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Weed_O_Whirler t1_j2sb2h7 wrote

Everything you're saying holds up, except for this one thing:

> a beam that cannot flex and is incredibly long

In physics, we often times talk about "rigid bodies" and make the assumptions that they are infinitely stiff, but that's just a "small thing, moving slow" assumption- where they appear to be completely rigid. In real life, materials are not completely rigid.

Now, you might think "that's just an engineering problem, we can just design something to be really stiff. But you can't. Information- including compressions and rotations, travel through a material at that material's speed of sound. Which makes sense if you think about it, sound waves are compressions passing through a material.

So, if you had a really long rod, and you started rotating it faster and faster, the rod would start to bend and then shear. And that's not an engineering problem- it's a real physics one.

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SW_Zwom t1_j2sdh2u wrote

I was actually asking myself this: If the rod did not bend at all - could we even move it? I mean without any pressure wave travelling through the material it could not accelerate. So wouldn't perfect rigidity require an infinite speed of sound inside the material? That would make it impossible, even in a "ideal" calculation...

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die_kuestenwache t1_j2t0rp6 wrote

In principle, yes. Infinite rigidity would mean infinite speed of sound. As soon as the velocity of any part of the rod approaches the speed of light, the torque necessry to further accelerate the rod would go towards infinity. However, as others have pointed out, infinite rigidity violates relativity as the speed of sound itself cannot exceed the speed of light.

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solenyaPDX t1_j2t00wt wrote

Even if we assume a non flexing support rod, you'd need to apply torque to the axle to move the object. The energy required to move the object wouldn't change, so you'd be looking at infinite torque necessary to turn the axle.

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