# gravity and kinetic energy

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If i were to jump off a very high cliff i would assume a steady increase in kinetic energy. If instead of jumping I pointed a flash light down and turned it on, being clueless I would assume the gravatational acceleration would be too small to effect a photons kinetic energy to any measurable degree. But, what if I pointed the flash light at a black hole. what then? If gravity causes a change in a photons direction of travel, implying that gravity does effect the photon what happens to the photons energy? C is th speed of light in a vacuum. What is a black hole to the photon? If all direction is a oneway street, is that the same as a vacuum? Do I need to put the word true, or perfect in front of vacuum?

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The force from a flashlight is P/c, where P is the power of the emitted light. That's 3.33 nanoNewtons per watt of power. The gravitational force on a 100kg person is just shy of a kiloNewton.  You've got almost 12 orders of magnitude difference there.

Where you point it won't matter — any effect on the photons after they leave the flashlight has no effect on you.

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17 minutes ago, swansont said:

The force from a flashlight is P/c, where P is the power of the emitted light. That's 3.33 nanoNewtons per watt of power. The gravitational force on a 100kg person is just shy of a kiloNewton.  You've got almost 12 orders of magnitude difference there.

Where you point it won't matter — any effect on the photons after they leave the flashlight has no effect on you.

Thank you. I am still trying to get used to the concept that gravity is not a force, so this answer tugs at my desire to still think of gravity as a force. Also, I didn't realize I had presented the thought that the photons would have an effect on me after leaving the flashlight, sorry if I did.

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4 minutes ago, jajrussel said:

Thank you. I am still trying to get used to the concept that gravity is not a force, so this answer tugs at my desire to still think of gravity as a force. Also, I didn't realize I had presented the thought that the photons would have an effect on me after leaving the flashlight, sorry if I did.

I don't see anything in the discussion that requires abandoning Newtonian physics. It's OK to think of gravity as a force.

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2 minutes ago, swansont said:

I don't see anything in the discussion that requires abandoning Newtonian physics. It's OK to think of gravity as a force.

Thanks,

Spoiler

Whats a spoiler? I was trying to find the spell checker? Didn't think I could hurt anything if I had only typed one word. Way out of my league here...

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On 11/15/2017 at 4:02 PM, jajrussel said:

If i were to jump off a very high cliff i would assume a steady increase in kinetic energy. If instead of jumping I pointed a flash light down and turned it on, being clueless I would assume the gravatational acceleration would be too small to effect a photons kinetic energy to any measurable degree. But, what if I pointed the flash light at a black hole. what then? If gravity causes a change in a photons direction of travel, implying that gravity does effect the photon what happens to the photons energy? C is th speed of light in a vacuum. What is a black hole to the photon? If all direction is a oneway street, is that the same as a vacuum? Do I need to put the word true, or perfect in front of vacuum?

If you were to jump off a very high cliff your momentum would be increasing steadily, and your velocity would be increasing steadily. Your kinetic energy would be those two things multiplied.

Now let's try to replace 'you' with  'photon' in that sentence above:

If a photon were to jump off a very high cliff its momentum would be increasing steadily, and its coordinate velocity would be decreasing. Its kinetic energy would be those two things multiplied.

The above sentence may be incorrect, as the kinetic energy would be decreasing, which does not sound quite right.

Let me try again:

The momentum of the downwards falling photon is increasing at the same rate as its potential energy is decreasing, and the coordinate velocity of the photon is decreasing at the same rate as its potential energy is decreasing. The product of the momentum and the coordinate velocity is unchanging.

That's much better.

Edited by Toffo
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18 hours ago, Toffo said:

If you were to jump off a very high cliff your momentum would be increasing steadily, and your velocity would be increasing steadily. Your kinetic energy would be those two things multiplied.

Now let's try to replace 'you' with  'photon' in that sentence above:

If a photon were to jump off a very high cliff its momentum would be increasing steadily, and its coordinate velocity would be decreasing. Its kinetic energy would be those two things multiplied.

The above sentence may be incorrect, as the kinetic energy would be decreasing, which does not sound quite right.

Let me try again:

The momentum of the downwards falling photon is increasing at the same rate as its potential energy is decreasing, and the coordinate velocity of the photon is decreasing at the same rate as its potential energy is decreasing. The product of the momentum and the coordinate velocity is unchanging.

That's much better.

You seem to have changed it, maybe too much. Does the photon jump off the cliff at c in your rearangement? There is not a lot of time before the photon hits the ground. Are you going to be able to calculate changes to the photons energy in that amount of time? Can it be done? There is a medium of atmosphere. Maybe the photon does change velocity on the way down due to the medium? I don't know. There is gravity. Maybe gravity will effect a small change? I would think a very small change.

I switched to the black hole, there is more time. Now maybe too much. I'm not sure. What happens to the photons energy when the medium no longr slows it down? I would think that at c the gravatational assist of the black hole would zero out. Again I don't know. It may be possible because of the angle of entry that the photon enteracts with the black hole similar to the way it would any other medium, and this thought just remended me that on TV you always see the ship banging into the side of the wormhole  , which is not exactly where I was going with the thought, but in a way maybe it helps to visualze the thought better.

I think I remember reading that a photons energy is its kinetic energy. Is that true?

Is it correct to say that a medium effects the photons velocity?

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3 hours ago, jajrussel said:

Is it correct to say that a medium effects the photons velocity?

There are many kinds of mediums. Below there is a road and three cars that are driving to the right, always at constant speed, we are looking from above. The road is the medium to the cars. When the cars enter the curvy part, they lose some rightwards pointing velocity and momentum. The road absorbs the momentum. When the cars leave the curvy part, the rightwards pointing momentum goes back from he road to the cars.

-----ooo--------------◠◡◠◡◠◡◠◡◠◡◠◡-------------------

Now let's say the  ''road'  is an optical fibre, and the 'cars' are photons.  The story is the same as above.

That was trivial. But what if the picture is a road profile where the bumps are hills?

Edited by Toffo
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8 hours ago, jajrussel said:

You seem to have changed it, maybe too much. Does the photon jump off the cliff at c in your rearangement? There is not a lot of time before the photon hits the ground. Are you going to be able to calculate changes to the photons energy in that amount of time? Can it be done? There is a medium of atmosphere. Maybe the photon does change velocity on the way down due to the medium? I don't know. There is gravity. Maybe gravity will effect a small change? I would think a very small change.

Small effect, but the effect is observable.

8 hours ago, jajrussel said:

Is it correct to say that a medium effects the photons velocity?

No. It affects light's velocity, but photons travel at c.

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On 11/19/2017 at 6:03 PM, swansont said:

No. It affects light's velocity, but photons travel at c.

I think that one of the reasons I get confused is be cause until you wrote this I thought of photons and light as inseperable as space/time, but with this statement you seem to have seperated the two?

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22 minutes ago, jajrussel said:

I think that one of the reasons I get confused is be cause until you wrote this I thought of photons and light as inseperable as space/time, but with this statement you seem to have seperated the two?

Photons are absorbed and emitted. They don't exist in that interim, so they don't have a speed. Light speed is measured by the time it takes for the signal to travel a certain distance. It can be c, or it can be slower. But the photons involved travel at c.

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20 minutes ago, swansont said:

Photons are absorbed and emitted. They don't exist in that interim, so they don't have a speed. Light speed is measured by the time it takes for the signal to travel a certain distance. It can be c, or it can be slower. But the photons involved travel at c.

Thanks...

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