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Conversion of units

  1. Apr 15, 2005 #1
    Hello. I am making a physics calculator, and I have to make a conversion Unit for the program.

    I need to know this (as an example)

    I need to convert Kilometers per hour to Meters per second. But no the usual way. I know that the conversion factor is 0.278 but I need to convert the "distance units" and the "time units" separately. Originally I though that I can do this.

    mps = kmph *1000 / 60

    I simply multiply by 1000 to convert the KM to M, and then I divide by 60 to convert the Hours to seconds. This was way off and I have no idea how to do this. :cry:

    Any ideas of how to convert these units without using the conversion factor of .278?
  2. jcsd
  3. Apr 15, 2005 #2
    1 hour is not 60 seconds, it's 60*60 seconds...
  4. Apr 15, 2005 #3
    Yep, im aware of that what is why I said

    But 1000 * 60 doesnt work
  5. Apr 15, 2005 #4
    If you were aware of the exact problem in your formula, what was the point of posting?

    Convert from km/h to m/s by multiplying by 1000/(60*60).
  6. Apr 15, 2005 #5
    Ah, I get it now. I have to convert it in this way for my conversion module (unit, class whatever).

    Thx a lot Muzza!
  7. Apr 16, 2005 #6
    Hmn, I seem to be having even more difficulties. Can someone please help me convert this unit of accleration in the same fashon as above. btw I want everything to be converted to scientific units

    10 kilometer per hour per minute = 0.0462962963 m / s / s

    I tried this
    10 / 1000 / 3600 / 60
    But that was way off.
  8. Apr 16, 2005 #7


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    [tex] 10 \mbox{Km}\cdot \frac{1000\mbox{m}}{1\mbox{Km}}\cdot\frac{1}{3600\mbox{s}}\cdot\frac{1}{60\mbox{s}} [/tex]

  9. Apr 16, 2005 #8
    hehe, I just figured it out.

    10 (1000 / 3600 / 60)

    hmn, the conversion procedures are going to be very easy to make. per = divsion

    Or I can use dextercioby's method of multiplying by the multiplicative inverse. It all works out the same way :smile:

    Thx everyone!
    Last edited: Apr 16, 2005
  10. Apr 17, 2005 #9
    For the future, just to make it clearer to you, write mps as ms-1. This could also be written as [tex]\frac{m}{s}[/tex] and here is where my point is.

    You need the metres to be kilometres. This is simple, divide by 1000 or multiply by the reciprocal of 1000. The seconds is just as simple to convert to hours but the conversion factor must be on the denominator because the unit says that the seconds are s-1 which is [tex]\frac{1}{s}[/tex], e.g.

    [tex]\frac{m}{s}[/tex] is converted to kmh-1 (kilometres per hour) by

    [tex]\frac{m \times \frac{1}{1000}}{s \ \times \frac{1}{3600}} = \frac{\frac{m}{1000}}{\frac{s}{3600}} = \frac{m}{1000} \div \frac{s}{3600} = \frac{m}{1000} \times \frac{3600}{s}[/tex]

    The m and the s are represented by the number 1 (as 1 second divided by 3600 will give you 2.78x10-4 hours or 1 metre divided by 1000 will give 1x10-3 kilometres. Both, we know, are right). Now add the factor x, which is the value you started with (for this example left say it is 50).

    [tex]x \times \frac{1}{1000} \times \frac{3600}{1} = x \times \frac{3600}{1000} = \frac{3600x}{1000}[/tex]

    Apply 50 as the x value and we now know that 50 metres per second (ms-1) is equal to 180 kilometres per hour (kmh-1).

    This can be done for all conversions. All you need to know is the units (in the correct form, shown above several times) and the conversion ratios. The rest is simple.

    Another example could be convert kilometres per hour into feet per second. I don't know why but lets do it.

    Kilometres to metres = 1km = 1 x 1000 = 1000m
    Metres to feet = 30.48cm to a foot (converted by knowing 2.54cm is equal to an inch). Therefore, 0.3048m is one foot.

    By doing more calculations you will find that 1 kilometre is equal to

    [tex]3280\frac{320}{381} \ feet[/tex].

    This means that the conversion factor is 1 kilometre =

    [tex]1 \times 3280\frac{320}{381} = 3280\frac{320}{381}[/tex].

    The hours can be converted to seconds by multiplying by 3600.

    Now we can find out the conversion factors.

    [tex]kmh^{-1} = \frac{km}{h} = \frac{km \times 3280\frac{320}{381}}{h \times 3600} = \frac{3280\frac{320}{381}}{3600} = \frac{3125}{3429}[/tex]

    That is the number you would need to multiply by to convert kilometres per second into feet per second. So 50 kilometres per hour is about 46 feet per second.

    The Bob (2004 ©)
    Last edited: Apr 17, 2005
  11. Apr 17, 2005 #10
    Hmn, I see what your saying. But I will impliment both meters per second and ms^-1 and m/s. I guess it would be more proper to use ms^-1 tho. I understand most of the examples you gave me, but I do tent to get lost in numbers and too much math. I have just one more question tho.

    I want to convert
    0.0462962963 meters per second per second to kilometers per hour per minute
    The result should be 10

    hmn, it seems that my conversion procedures are going to be slighly better than google's :D


    btw I have one more question. In my software I had my procedure convery 30 kilo yards per hour per gram
    into SI units, and the result was
    7620 Meters Per Second Per Kilogram

    Please tell me that this is the right anwer, I hope I don't have to spend the next few hours debuging the code. And I simply don't trust google when it says http://www.google.com/search?hl=en&...q=30+kilo+yards+per+hour+per+gram&btnG=Search
    Last edited: Apr 17, 2005
  12. Apr 17, 2005 #11


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    It's not logical to use SI prefixes with English units.There's no such thing as kiloyards...

  13. Apr 17, 2005 #12
    1 Kiloyard = 1000 Yards. I know its not logical, I am just "debugging" my code. So lets just assume a Kiloyard exists :approve:

    I think that this is right
    30 * (1000 * 0.9144 / 3600 * 1000)
  14. Apr 17, 2005 #13


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    It should be

    [tex] 30,000 \ \mbox{yd}\cdot\frac{0.914 \ \mbox{m}}{1 \ \mbox{yd}}\cdot\frac{1}{1 \ \mbox{hr}}\cdot\frac{1}{\frac{3600 \ \mbox{s}}{1 \ \mbox{hr}}} \cdot \frac{1}{1g}\cdot \frac{1}{\frac{1 \ \mbox{Kg}}{1000 \ \mbox{g}}} [/tex]

  15. Apr 17, 2005 #14
    Do you think you can give me a simpler ASCII translation? Hmn, I have been working with these conversion numbers for hours and I just can't get it yet :(

    I have made to procedures to convert "to SI units" but now I have to convert "from SI units"
  16. Apr 17, 2005 #15


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    It's the other way around.Simply inver the fractions altogether (the numbers and the units).

    Give an example.

  17. Apr 19, 2005 #16
    Using them for your convertion table is fine but you need to remember that what I have said will allow you to change the units easily. Using m/s does not show what I have explained and it really should be ms-2 to be accurate.

    It is 10.

    The Bob (2004 ©)
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