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I mentioned the gravitational slingshot when we were talking about gravity and work, Vernon. See http://www.thenakedscientists.com/forum/index.php?topic=27444.msg291021#msg291021 [nofollow].
A better example for "never actually touch" would have been a magnet.
The reason a nail is attracted to a magnet is because its in a magnetic field, which is local to the nail. It's similar for a gravitional field. A field has energy, and is a non-uniform disposition of the space/energy caused by one object that has some effect upon another.
All I can do is reiterate that when you fire a cannonball up into the sky at 11.2 km/s it escapes the Earth's gravitational field, which is then diminished.
You really should look at the example I gave which explains why the energy goes into the cannonball rather than the far more massive Earth which has no detectable recoil.
Can we consider the question in terms of the Earth and an object of significant mass, for example, 10% of the mass of the Earth? Although the forces involved might be a little greater, I would not think the underlying physics would differ in the slightest. A model that only works for cannonballs of negligible mass (relative to the mass of the Earth) does not strike me as terribly viable.
When you alter the mass ratios such that one is very much bigger than the other, very little of the kinetic energy goes into the larger mass. See http://www.physicsclassroom.com/Class/energy/U5L1b.cfm [nofollow] re gravitational potential energy - there is no mechanism by which the kinetic energy of a vertically-fired cannonball is transferred to the earth or the earth's gravitational field as the cannonball slows down and reaches its final height.
there is no mechanism by which the kinetic energy of a vertically-fired cannonball is transferred to the earth or the earth's gravitational field as the cannonball slows down and reaches its final height.
Vernon, the kinetic energy becomes potential energy, but it's cannonball potential energy.
It isn't transferred out of the cannonball.
I've never claimed that black holes do not exist, ...
Note that what I'm saying is no hypothesis, it's in line with educational establishments such as http://www.physicsclassroom.com/Class/energy/U5L1b.cfm [nofollow].
Geezer has one important thing right about requesting us to talk about a system in which the masses are more nearly equal to each other; you will be unable to assign the potential energy to just one part of it.
Since the cannonball has escape velocity it leaves the earth’s gravitational field and escapes the system. Conservation of energy tells us the cannonball takes the energy away with it ...
Since the cannonball has escape velocity it leaves the earth's gravitational field and escapes the system.
Gravitational fields don't stop at some arbitrary boundary. They just get weaker. At escape velocity the cannonball and the Earth are still mutually attracting each other, but the KE of the cannonball is great enough that the Earth is not able to "pull it back". Consequently, it's not valid to say that the cannonball has escaped the system. In fact, I don't think it's possible to define any point at which the cannonball has left the system. No matter how hard we try, there is always going to be some interaction. It may be an incredibly weak interaction of course, but it will not go to zero.
Geezer: when an object has escape velocity, as it gets further away its velocity reduces because the Earth's gravity pulls it back. But the velocity reduces less than the gravity reduces. So it always gets further away. It never comes back.
I re-read what you said Geezer, and now understand what you meant. Apologies for misunderstanding. I'd say when an object achieves escape velocity, it has not given up its potential energy. Can I add though that this potential energy is only relative to us rather than something absolute. If we were on the surface of a different planet with a different gravitational field, we'd say the object had some different amount of potential energy.
Geezer, physicists typically work with both "ideal" cases and "real" cases. They know the difference and aren't afraid to say which case they are talking about in any given situation. The first two of Newton's Laws of Motion, for example, represent ideal cases in which there is no air resistance or other forms of friction. So, in the case of an object that we know has an escape velocity from some large gravitating body, there is no reason why we cannot talk about the object as having actually escaped from that body; we can mentally remove the body from our discussions of the escaped object, even if we have to pretend that the object has reached infinite distance from the large gravitating body (because we know it can, given infinite time).
Quote from: HankRearden on 19/07/2009 23:09:39So matter can basically be thought of as just potential energy because matter can be turned into energy and theoretically vice-versa, does that mean when I lift up a plate and increase it's potential energy, i've technically increased its mass?The mass is increased, but not the mass of the plate: the mass of the system Earth-plate.
So matter can basically be thought of as just potential energy because matter can be turned into energy and theoretically vice-versa, does that mean when I lift up a plate and increase it's potential energy, i've technically increased its mass?
...the rule of thumb is that the more mismatched the masses are, the more energy goes into the smaller mass.