1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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.

  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!
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Interesting question from AP student
  1. Interesting Question (Replies: 28)