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Turning on a flashlight at 98 of c

  1. Mar 21, 2013 #1
    Hi, this is my first post so if i made any noob mistakes have at me

    I can best formulate my question in the form of this
    If I'm driving a car at 100kph, and someone the kleenex box form the back seat at 30kph, then it moves at 30kph relative to me but in actuality is moving at something like 130kph

    now if i was in a vehicle that travel at 98% of c, and I turned on a flashlight then would the only be traveling at 2% faster then me? is my understanding correct, and what kind of effects would I see/feel
  2. jcsd
  3. Mar 21, 2013 #2


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    Hi there
    welcome to PF :)

    its only moving at 130kph relative to someone at rest and not in the car
    to everyone in the car it is moving forward at 30kph

    no, cuz in your timeframe you are at rest and the light from the flashlight is still travelling away from you at its normal velocity, c.
    Its only when some one else, in their time frame, views you and your space ship do they see you travelling at 98% c. From your point if view, you are stationary and everything else is moving past you at 98% c.
    And even then, the beam of light from the flashlight is still moving at only c, NOT c + your velocity.
    ( thats for reasons tied up in relativity, that I'm not well versed on ;) )

    hopefully that made sense :)

    Last edited: Mar 21, 2013
  4. Mar 21, 2013 #3
    thanks that did make some sense but I want to clarify something.
    I was under the impression even in I'm at rest in the car, for purposes of total energy Im moving at 100 kph. thus
    Now as I understand that Kleenex box has to travel at 130 relative to a person at rest in order to appear to travel at 30kph compared to me?
  5. Mar 21, 2013 #4
    You are correct. At these speeds you can simply add and subtract the velocities. However, if you apply relativity, you will find that the the cleenex box is actually moving just a bit off of but not exactly 130 kph. Probably too little to even measure.

    As you get closer to the speed of light though, it is quite measurable and quite pronounced, such that if you're travelling at 98% c and throw a kleenex box forward at 98% c, an outside observer will measure the kleenex box as moving only perhaps 99.5% c (I don't know the exact number...too much of a layman). But never c or never more than c.

    Now, obviously if the velocities don't simply add up, something else has to give. That something else is time. The clock actually ticks slower for you, travelling at 98% c than it does for the outside observer.

    This also means that c is always c for everybody, whether they are on earth or on a almost-at-the-speed-of-light spaceship. The differences will be in how fast their clocks tick. For an actual light beam, the clock stops.
  6. Mar 21, 2013 #5


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    Look up relativistic "velocity addition":

    http://hyperphysics.phy-astr.gsu.edu/%E2%80%8Chbase/relativ/einvel.html [Broken]
    Last edited by a moderator: May 6, 2017
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