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KillerCodeMonky t1_j8i455z wrote

Reply to comment by Stillcant in Light traveling through a medium that slows it. Does the same photon emerge? by TheGandPTurtle

Here on earth, a ball resists being lifted due to gravity. It also returns all that energy when it's dropped again.

A wave that lifts a bunch of ping pong balls on it's leading edge, then drops them on it's trailing edge where the energy is returned to the wave, has done net zero work.

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grahampositive t1_j8ici68 wrote

This is where the analogy breaks down though. A water wave lifting a ping pong ball and returning it to it's initial position has lost energy. The ping pong ball pushes against air as it moves upwards, and the resulting ball-air collisions generate heat which is lost to entropy.

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KrombopulosThe2nd t1_j8i7auz wrote

Well here on earth, is it possible to lift anything with 100% efficiency? Shouldn't there be some loss of energy

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KillerCodeMonky t1_j8ia5jn wrote

Yes of course these are not perfect processes. Otherwise an undersea earthquake would create a tsunami on all of its coastlines. Energy is lost or made non-coherent in a variety of imperfections, including heat and scattering. In the case of ocean waves, they are typically created and recharged by the wind as they move along.

I mostly wanted to make the point that a wave is not displacing its medium, but simply moving through it. A wave is nothing but water and energy. There's nothing there displacing the water to do work. The water moves around due to the energy, but in a way which is generally neutral in terms of work actually done. A wave hitting the shoreline really just transfers from moving through the water to moving through the land.

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DamionFury t1_j8ijw6m wrote

Work can turn out to be one of the less intuitive aspects of physics. For example, magnetic fields cannot do any work because they act orthogonally to the direction of motion, yet it certainly looks like work when you use an electromagnet to lift an object and make it float. I wish I could remember the explanation my Electromagnetism professor gave me for what is actually doing the work in that scenario.

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Parrek t1_j8j1x62 wrote

Usually the battery or another power source maintaining the magnetic field. Otherwise, the back current induced by the object being picked up would cancel everything out

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DamionFury t1_j8j6cgr wrote

Ah yes. That's right. I also remember we talked about the case of permanent magnets and objects suspended above them. I don't remember what his answer was and I wouldn't be surprised if he told me to try to work it out for myself because my questions were holding the class up.

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kkngs t1_j8l4jhj wrote

If the object is just being suspended by the permanent magnet, being held still against the force of gravity, then no work is being done. It’s not conceptually different than being held up by a shelf.

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agoodpapa t1_j8jpira wrote

Not sure. The ping pong balls have been moved and therefore have absorbed energy.

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KillerCodeMonky t1_j8jr7mn wrote

Movement perpendicular to and away from the earth will convert to potential gravitational energy, which is then released when the ball moves back towards the earth.

In addition, you're assuming that the balls are passive participants. In actuality, from the waves perspective, they are actively resisting being lifted, and actively attempting to fall, limited by their buoyancy in the water. When the wave causes the water to fall away from the balls, they are falling into the water on that trailing edge.

Finally, conservation of energy dictates that that energy does not just disappear. If the energy goes into the ball as movement, something has to stop that movement. That something is going to be either gravity if moving up, or the water itself in any other direction. So the energy is moving from the water, into the ball, back into the water as it resists the ball displacing it to move.

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