Science Background Information
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Gravity is one of the fundamental forces in the Universe.  Although gravity is the weakest of all the forces in the Universe, it is the most important force in the study of astronomy.  Originaly defined by Newton, and refined by Einstein, gravity is essentially the natural force of attraction between any two objects.  Two factors determine the magnitude of the gravitational force between two objects: (1) their masses and (2) the separation distance between them.  The size of the force is proportional to the product of the masses of the two objects.  For example, the force doubles in size if either of the masses is increased to twice its mass.  On the other hand, the force grows weaker if the two objects are moved farther apart. In fact, it is inversely proportional to the square of the distance between them, so that if the distance is tripled, the force is only one ninth as strong.

The gravitational force of the sun, acting on the earth, keeps the earth in its orbit, preventing it from traveling away into interstellar space.  The gravitational force of the earth, acting on us, holds us to the earth's surface.  The gravitational attraction between a person and the earth is proportional the person's mass and inversely proportional to the square of the planet's radius (distance from the person to the center).  This number for gravitational attraction is called your weight!

Every planet has mass and so every planet exerts a gravitational force on nearby objects.  We say that planets have gravity.  However, what we really mean is that there is a gravitational force of attraction between the planet and a person standing on the planet's surface.  This force depends on the visitor's mass, the planet's mass, and the planet's radius.  Accordingly, people have different weights on different planets.

For example, a person on the moon weighs only about 1/6 as much as on earth.  The moon's radius is 25% earth's radius and the moon's mass is 8% of earth's mass.  So, if a student weighs 150 lbs on earth, she would weight only (1/6) * 150 lbs = 25 lbs on the moon.

 Common Misconceptions Among Middle School Students  the moon has no gravity planets with thin atmospheres have little gravity planets distant from the sun have less gravity gravity is stronger between the most distant objects space shuttle astronauts are weightless because there is no gravity above earth  NOTE: --in fact, space shutttle astronauts only appear weightless because they are in free fall around the earth.
Gravity does more than just keeping planets orbiting the sun and causing people to have weight, gravity also causes tides.  In simple terms, the tides are caused by the gravitational attraction between the moon and earth's oceans AND by the motion of earth through outer space.

On-Line Resources

• Central Force Applet
• NASA Observatorium Orbital Simulators (1) (2) and (3)
• How Old Are You on Other Planets Calculator