4 ms·

It would have been really interesting to ask people to draw a free-body diagram listing out each of force acting on the rock, prior to asking the question of wh

by ghayes 13y ago

It would have been really interesting to ask people to draw a free-body diagram listing out each of force acting on the rock, prior to asking the question of what would happen when dropped. It would be hard to make up a force besides lunar gravity and any Physics 101 student should now answer correctly by analytic reasoning.

marvin 13y ago
They would get it wrong, though. Even the physics 101 students probably wouldn't draw the gravity of the sun, the gravity of the earth and the Milky Way. If we're discussing reasoning from first principles, is it really so intuitive to disregard the gravity of the Earth in this experiment? Why can you disregard it and still get the correct result?
- jjoonathan 13y ago
  Exactly right: you have to recognize that the moon is in orbit/freefall with respect to Earth, which means there is no gravitation near its center of mass and only tidal forces away from the center of mass. Then you have to estimate that the tidal forces on the astronaut will be small compared to the moon's gravitational forces on him. None of this even counts centrifugal forces from the moon's rotation or effects of the sun. Nested non-intertial reference frames play merry hell with the simplicity of a FBD sans prior knowledge (or solid logical reasoning) about what can be neglected. Even with a physics degree, my kneejerk reaction didn't come from first principles. I, too, was doing "stupid pattern matching" -- the only initial difference between me and the floating-pen-people was that I happened to know that gravitational acceleration could be reasonably approximated as constant on large astronomical bodies (moon is g/6). It took a few seconds for the "otherwise the planetary body would disintegrate" argument to occur to me and even longer for the formal mathematical picture.

4 ms·

It would have been really interesting to ask people to draw a free-body diagram listing out each of force acting on the rock, prior to asking the question of wh

by ghayes 13y ago

marvin 13y ago
They would get it wrong, though. Even the physics 101 students probably wouldn't draw the gravity of the sun, the gravity of the earth and the Milky Way. If we're discussing reasoning from first principles, is it really so intuitive to disregard the gravity of the Earth in this experiment? Why can you disregard it and still get the correct result?
- jjoonathan 13y ago
  Exactly right: you have to recognize that the moon is in orbit/freefall with respect to Earth, which means there is no gravitation near its center of mass and only tidal forces away from the center of mass. Then you have to estimate that the tidal forces on the astronaut will be small compared to the moon's gravitational forces on him. None of this even counts centrifugal forces from the moon's rotation or effects of the sun. Nested non-intertial reference frames play merry hell with the simplicity of a FBD sans prior knowledge (or solid logical reasoning) about what can be neglected. Even with a physics degree, my kneejerk reaction didn't come from first principles. I, too, was doing "stupid pattern matching" -- the only initial difference between me and the floating-pen-people was that I happened to know that gravitational acceleration could be reasonably approximated as constant on large astronomical bodies (moon is g/6). It took a few seconds for the "otherwise the planetary body would disintegrate" argument to occur to me and even longer for the formal mathematical picture.