SW_Zwom t1_j2sdh2u wrote on January 3, 2023 at 4:59 PM

Reply to comment by Weed_O_Whirler in If an object is placed at the end of of a VERY long bar that is connected to an axel, Could rotating the bar close to the axel make the object at the other move extremely fast (like light speed)? by Swade060504

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...

die_kuestenwache t1_j2t0rp6 wrote on January 3, 2023 at 7:23 PM

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.