Can someone smart or good with physics answer this dumb Q I came up with?

[Zeze]

I thought a bit & came back. Assuming the perfectly rigid stick, etc., why would he have to push the stick? The gravitational force of attraction at the star's end is certainly greater than the gravitational force of attraction at the Earth end.

That's not the point of the question...

 

[OILFIELDTRASH]

That's true, but it implies that energy is being added to its orbit, not taken away.

So now we have tides that are absorbing energy, and a moon that is absorbing energy.

Where's it coming from?

:biggrin:

Dark energy?

 

[SunnyD]

Then assuming you have enough upper arm strength to move a 2.5LY stick. And the stick isn't buckling, bending, etc.

The stick still has intertia - 2.5 LY of mass. That's a lot of inertia.

You wouldn't be moving it, you'd be compressing it. The other end of the stick would move that 5", but only after a period of time determined by the material's wave propagation speed.

Wouldn't the correct answer with nearly any existing material essentially be "Never"?

I'm fairly certain that the compression wave would dissipate within the material far before it reaches the end of the stick due to various other energy-loss inducing forces. In fact, as alluded to earlier, given a "human" pushing on the stick, I doubt it would even move in the first place. But still, there's a lot of variables to the original question that need to be answered before the question can even remotely become possible to test, even hypothetically.

 

[OILFIELDTRASH]

Nvm

 

[SlitheryDee]

Hey OP, I have a better question for you. I actually struggled with this one for a while and I think I even created a thread here about it. You can answer it with some internet research.

Every day, the tides come in, and they go out. As they go in and out, they produce friction between water and the surfaces the water is flowing against. In some cases, the tides are captured to produce electricity.

BUT

We know that energy can neither be created nor destroyed, only conserved or transformed from mass or to mass. So... where does all this tidal energy come from?

The answer might surprise you. It surprised me!

Well we know that the direct cause of tides is the gravitational pull of the moon. I don't think that counts as a source of energy though. What causes the tides to continuously be pulled in different directions is the rotation of the moon around the earth though which causes it's gravitational force to change directions relative to the earth. So the source of the energy of the tides is the actual orbital force of the moon being bled off through it's gravity acting on the earth. Whatever initial push there was that gave the moon it's velocity is therefor the ultimate source of tidal force. I guess you could trace that back to the big bang, like everything else.

 

[Puppies04]

Your stick would melt.

 

[Zeze]

Wouldn't the correct answer with nearly any existing material essentially be "Never"?

I'm fairly certain that the compression wave would dissipate within the material far before it reaches the end of the stick due to various other energy-loss inducing forces. In fact, as alluded to earlier, given a "human" pushing on the stick, I doubt it would even move in the first place. But still, there's a lot of variables to the original question that need to be answered before the question can even remotely become possible to test, even hypothetically.

So... if the stick was moved 5" from earth... where did the 5" go?

 

[OILFIELDTRASH]

He meant the gravitational pull that gave the moon an orbit is from the sun?

The gravitational pull is from the earths gravity. I was talking about the spinning but after some research the earth spinning comes from the way it's created. That energy was set in motion at it's creation and will not slow down due to vacuum of space.

 

[Ruptga]

Juheezus, people! The fifth post answered the thread. Here it is again for you knuckleheads that missed it.

Then assuming you have enough upper arm strength to move a 2.5LY stick. And the stick isn't buckling, bending, etc.

The stick still has intertia - 2.5 LY of mass. That's a lot of inertia.

You wouldn't be moving it, you'd be compressing it. The other end of the stick would move that 5", but only after a period of time determined by the material's wave propagation speed.

 

[njdevilsfan87]

That's true, but it implies that energy is being added to its orbit, not taken away.

So now we have tides that are absorbing energy, and a moon that is absorbing energy.

Where's it coming from?

:biggrin:

The Earth's rotation.

 

[OILFIELDTRASH]

Well we know that the direct cause of tides is the gravitational pull of the moon. I don't think that counts as a source of energy though. What causes the tides to continuously be pulled in different directions is the rotation of the moon around the earth though which causes it's gravitational force to change directions relative to the earth. So the source of the energy of the tides is the actual orbital force of the moon being bled off through it's gravity acting on the earth. Whatever initial push there was that gave the moon it's velocity is therefor the ultimate source of tidal force. I guess you could trace that back to the big bang, like everything else.

But the tides are nightly. The energy is coming from the earth rotating completely every 24 hours. The moon takes roughly a month to orbit us.

 

[Fritzo]

Then assuming you have enough upper arm strength to move a 2.5LY stick. And the stick isn't buckling, bending, etc.

*insert penis joke*

The stick still has intertia - 2.5 LY of mass. That's a lot of inertia.

Correct. The impact would likely destroy the planet. Nice work, Mr. Death Star.

 

[SlitheryDee]

But the tides are nightly. The energy is coming from the earth rotating completely every 24 hours. The moon takes roughly a month to orbit us.

I forgot that. That's what I get for living miles away from anything that demonstrates noticeable tides. Well then it is some combination of the earth's rotational force and the orbital velocity of the moon. Same principal I suppose.

 

[SunnyD]

So... if the stick was moved 5" from earth... where did the 5" go?

Compression and deformation within the material along the length of said stick? Some of the energy would simply dissipate. The stick would bulge, bend or buckle in various spots.

A simple (semi-related) example for you would be to take a (long) string, tie it to something in place at one end, stand far enough away and throw a wave on it. If you had the equipment to measure the amplitude of the wave along the string, you'd find that it would lose some between beginning and end.

Compression waves propagate a lot better, but they still dissipate through a medium just the same. The energy will get absorbed, retransmitted and/or reflected as the wave hits the atoms in your stick's medium along the way. The energy will also get converted into various forums, but usually heat. And heat is a very efficient way for a material to dissipate energy, especially in the vacuum of space.

 

[Blackjack200]

Dark energy?

The tides drag on the Moon's orbit, so the most accurate answer is that the tides are slowing the Moon's orbit down, and if nothing else was affecting the Moon's orbit it would eventually slow down enough to fall into the Earth.

But there is something else affecting the moon's orbit. Periodic differences in the speed of the Earth's rotation and the moon's orbit transfer energy from the Earth's rotation to the moon's orbit and that energy overpowers the drag of the tides and drives the moon into ever higher orbits.

So in my opinion, the most useful way to imagine it is that the energy from the tides actually comes from the inertial energy of the rotation on the earth. In other words, the tides are making our days longer.

http://en.wikipedia.org/wiki/Tidal_acceleration

 

[arkcom]

The stick's so heavy the earth moves back 5 inches instead.

 

[JTsyo]

The Earth's rotation.

This. Conservation of angular momentum says as the moon moves away, earth has to slow to keep the system's angular momentum the same. This means in future we get longer days, yay. We're gaining about 23 microseconds per year. It'll take about 50 billion years for the moon to reach a location where the system would be tidally locked but of course the sun goes out way before then.

 

[silverpig]

I am disapoint with ATOT. It took over 50 posts for someone to mention "speed of sound".

If we make this more realistic and have a 1000 ft long piece of rebar laying on a long flat table, and you give one end a whack with a sledgehammer, the other end won't move instantly. There'll be a delay of something like 1/20th of a second before the other end moves.

 

[Slew Foot]

Only if youre standing on a treadmill

 

[BarkingGhostar]

I'm late to this post and only read the first few replies. I would have a problem with the other end of the stick being only 5" from touching it. Is this stick made of something special since being that close to a star would set it on fire, and then some.

Also, isn't the nearest star 4.29 LY away?

 

[phucheneh]

Juheezus, people! The fifth post answered the thread. Here it is again for you knuckleheads that missed it.

'the fuck? What is with the answers of 'well, if we assume the stick is perfect, it would distort.' Uh...no. You're negating the point of qualifying that the stick, being an impossible fictional item, has its own properties that are in line with the behavior of any 'normal' stick. I.e. you're not worried about the mass of the stick causing it to compress when you push it. Especially because that thought would be to invite gravity to the party, and the gravitational forces at play are essentially incalculable.

This makes me want to punch babies.

The stick moves, people, just like my penis did into your moms.

 

[phucheneh]

I am disapoint with ATOT. It took over 50 posts for someone to mention "speed of sound".

If we make this more realistic and have a 1000 ft long piece of rebar laying on a long flat table, and you give one end a whack with a sledgehammer, the other end won't move instantly. There'll be a delay of something like 1/20th of a second before the other end moves.

To again refute this silliness...pretend you're moving the stick ever so gently. Again, much like something involving my penis and someone's mother.

But, like, ridiculously slowly. Say it takes a year to move the stick five inches. Are you still going to argue that some kind of shockwave has to propagate through the damned thing?

Also, go back to my example of the tube full of ball bearings, and assume that, because we're traversing empty space, you do NOT have to fight the mass of them, anyway. Are you still waiting for this 'wave' to reach the last bearing?

I would just say 'pretend the stick has zero mass,' but then there would be 'oh noes that is unpossible!' complaints.

 

[Ruptga]

'the fuck? What is with the answers of 'well, if we assume the stick is perfect, it would distort.' Uh...no. You're negating the point of qualifying that the stick, being an impossible fictional item, has its own properties that are in line with the behavior of any 'normal' stick. I.e. you're not worried about the mass of the stick causing it to compress when you push it. Especially because that thought would be to invite gravity to the party, and the gravitational forces at play are essentially incalculable.

This makes me want to punch babies.

The stick moves, people, just like my penis did into your moms.

I'm sorry you have trouble with reading, but here it is again, this time just the important part.

The other end of the stick would move that 5", but only after a period of time determined by the material's wave propagation speed.

That answer is completely true to the spirit of the OP, and is in line with what you just said.

A more literal interpretation would be that the effects of us hitting the stick would be completely overshadowed by the star sucking the stick into itself. That motion would also move down the stick according to the material's wave propagation speed. The stick would be vaporized as it fell into the star, and if its thickness were appreciable at all, so much volume of stick would eventually fall into the star that the star's composition would be appreciably changed. What the star does next is mostly determined by what the stick is composed of.

 

[Brigandier]

To again refute this silliness...pretend you're moving the stick ever so gently. Again, much like something involving my penis and someone's mother.

But, like, ridiculously slowly. Say it takes a year to move the stick five inches. Are you still going to argue that some kind of shockwave has to propagate through the damned thing?

Also, go back to my example of the tube full of ball bearings, and assume that, because we're traversing empty space, you do NOT have to fight the mass of them, anyway. Are you still waiting for this 'wave' to reach the last bearing?

I would just say 'pretend the stick has zero mass,' but then there would be 'oh noes that is unpossible!' complaints.

I nominate you to be the physics hulk.

 

[phucheneh]

A more literal interpretation would be that the effects of us hitting the stick would be completely overshadowed by the star sucking the stick into itself. That motion would also move down the stick according to the material's wave propagation speed. The stick would be vaporized as it fell into the star, and if its thickness were appreciable at all, so much volume of stick would eventually fall into the star that the star's composition would be appreciably changed. What the star does next is mostly determined by what the stick is composed of.

oh jesus god allah christ.

edit: Oh, and on the first part...no. It's not a signal. You're not broadcasting to the distant planet/star/whatever. It's a solid chunk of matter than is already in place, and you are physically moving it, not sending a signal. You're not waiting on electrons to jump between atoms, or otherwise limited by the speed of light, because nothing is moving that quickly.

I understand the argument; yes, a wave can travel through a solid. But it's a magic stick, FFS.