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Interesting question from AP student

by Drew boxer
Tags: interesting, student
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Drew boxer
#1
Apr25-12, 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,
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Khashishi
#2
Apr25-12, 02:15 PM
P: 887
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.
ZxcvbnM2000
#3
Apr25-12, 02:48 PM
P: 64
Quote Quote by Khashishi View Post
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.
So if what you say is true then the bar would be a couple of inches shorter for a little while ? I fail to understand how that can happen :S

Khashishi
#4
Apr25-12, 03:06 PM
P: 887
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 gently--the compression wave is just much smaller in amplitude.
Travis_King
#5
Apr25-12, 03:37 PM
P: 841
Not to mention the inertia of a 2 lightyear long steel bar!


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