
#1
Apr2512, 02:03 PM

P: 1

I'm a physics teacher and one of my AP students posed the following question: "if there was a steel bar 2 light years long and you moved it forward a couple inches, would the other end move forward a couple inches immediately or would there be a delay?". We'd been discussing electric fields, so that got him thinking. I thought it was interesting and wondered how others would have answered.
Thanks, 



#2
Apr2512, 02:15 PM

P: 832

There would be a delay. The movement would propagate down the bar at the speed of sound in the bar, which is [itex]\sqrt{Y/\rho}[/itex], square root of Young's modulus over density.




#3
Apr2512, 02:48 PM

P: 64





#4
Apr2512, 03:06 PM

P: 832

Interesting question from AP student
It's not surprising that the length of the bar will change. If you strike a bar, it will compress a little and send a compression wave down the bar. It's no different if you move it gentlythe compression wave is just much smaller in amplitude.




#5
Apr2512, 03:37 PM

P: 763

Not to mention the inertia of a 2 lightyear long steel bar!



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