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Interesting question from AP student

  1. Apr 25, 2012 #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. jcsd
  3. Apr 25, 2012 #2
    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.
     
  4. Apr 25, 2012 #3
    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
     
  5. Apr 25, 2012 #4
    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.
     
  6. Apr 25, 2012 #5
    Not to mention the inertia of a 2 lightyear long steel bar!
     
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