Ann wrote:Think of it like this. Is the Earth falling into the Sun? Clearly not.
That's certainly one way to put it. But I think it also needs to be pointed out that the Earth very much
is falling into the Sun- as are all the planets, and in some sense, everything in the observable Universe.
Free fall is called that for a reason: you experience no weight when you are falling freely towards the body that produces the gravitational field. The radial velocity of the Earth towards the Sun is exactly what Newtonian mechanics requires it to be. The important point is that we are not getting any
closer to the Sun as we fall towards it, because we are also moving tangentially at just the right speed to keep our distance constant (well... not quite, since the orbit isn't perfectly circular, but that can be ignored for the purposes of this discussion).
Okay, now that the Sun had turned into a black hole, would the Earth fall into it? No, not at all. The Earth would continue orbiting the black hole in the same way it that it had orbited the Sun. There would be no difference. In fact, anything that is orbiting the Sun today without falling into it would continue orbiting it in the same way after it had turned into a black hole.
I've always like this example. It goes completely against most people's intuition, and is therefore a useful tool. The problem is, so many people have seen movies featuring great, sucking black holes that they have a totally unphysical model of them running in their minds.
Is nothing falling into the Sun, then? Yes indeed, some comets are falling into the Sun.
To be clear, there is nothing fundamentally different about the orbits of comets like Lovejoy and of planets like the Earth. In all cases, the bodies are falling into the Sun, but have a tangential velocity that keeps them from falling directly towards the
center of the Sun. Comet Lovejoy is in a highly eccentric (oval) orbit; Earth is in a nearly circular orbit. So over a complete orbit of Earth, the distance to the Sun doesn't change much; for a complete orbit of Lovejoy, it does. If you were treating this example in a typical physics problem, nothing would ever be able to actually hit the Sun, because it would be treated as a dimensionless point. Of course, in reality, the Sun has a finite radius, so if a body has a sufficiently eccentric orbit, it can actually intersect some part of the Sun (not the center), which is what happens with sungrazing comets sometimes.
[quote="Ann"]Think of it like this. Is the Earth falling into the Sun? Clearly not.[/quote]
That's certainly one way to put it. But I think it also needs to be pointed out that the Earth very much [i]is[/i] falling into the Sun- as are all the planets, and in some sense, everything in the observable Universe.
Free fall is called that for a reason: you experience no weight when you are falling freely towards the body that produces the gravitational field. The radial velocity of the Earth towards the Sun is exactly what Newtonian mechanics requires it to be. The important point is that we are not getting any [i]closer[/i] to the Sun as we fall towards it, because we are also moving tangentially at just the right speed to keep our distance constant (well... not quite, since the orbit isn't perfectly circular, but that can be ignored for the purposes of this discussion).
[quote]Okay, now that the Sun had turned into a black hole, would the Earth fall into it? No, not at all. The Earth would continue orbiting the black hole in the same way it that it had orbited the Sun. There would be no difference. In fact, anything that is orbiting the Sun today without falling into it would continue orbiting it in the same way after it had turned into a black hole.[/quote]
I've always like this example. It goes completely against most people's intuition, and is therefore a useful tool. The problem is, so many people have seen movies featuring great, sucking black holes that they have a totally unphysical model of them running in their minds.
[quote]Is nothing falling into the Sun, then? Yes indeed, some comets are falling into the Sun.[/quote]
To be clear, there is nothing fundamentally different about the orbits of comets like Lovejoy and of planets like the Earth. In all cases, the bodies are falling into the Sun, but have a tangential velocity that keeps them from falling directly towards the [i]center[/i] of the Sun. Comet Lovejoy is in a highly eccentric (oval) orbit; Earth is in a nearly circular orbit. So over a complete orbit of Earth, the distance to the Sun doesn't change much; for a complete orbit of Lovejoy, it does. If you were treating this example in a typical physics problem, nothing would ever be able to actually hit the Sun, because it would be treated as a dimensionless point. Of course, in reality, the Sun has a finite radius, so if a body has a sufficiently eccentric orbit, it can actually intersect some part of the Sun (not the center), which is what happens with sungrazing comets sometimes.