= M m

d 2

Note that G is a gravitational constant = 6.670 x 10-11 N-m2/kg2.

This relationship has come to be known as Newton's Law of Universal Gravitation. It is "universal" because all objects in the universe are attracted to all other objects in the universe according to this relationship.

Hence, the force of gravity explains the motions of the planets.

The Sun's gravitational force prevents the planets from moving in straight lines.

According to Einstein, gravity is actually the curvature of space around the mass of an object. As a lighter object (like a planet) approaches a heavier object (like the Sun) in space, the lighter object follows the lines of curved space, which draws it near the heavier object. To understand this concept, imagine space as a huge stretched sheet. If you were to place a large heavy ball on the sheet, it would cause the sheet to sag. Now imagine a marble rolling toward the ball. Rather than traveling in a straight line, the marble would follow the curves in the sheet caused by the ball's depression.

Source: http://facstaff.gpc.edu/~pgore/Earth&Space/GPS/gravity.html and

Practice Test Questions and Scoring Guides:

Test Question:

Describe how the planets’ gravitational pull keeps satellites and moons in orbit around them.

Scoring Guide:

A Satellite remains in orbit because of a balance between the satellite's velocity (speed at which it would travel in a straight line) and the gravitational force between the satellite and Earth. (1 point)

Were it not for the pull of gravity, a satellite's velocity would send it flying away from Earth in a straight line.

Were it not for velocity, gravity would pull a satellite back to Earth. (1 point)

What holds the Earth and other planets in orbit around the Sun?

• The Sun’s gravitational pull holds the planets in orbit around the Sun.

How do planets move around the Sun? What is the name of this path?

• Planets revolve or orbit around the Sun in an elliptical path.

Why don’t planets travel off into a straight line?

• A planet remains in orbit because of a balance between its velocity (speed at which it would travel in a straight line) and the gravitational force between the planet and the Sun.

Why do planets have greater accelerations than the Sun?

• Planets have greater accelerations because of their smaller masses. a = F/m.

What keeps satellites and moons in orbit around planets?

a) A planets mass keeps satellites and moons in orbit around them

b) A planets gravitational pull keeps satellites and moons in orbit around them

c) A planets composition keeps satellites and moons in orbit around them

d) A planets atmosphere keeps satellites and moons in orbit around them

Earths gravity pulls any object on or near the Earth toward it(including natural and artificial satellites).

a) True

b) False