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Relative Motion

  1. Sep 15, 2009 #1
    Before I start studying the relativity theories, I have to grasp this concept:
    Suppose two cars are moving with constant velocities in any direction and a ball is thrown straight up and comes back straight down. This will happen in both cars.
    But what if a person in one car observes the experiment done in the other car, will he observe the same?
    ALSO plz explain this in terms of frame of references.
  2. jcsd
  3. Sep 18, 2009 #2
    Someone observing the ball from outside the car - i.e. in a different frame of refernce - will indeed see the ball go straight up and down **with respect to the car** (this is the key bit - they won't see the ball land in the back seat or something daft... they will see it land in the same place as an observer inside the car will see it)

    Where it gets interesting is when you try it with a beam of light - so instead of a ball, you send a beam up from the bottom of the car to a mirror at the top of the car and back down again. So far so good...

    Outside the car you will still see the beam go straight up and down **with respect to the car**, but with respect to you, the car has moved forward in the interviening time and so you see the beam of light travel further than an observer in the car would see it (inside the car: straight up and down... from outside the car: up, down and along a bit).

    So... two observers in different frames of reference, see the same beam of light travel different distances. Not a problem until the observers measure the speed of light. Both of them will come up with the same speed.

    Same speed, same light beam, different distance travelled. Something has gone wierd with time. And space :-)
    Welcome to the highly entertaining world of relativity!

    Last edited: Sep 18, 2009
  4. Sep 18, 2009 #3
    Also, if you are about to start studying relativity, please keep in mind something that I've found isn't made very clear in relativity textbooks and is a very common misconception among new students:

    The effects of relativity are not simply a result of light "lag". Although the finite speed of light does result in interesting observed effects, the real study of relativity has to do space and time itself being experienced in a relative way. In fact, even after having corrected measurements of observations for light lag, two observers can still disagree on the simultaneity of two events.
  5. Sep 18, 2009 #4
    Good point! The effects of lag are interesting in their own right, but the more you get into the subject, the more fascinating - not to mention mind bending - it becomes.

    Have you ever wanted to store a 10 metre pole in a 5 meter shed? Well travel fast enough and you can do. For a fraction of a second anyway:-)
  6. Sep 18, 2009 #5
    What is light "lag"?
  7. Sep 19, 2009 #6
    Me being lazy.... I meant the difference in observation caused by two observers being different distance from an event :-)

    Many reletavistic effects can be explained by this, but many others have to be explained through changes to spacetime.
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