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View Full Version : Thought experiment, a really long lever


Aerozord
01-13-2012, 06:47 PM
so last night I was thinking of the quote "give me a long enough lever and I will move the world" and went, what if you did have a really long lever.

This led me to the following thought experiment

You have a lever thats, lets say 100 million lightyears long, and made of a perfect solid that doesn't bend or brake. Fulcrum at one end and force applied directly above it. You have nigh-limitless force to apply to it.

Now what would happen? Because of the length even an incredibly minute amount of acceleration would cause the other end to break the lightspeed barrier. Made me wonder if there is a minimum amount of force required to break inertia, and if so would that make the lever immovable in this manner do to limit on acceleration at the other end?

Other is what kind of odd effect that would have on the center of gravity and equation for a lever, since the kinetic energy would increase exponentially and center of gravity would change due to the changes in mass. In fact would it be an instantaneous or a progressive change?

Amake
01-13-2012, 06:57 PM
My immediate thought is, there's going to be some odd bendy effects along the lever as it approaches the speed of light. It should get weird within just one light year of the fulcrum as we remember the movement of the lever can only propagate from one end to the other at the speed of light. To accommodate an unbendable lever I imagine space will have to bend around it somehow.

Interesting idea. I'm a little sleep deprived to consider it further right now though.

Mr.Bookworm
01-13-2012, 07:24 PM
Now what would happen? Because of the length even an incredibly minute amount of acceleration would cause the other end to break the lightspeed barrier.

No, it wouldn't. The speed of light in a vacuum (c) is an absolute. You would get a lever going some enormous fraction of c, but you would never reach c (doing so requires infinite force). You would need more and more force to move the lever as it approaches c.

On what actually happens to the lever, you get weird time shit on the other end, and it becomes really fucking heavy at the other end. Of course, it takes you 100 million years to see it happening.

Made me wonder if there is a minimum amount of force required to break inertia, and if so would that make the lever immovable in this manner do to limit on acceleration at the other end?

Uh. What. No, you can't "break" inertia. You would need an increasing amount of force to move the lever, due to the lever's increasing mass as it approaches c.

Sithdarth
01-13-2012, 07:41 PM
The existence of said lever requires that the bonds between the atoms in said lever be of infinite strength. Thus the question is meaningless because the assumptions you have to make to allow for the existence of the lever, perfect rigidity, directly alter the properties you are attempting to explore, the motion of the lever. Such thought experiments are absolutely useless because you can't accurately explore a set of properties while simultaneously postulating an impossibility that effects those properties.

Aerozord
01-14-2012, 12:27 AM
The existence of said lever requires that the bonds between the atoms in said lever be of infinite strength. Thus the question is meaningless because the assumptions you have to make to allow for the existence of the lever, perfect rigidity, directly alter the properties you are attempting to explore, the motion of the lever. Such thought experiments are absolutely useless because you can't accurately explore a set of properties while simultaneously postulating an impossibility that effects those properties.

ok, then what if it wasn't perfectly rigid, would the forces just rip it apart?

Sithdarth
01-14-2012, 12:41 AM
Pretty much. I'd wager it's own weight would probably rip it apart. But it was actually mathematically proven that in Relativity there can be no such thing as a rotating rigid object at relativistic speeds. Relativistic effects make it impossible.

Seil
01-14-2012, 01:17 AM
I thought it was just a figure of speech.

Sithdarth
01-14-2012, 01:44 AM
This is for a dis but the same arguments apply for really any rotating object such as a lever. (http://math.ucr.edu/home/baez/physics/Relativity/SR/rigid_disk.html)