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Homework Help: I'm having a lot of trouble converting units

  1. Jan 24, 2012 #1
    For some reason I can't seem to be able to convert units without getting all mixed up. I can convert feet to inches to cm fine, but I can't convert units in a more complicated problem; for example, converting ft/sec to km/hour. I just get really mixed up and lose track of the numbers.

    ] I looked at multiple videos on youtube and I get it, but when I look at a problem its like gibberish to me. I know this is really easy math (I mean, I'm in calculus now so it shouldn't even be a problem, multiplying fractions) but I just can't do it. It's like I'm only dyslexic whenever I look at a unit converting problem...anyone have any advice for me?
  2. jcsd
  3. Jan 24, 2012 #2


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    Hi Alexander,

    Transforming units is not at all easy, many people have problems with it. I try to explain it on the way it is usual in my place.

    Imagine a meter rod, with cm scaling. The length of 1 m is 100 times the length of 1 cm: 1 m is the same as 100 cm, is not it? Whenever you see "m" replace it with "100 cm".

    1.243 m = 1.243 (100 cm) = (1.243*100) cm = 124.3 cm.

    At the same time 1 cm is one hundredth of 1 m. 1 cm = 0.01 m. If you need to convert cm-s to m, replace cm by (0.01 m).

    235 cm = 235 (0.01 m) = (235*0.01) m=2.35 m.

    One hour is 3600 seconds. You convert hours to seconds by replacing "hour" by "3600 s" So 0.5 hours = 0.5 (3600 s) = (0.5*3600) s =1800 s.

    Velocity is length over time. You have velocity in km/hours and want it in m/s- units. Say v=90 km/hour

    1 km =1000 m, 1 hour = 3600 s.

    90 km/hour=90 (1000 m) /(3600 s) = [itex]90\frac{1000 m}{3600 s}= \frac{90}{3.6}\frac{m}{s}=25 \frac{m}{s}[/itex]

    When transforming backwards, use that 1 m = 0.001 km and 1 s = (1/3600) hour.

    20 m/s = [itex]20 \frac{0.001 km}{\frac{1}{3600}hour} =20*\frac{0.001*3600 km}{hour}=20*3.6 \frac {km}{hour}[/itex]=72 km/hour.

    Now foot. 1 foot = 304.8 mm. You want to convert 10 ft/sec to km/h. (The standard abbreviation of seconds is "s"in the SI system of units)
    We do it in two steps, first transforming ft/sec to m/s

    You know that 1 m =1000 mm, so 1 mm = 0.001 m. 1 ft = 304.8 mm =304.8* (0.001 m)= (304.8*0.001)m = 0.3048 m. So 10 ft/sec =3.048 m/s. That you can transform to km/hours by replacing "m" with 0.001 km and "s" with (1/3600) hour.

    10 ft/sec = 3.048 m/s = 3.048[itex]\frac{0.001 km}{\frac{1}{3600}m}[/itex]=3.048 *3.6 km/hour=10.97 km/hour.

  4. Jan 24, 2012 #3
  5. Feb 2, 2012 #4
    Thanks ehild! I just went over your examples again and again and I think I can do it now! Your way of thinking was different from what i was trying and it helped.

    @jobrag- I need to know how to convert without a calculator, but thanks anyway!
  6. Feb 2, 2012 #5


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    I am pleased to know that you :cool: understood unit conversion. It is not at all simple!

  7. Feb 2, 2012 #6
    just remember this... always put the unit you are trying to convert in such a way that it will be cancelled out and the unit left will be the one you are looking for..

    for example.

    we know that 1inch = 2.54cm

    if you want to convert 41.5 cm - inch

    41.5 cm x [itex]\frac{1 inch}{2.54 cm}[/itex] = [itex]\frac{41.5}{2.54}[/itex] inch

    = 16.34 inches.

    as you can see... i put cm at the denominator of the conversion factor so it can be cancelled and the unit left is inch.

    now for another example

    m/s - km/hr

    1km = 1000m

    1 hr = 3600 s

    if you want to convert

    5 [itex]\frac{m}{s}[/itex] - km/hr

    5 [itex]\frac{m}{s}[/itex] x [itex]\frac{1km}{1000m}[/itex] x [itex]\frac{3600 s}{1hr}[/itex]

    [itex]\frac{5 (3600)}{1000}[/itex] [itex]\frac{km}{hr}[/itex]

    = 18 [itex]\frac{km}{hr}[/itex]

    as you can see.. i converted the units one unit at a time.. i deal with meter first... I converted it to km. then after that i converted sec to hr... as you can see also, i put the units in the conversion factor in such a way that you can cancel out the unit you want to convert..

    ==I hope this helps. == :))
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