Cool PhET Simulation of Planetary Orbits

Check out this PhET simulation for a planet orbiting a star. Experiment with it by changing masses, adding a moon, etc. Very useful to see orbital motion in action!

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How to Find Gravitational Potential Energy when Gravity Changes (i.e. large heights)

We look at how to do gravitational energy the 'right way.' This is because gravity really is a non-constant force, and U = mgh is simply an approximation. While it works when the height is small compared to the radius of the earth, it does not work for space launches, orbits, and so on. Let us see what happens and how to do this, starting with Newton's law of gravity and our old gradient friend, F = -dU/dr. We will also see how this gets us escape velocity and Schwartzschild radius (for black holes).