Tuesday, December 11, 2007
Free Fall Physics
One of the things that confuses people about space flight is the whole "weightless" or "zero-G" thing. I was once discussing space flight with a man on an airport bus. He was a Ph.D. biochemist and had just attended a shuttle launch at KSC. He was pretty interested in space and said he had read a few articles and books, so I was surprised when he asked me, "At what altitude does gravity fall to zero so the astronauts can be weightless?" I explained that while gravitational force does fall off with increasing distance (squared), it is only slightly lower at the shuttle's typical orbital altitude than it is at the surface. The astronauts are not really in zero-G or weightless, but rather in a continuous free-fall around the Earth, and that this is essentially what an orbit is, just a really long fall as gravity pulls the spacecraft and astronauts toward the center of the Earth, and the Earth's surface "bends away." He was surprised and even somewhat skeptical of this. I guess the common terms "weightless" and "zero-G" are pretty misleading and influential. Free-fall is a better term.
This is explained in many places, starting with Newton's Principia in 1686-87. I even have a discussion in my Go Play In Space. But one of the best explanations I've seen is in a video of a physics lecture by Richard A. Muller, professor of physics at UC Berkeley. His course "Physics for Future Presidents" (Physics 10, for non-majors) is one of the most popular introductory classes at Berkeley, and his lectures are available as live webcasts and on YouTube. He also wrote a textbook for the class (several chapters are available on his teaching web site). The video above is lecture #3 on gravity and satellites (the orbit explanation starts at around 20 minutes). He does a few demos, but it's mostly blackboard and talking, with many real-world examples and great analogies and explanations. I've only watched this one lecture so far (69 minutes), but he's obviously really good. It's great that stuff like this is on the web and free to all (he has people all over the world auditing his webcast lectures and listening to his podcasts, which are also available free on iTunes).