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

What would happen?

  1. Jan 5, 2005 #1
    What would happen???

    In the following hypothetical situation...

    If Bob was in a space ship that was moving 0.9c and bob (who can accelerate very quickly and not feel the effects...) desides to move in the opposite direction at 0.7c. From an observer who is relativly stationary what would bob look like (ie lenght contradiction). Also how would Bob's time be diffrent from the ships and the observer?

    Derek Mohammed
  2. jcsd
  3. Jan 5, 2005 #2
    Hello Derek,
    You must give the reference frames or the questions are meaningless.... 0.9c wrt who?.... 0.7c wrt who? ...stationary wrt who?????
  4. Jan 5, 2005 #3


    User Avatar

    If the spaceship is moving at .9c then the ship would have a strong accelerated force pushing down on bob, making it very hard for him to move in the oposite direction. It's like when you jump up here on earth, gravity forces you down.

    Oops, I should have read the question better, he doesnt feel the effects. Sorry.
    Last edited: Jan 5, 2005
  5. Jan 5, 2005 #4
    I can tell you the relavistic effects of traveling at .9c and .7c, but I don't get the logic behind your question.
  6. Jan 5, 2005 #5


    User Avatar
    Staff Emeritus
    Science Advisor

    " From an observer who is relativly stationary"

    Relative to whom? Bob or bob?
  7. Jan 5, 2005 #6


    User Avatar

    haha, bob isnt the same person?
  8. Jan 6, 2005 #7
    OK I am soooo sorry. Bob and bob are the same person! Sorry.

    Well What I mean is: When the ship is travelling at .9c bob is going to experiance some relavalistic effects, right? So if he is already experiancing those effects what will happen if he moves in the opposite direction at 0.7c. Will the effects negate each other and bob will only experiance 0.2c relavalistic effects.

    (when I say relavalistic effects I mean time dialation and length contradiction relative a stationary observer. ie the diffrence in time from soemone watching)

    I hope that helps... Again I apologize...
  9. Jan 6, 2005 #8


    User Avatar
    Staff Emeritus
    Science Advisor

    Assuming that Bob is moving at 0.9c relative to the observer, then accelerates to 0.7c in the opposite direction, again relative to the same observer, the observer will see Bob experiencing the relativistic effects of the 0.7c speed. Relativistic effects depend only on the instantaneous relative velocity, not how you get to that velocity.
  10. Jan 6, 2005 #9
    So would the relavalisitc effects be compounded? (ie moving at 0.9c and then moving at 0.7c???)
  11. Jan 7, 2005 #10
    No, HallsofIvy has said it, it will be contracted according to the velocity that has in that instant measured by the stationary observer. E.g, if the stationary observer measures a velocity of 0.7 c, that is the quantity that enters in the Lorentz factor

    There's a formula for composing velocities, but is for another different situation; is when an observer A measures the velocity of observer B, and at the same time the velocity of A is being measured by and observer C
  12. Jan 7, 2005 #11


    User Avatar
    Science Advisor

    Are you saying bob "moves in the opposite direction at 0.7c" relative to an observer on board the ship, or relative to an observer on earth who sees the ship moving at 0.9c? If the first, then the observer on earth will see bob moving at 0.2c; if the second, then he'll see bob moving at 0.7c, by definition. Either way, the amoung of length contraction and time dilation he'll see in bob will be based on how fast bob is moving in his frame. But remember, length contraction and time dilation are relative, not absolute--an observer on the ship would see a different amount of length contraction and time dilation in bob, and bob himself would see no length contraction and time dilation in himself (but he would see the other two observers slowing down and contracting).
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook