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How fast am i going?

  1. Oct 10, 2013 #1
    My wife and I have had a debate going for some time now.

    I say that I am traveling at 77 mph and she says I am traveling at 65 mph. Who is correct?

    Thank you for your time.
  2. jcsd
  3. Oct 10, 2013 #2


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    You are correct ... until you bump into the front of the bus.

    You can verify this by noting that a car driving 77 mph (your wife can be the driver) passing the bus will be appear stationary for you.

    This is Galileo's relativity in action.
  4. Oct 10, 2013 #3

    Doc Al

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    How fast you are traveling only makes sense if you specify how fast relative to what. Relative to the road, you'd be traveling at 77 mph. Relative to the bus, you'd be traveling at 12 mph. Not sure what your wife had in mind with 65 mph. (Relative to someone running alongside the bus at 12 mph, you'd be traveling at 65 mph.)

    Short answer: You are correct.
  5. Oct 10, 2013 #4
    Thank you for the fast response(s).
    Last edited: Oct 10, 2013
  6. Oct 10, 2013 #5


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    Man I hope we didn't ruin the atmosphere at home for you :-)
  7. Oct 10, 2013 #6
    Neither, though you are much closer then she is. Your true speed is about 8.46 billionths of an inch less then 77 miles per hour, due to relativistic effects.
  8. Oct 10, 2013 #7


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    Nitpicker :smile:
  9. Oct 11, 2013 #8


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    If we are nitpicking, how much is 77 miles per hour - 8.46 . 10-9 inch? Can I eat that?
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