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Problem does not make sense km/h at m/s?

  1. Mar 2, 2014 #1
    problem does not make sense km/h at m/s??

    1. A van is traveling at 80 km/h at 25 m/s undergoes uniform deceleration when the brakes are applied, slowing to 50 km/h at 10 m/s after 2.1 s. (a) what is the deceleration? (b) How far does it travel during this period? (c) How much farther does it travel before stopping if deceleration remains constant?

    Variables= i dont understand what is meant by 80km/h at 25m/s and 50km/h at 10m/s
    vi=25m/s? vf=10m/s? t=2.1s 1)find=-a? 2)displacement=? 3)at vf=0 displacement?

    2. Relevant equations
    1)i used d=(vi+v2)/2 x t
    2)vf=vi+at to find acceleration
    3)know how to compute but didnt because im unsure of my answer what to do with 80km/h what does this mean??? please help

    3. The attempt at a solution
  2. jcsd
  3. Mar 2, 2014 #2


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    Gold Member

    Welcome to PF :smile:
    Is that really the question?Name of the text book?
  4. Mar 2, 2014 #3

    yes thats the questions its a homework problem from a document i dont understand what he means by km/h at m/s ?
  5. Mar 2, 2014 #4


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    IMO, it looks like a rather clumsy attempt at converting 80 km/h to the equivalent velocity of 25 m/s, etc..
  6. Mar 2, 2014 #5
    yes, thats what i initially thought, i was thinking that at an initial velocity of 85km/h it changes 25m/s but that does not even make sense because how can velocity change a velocity wouldnt it have to be aceleration
  7. Mar 2, 2014 #6


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    I think you should use either km/h or m/s in that problem.
  8. Mar 2, 2014 #7
    ya thats what i wind up doing thanks man
  9. Mar 2, 2014 #8


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    It's weird that 80 and 50 kph convert to 22 and 14 m/s, which is off from the m/s values given in the problem. Or, 25 and 10 m/s would convert to 90 and 36 kph. So we would expect two different answers, depending on whether the kph or the m/s values are used -- unless the distances turn out to be the same by sheer coincidence.

    As SteamKing said, it's a clumsy attempt at conversion, if that was the the intent.
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