Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Rigid body movement

  1. Oct 4, 2011 #1
    Assume a stick
    of length 15*3*10^8 metre.
    One end points towards
    me, at the other end we
    place a ball very near to
    the end of the stick. If i
    push the end pointing
    towards me with some
    force, instantaneously the
    ball will be moved(~). At
    the same time, a light
    beam emanating
    alongside me would take
    15 seconds to reach that
    ball. How can i change the
    information of a space
    even before the light beam
    reaches out to that place
    (~)? This question appears
    to be childish, pardon my
    innocence, explain me
    please. I've been baffled
    by this question since my
    childhood. I know this
    question is a flaw or
    immature way of
    understanding a concept,
    please find out the
    misconception I've been
    thinking of.

    P.S: I'm NOT gonna
    SHAKE, or BANG, or MAKE
    VIBRATE the stick. This situation is slightly analogous to newton's gravity model which contradicts with light's speed limit later overturned by einstein's general theory of relativity, but not completely i think.

  2. jcsd
  3. Oct 4, 2011 #2

    Doc Al

    User Avatar

    Staff: Mentor

    When you push one end of the 'rigid' body, it takes time for the other end to move. It's not instantaneous.
  4. Oct 4, 2011 #3
    That means the ball will move 15 seconds later after you pushed the rod? The force itself is also travelling at the speed of light?
  5. Oct 4, 2011 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    No, at the speed of sound. DaleSpam posted a nice FAQ reply to this question yesterday.

    https://www.physicsforums.com/showthread.php?t=536289 [Broken]

    If you hit the surface closest to you so hard that it's forced to move faster than the speed of sound in the material, it will probably break. It will also create a disturbance that propagates faster than the speed of sound for a little while (but obviously never faster than the object that hit it), and eventually turns into a plain old sound wave when it has lost enough energy.
    Last edited by a moderator: May 5, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook