Gravity 
Johannes Kepler came up with three laws that described how planets moved. These three laws were based on observations of the planets, but Kepler didn't know why they moved the way they did. Kepler's first law is that the orbits of planets are elliptical. Kepler's second law is that the orbits of planets sweep out equal areas in equal times. Kepler's first two laws are illustrated well by this animation. Kepler's third law is that there is a special relationship between the average radius of an orbit and the time it takes for one orbit. It took almost 100 years before Newton came along and explained why the planets moved the way they do. He realized that the gravity that exists on Earth must extend out into space, and that it must be gravity that causes the the planets to move the way they do. Not only was Newton able to explain all three of Kepler's law, but he was able to show that using Kepler's third law you could find the mass of the sun and of any planet with a moon. Using either of these two animations and Kepler's third law it is possible to find the mass of Jupiter. Even better than the animations is an open source program called Celestia. Newton's Theory of Universal Gravitation can be used to show why all things fall at the same speed, even a feather and a hammer. Newton also described orbits and how it might be possible to put satellites in orbit using a large cannon. There is a common misconception that astronaut in the space shuttle experience zero gravity. There is actually almost as much gravity in a space shuttle orbit as there is on Earth. What the astronauts experience is actually free fall. As long as we are talking about space, here are cool videos of the moon and Mars, and a picture of the Earth and moon that is drawn to scale (imagine being in a tiny space ship right between the two). And finally a video showing how gravity from the moon creates tides. The friction from the tides is actually slowing the Earth down, but angular momentum has to be conserved, so the moon is moving farther away.




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