At 8/18/09 06:11 AM, reverend wrote:
At 8/18/09 05:33 AM, Carmoil wrote:
NOT true. . . . In the stop light example the force we're dealing with is gravity. When everything is still it's easy to see the net 0 force. But when the rope breaks and the light is falling the net is still 0. Again disregarding air resistance for simplicity, the two basic forces we have are the gravity of the earth and the gravity of the falling light. We tend to think of only the light as falling, but actually the light is pulling the earth towards itself the same amount as the earth is pulling the light towards itself.
In short, you are referring to Newton's law of Universal Gravitation.
Right, and gravity is a force between two objects, so you can basically think of the gravity of object one on object 2 as a vector equal to X and the gravity of of object 2 on object 1 as a vector equal to -X, with a total net of 0, making it still apply to the OP's question.
The light and the earth are falling towards each other with the same energy, but the energy it takes to move the light is almost infinitesimally small compared the how much it takes to move the earth, so the only movement we perceive is that of the light.
And that is the reason it's often omitted in Physics problems like the example I did. It's so small that it's practically irrelevant. I would even go as far to say that the air drag creates much more measurable Force than the light's Force on the Earth. The only time you would only need to factor that in is when the masses are close to equivalent.
Right again that it is usually negligible, except when talking about conservation of energy, because if you disregard the movement of the earth towards the light you end up with a non-zero net energy, when in reality the movement of the earth has equal energy and opposite direction as that of the light, creating a net 0. By using an example where the main force at work is gravity, and disregarding the gravity you completely bypass the point of the question the OP is asking. Are you just trying to confuse him?
And true air resistance is going to be more "measurable" than the movement of the earth in a real life situation, but that's besides the point of this thread.
... I know what I'm talking about better than these other guys, but I'm no expert.
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For conservation of energy to be calculated in a real world situation it's a hell of a lot more complicated than what either of us are talking about, because a net zero includes all forms of energy: potential, kinetic, chemical, and thermal. In your light example the light has potential energy before it falls, which turns into kinetic as it accelerates, and on impact a lot of that is going to turn into thermal, but the earth also has potential energy which becomes kinetic as it accelerates towards the light. Anyways, I'm done talking about this unless you make a really good point.
