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Homework Help: Conversion of km/h to m/s

  1. Nov 12, 2006 #1
    how do u convert the following
    MJ to J
    km/h to m/s
    25 kN to ? (i dont know what)
  2. jcsd
  3. Nov 12, 2006 #2
    I usually use fractions. If I wanted to convert 10 ft/s to inches/min, for example, I would do:

    10 ft/s * 12 inches/1 ft * 1 min/60 s = 2 inches/s

    Since there are 12 inches in a foot, and 60 seconds in a minute, this is just multiplying by 1, in a creative way.

    Just use this idea, and you will be converting units in no time at all.

  4. Nov 12, 2006 #3


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    Surely you have some idea as to how to do this.

    Am I correct in reading the first to be converting MegaJoules to Joules.
    You should be able to figure that out with a bit of thought.

    Lets go through the conversion of [itex] \frac {km} h [/itex] to [itex] \frac m s [/itex]

    There are 1000 m per km and 3600 s per h so

    [tex] \frac {km} h \times \frac {1000m} {1km} \times \frac {1h} {3600s} [/tex]

    examine the units in this expression, note that km appears in both the numerator and denominator, so can be canceled, leaving only m. The same is true for h. Do the arithmetic.
    Last edited: Nov 12, 2006
  5. Nov 12, 2006 #4

    i ve tried all i know is a MJ a is a like 1000J a second but it doesnt work out for my answers
  6. Nov 12, 2006 #5
    1000 J/s is a Kilo Watt. Mega is not 1000, and there is no second in kilo or in your problem.

    Look in your physics book for the definitions of metric prefixes.

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