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From: Clark in MI
Category: General Category
Remote Name: 65.216.75.240
Date: 27 Oct 2004
Time: 01:18:00 PM
Yes, you're right in that the force of gravity an object at the same height as a spacecraft in low orbit is experiencing is only slightly less than that experienced by a person standing on the earth's surface. Gravity is a force that is inversely proportional to the square of the distance away from the center of mass that is exerting the gravitational attraction (the old "inverse square law"). Thus, the relative difference in distance from the earth's core is minor between a low orbit and the earth's surface as compared to the overall distance to the earth's core. However, there is a huge difference in the perceived gravitational force between a body at rest and a body that is falling. A falling body "appears" to be weightless relative to its surroundings. That's one of the ways they train astronauts for weightlessness (and also the primary way they achieve the weightless effect in movies); they simply take people up in a big jet, then let it drop back to earth in a dive that matches the gravitational acceleration of 32 feet per second per second. The weightless experience lasts less than a minute (you've got to pull out of that dive before you hit the ground), but it's real nonetheless. For our spacecraft that is moving in a low orbit around the earth, its rate of speed exactly matches the threshold of escape velocity for that height above the earth. That speed counteracts the force of gravity which, for an object in orbit, is its centripetal force. Since the centripetal force is perpendicular to the direction of travel, gravity does not do WORK on the orbiting object if it is in a circular orbit. In equilibrium between the earth's gravitational pull, and the speed that neutralizes the centripetal force, in essence the spacecraft is continuously falling; ergo: weightless for all objects moving relative to it, like the astronauts and the growing balsa aboard the ISS. For more on this you can refer to the HyperPhysics web page (which is a really, really good reference for all you students of Physics and Math out there). The direct link to the circular orbit section is: http://hyperphysics.phy-astr.gsu.edu/hbase/orbv.html#co From there you can explore the rest of the site...