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Mybe stupid but i would like to know (vehicles in relative motion)

  1. Dec 14, 2014 #1
    So we have moving truck at 100mph, on truck is truck moving 100mph, then on this truck is truck moving 100mph, and on this truck is car moving 100mph.
    Then the last car is moving 400mph in space but only 300 from first truck?
     
    Last edited: Dec 14, 2014
  2. jcsd
  3. Dec 14, 2014 #2

    Danger

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    The car is. The last truck is moving 300 in space and 200 from the first.
    That, of course, is assuming that they are all moving in the same direction.
     
  4. Dec 14, 2014 #3

    yah i meant the CAR :P sry
     
  5. Dec 15, 2014 #4

    CWatters

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    The second truck is moving 100mph relative to what?

    Don't forget the first truck is on a planet that's orbiting the sun at around 67,000 miles an hour and the solar system is orbiting the galaxy at about 514,000 miles an hour. How fast is that car actually going?
     
  6. Dec 15, 2014 #5
    Oh i missed that, i meant relative to our mother earth.
     
  7. Dec 15, 2014 #6

    phinds

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    Yes, but not "in space", rather "relative to the ground". ALL motion is relative and when you say that something is "moving" it doesn't mean anything unless you say RELATIVE TO WHAT.

    We normally leave that out in everyday life because the context normally makes it clear what you are referring to. For example, when you said the first truck was moving at 100mph, that is technically a meaningless statement because you did not say relative to what, but we all know that you meant "relative to the highway".

    In physics we don't make those assumptions because it can lead to confusion.
     
  8. Dec 15, 2014 #7

    Danger

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    I did, in assuming that ajlen meant Earth when s/he said space. :redface:
     
  9. Dec 15, 2014 #8
    +1 to that :)
    Wont miss out that anymore: "relative to"
     
  10. Dec 15, 2014 #9

    CWatters

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    I don't think you did :)

    If they are all going 100mph relative to the earth then the final car is also going 100mph relative to the earth (and 0mph relative to all the trucks).

    If they are all going 100mph relative to the one they are on then the final car is going 400mph relative to the earth (and 300mph relative to the first truck).
     
  11. Dec 15, 2014 #10
    basically i said, truck moving 100mph, and truck on him moving 100mph obviously relative to truck below...etc.
    so the car is 400mph to earth
     
  12. Dec 15, 2014 #11

    Danger

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    Hang on here! I just re-read the original post. It doesn't matter to what the first truck is relative. Every other element of the question is referenced to that same "unknown", via the preceding element, regardless of what it is.
     
  13. Dec 15, 2014 #12

    phinds

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    I think you need to read it again and maybe think about it. Did you perhaps overlook the statement "the last car is moving 400mph in space"
     
  14. Dec 15, 2014 #13

    Danger

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    I didn't overlook it; it just seems extraneous. Maybe this illustrates the difference between competing viewpoints which to me (and I could very well be wrong) are equally valid. From my perspective, the wording of the question demonstrates a clear, logical chain of events (cause and effect) that demands the acceptance of the same reference frame for the first truck and the car. If someone asks for the muzzle velocity (properly muzzle speed) of a bullet, it's pretty much guaranteed that he means relative to the gun, not relative to someone on a boat travelling opposite to the train that the gun is on.
     
  15. Dec 15, 2014 #14
    Relative to what ?
     
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