We've all heard of gravity. It's what causes objects to fall, right? Well that's the everyday notion but we'll look at gravity from a more scientific approach. A gravitational force exists between all objects. Any two objects that you can think of are attracted to one another. How strong that attraction is, however, depends on two important quantities. They are the mass of the objects and the distance between them.

Johannes Kepler believed that the sun exerted a force on the planets. He studied and analyzed data for several years to develop three laws to describe the motion of every planet as they revolve around the sun.

http://www.kepler.arc.nasa.gov/johannes.html

Kepler's First Law http://zebu.uoregon.edu/~soper/Orbits/kepler1.html  states that the orbit of each planet is elliptical (see shape below) with the sun at one focus. A focus is one of two points used to make an ellipse. You can draw an ellipse by attaching a string and two tacks to a piece of cardboard and then tracing out the ellipse by moving the pencil just inside the string held tight.

http://library.thinkquest.org/3461/kepler_m.htm

Kepler's Second Law http://zebu.uoregon.edu/~soper/Orbits/kepler2.html states that an imaginary line from the sun to a planet sweeps out equal areas in equal time intervals. That means that Earth moves faster when it's closer to the sun and slower when farther away.

http://hyperphysics.phy-astr.gsu.edu/hbase/kepler.html#c6

Check out the Physics Applet at http://www.walter-fendt.de/ph14e/keplerlaw2.htm to see Kepler’s Second Law in action.

Kepler's Third Law states the ratio of the squares of the periods (the time needed for one trip around the sun) of any two planets is equal to the ratio of the cubes of their mean distances from the sun.
http://scienceworld.wolfram.com/physics/KeplersThirdLaw.html

The period is the time for one complete cycle of motion. Earth has a period of 24 hours. That's how long it takes Earth to make its complete cycle around the sun.

The third law may be easier to understand from the useful equation below. T represents the period and r is the distance of the planet from the sun.

Find more about the life and work of Johannes Kepler at http://kepler.nasa.gov/johannes/#anchor784359.

Although Kepler's Laws helped to understand the motions of the planets, they didn’t explain why those motions existed.

Isaac Newton studied planetary motion and came to the conclusion that any two objects in the universe exert gravitational attraction on each other. Based on his findings he proposed his law of universal gravitation. This law states that objects attract other objects with a force that relates to mass and distance. For objects such as you and a classmate, the force of gravitational attraction is very small and insignificant. You would have to add to your mass quite a bit in order to see the attraction. However, we can easily see our attraction to Earth's surface due to the earth's large mass.

To calculate this gravitational force between any two objects, multiply the mass of the first object by the mass of the second object by the universal gravitation constant and then divide that number by the squared distance between the objects.
Here’s a diagram along with the equation.

In 1798, Henry Cavendish experimentally determined the value for G.

Video Instruction
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• Newton’s Law of Gravitation #1 [8:37]

• Newton’s Law of Gravitation #2 [8:16]

• Newton’s Law of Gravitation [6:08]

for Students, Parents and Teachers

Now let's do Practice Exercise 1-5 (top).

Next Page: Applying Forces to an Object (Newton's 3rd Law) (top)