# Kinetic energy and blackholes

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I'm currently studying energy transfers e.g (very basically) chemical energy used for lifting something onto a shelf, which transfers to kinetic energy and once on the shelf has potential gravitational energy due to the height increase relative to the earth. No energy has been lost but conserved.

Now if hypothetically a black hole appeared below the shelf...and the object fell into it...with the absence of a mass to transfer the energy from the object, wouldn't the gravity be applying constant work (constant acceleration) and obviously this would increase the kinetic energy...I can't fathom how the rules of energy convservation apply to black holes.

Surely the energy would be increasing exponentially, what am I missing about black holes that preserves energy conservation...it seems there is just free energy when gravity has no stopping block (for want of a better term.)

I guess classical mechanics and black holes aren't compatible ?

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Gravity doesn't apply a constant acceleration, it's:

$g=- \frac {GM} {r^2}$

TBH I don't know much about black holes, but IIRC classical mechanics breaks down...

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I dont think I understand what the problem is here. How is the amount of energy being increased?

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Gain in kinetic means loss in potential...

Just because it's a black hole, it doesnt act any differently than say the earth. (Not taking relativity into account)

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Gain in kinetic means loss in potential...

Just because it's a black hole' date=' it doesnt act any differently than say the earth. (Not taking relativity into account)[/quote']

I realise this, but an object hitting the earth displaces it's energy through sound, and internal energy, and that's where it ends for the object, so I'm not sure a gain in kinetic and loss in potential is applicable with blackholes. I already stated a loss in kinetic means a gain in potential with my example.

As Klaynos said though, I slipped up with thinking gravity has a constant acceleration, it's any object has a constant acceleration toward the earth...that's really basic (going back to Galileo) physics (slaps forehead.)

So thanks for your responses, but I was being an utter dim wit...had a lot on my mind this week, not thinking straight.

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So thanks for your responses, but I was being an utter dim wit...had a lot on my mind this week, not thinking straight.

I know that feeling well

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I know that feeling well

Thanks Klaynos, that made me feel better. The whole reason you can't apply classical mechanics with black holes is that your dealing with the speed of light, as light can't escape black holes. So for a particle to have escape velocity from a black hole it would have to exceed the speed of light, which is impossible.

The problem I have is...if a particle (even a photon) can't escape the gravitational pull of a black hole, then there must be acceleration within the black hole that exceeds light, otherwise particles would just freeze or hang in suspension around the black hole...maybe.

So my reasoning is that under black hole conditions, the work is provided by gravity...and potential energy goes out the window...if we presume that gravity is just a product of the shape (for want of a better word) of space-time. So anything entering a black hole would consistently gain kinetic energy, because even a particle of no mass cannot escape, therefore even photons gain more kinetic energy as they are pulled into the blackhole. Otherwise they would just be suspended around the event horizon.

Sorry if this poorly worded...I was studying till 2 am last night, my brain feels a little fried.

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Thanks Klaynos, that made me feel better. The whole reason you can't apply classical mechanics with black holes is that your dealing with the speed of light, as light can't escape black holes. So for a particle to have escape velocity from a black hole it would have to exceed the speed of light, which is impossible.

Klaynos, just for the record, that explanation wasn't aimed at you. I'm sure you're well aware of the problem with classical mechanics and the speed of light.

I think I might wait until I've got the basics in my head first, before I start even thinking about black holes. I just find them far too interesting, to not talk about them.

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