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Homework Help: SI unit conversion HELP

  1. Sep 6, 2012 #1
    1. The problem statement, all variables and given/known data
    This is the question
    Still think that unit conversion isn't important?
    Here is a widely publicized, true story about how failing to convert units resulted in a huge loss. In 1998, the Mars Climate Orbiter probe crashed into the surface of Mars, instead of entering orbit. The resulting inquiry revealed that NASA navigators had been making minor course corrections in SI units, whereas the software written by the probe's makers implicitly used British units. In the United States, most scientists use SI units, whereas most engineers use the British, or Imperial, system of units. (Interestingly, British units are not used in Britain.) For these two groups to be able to communicate to one another, unit conversions are necessary.

    The unit of force in the SI system is the newton (N), which is defined in terms of basic SI units as 1N=1kg*m/s^2. The unit of force in the British system is the pound (lb), which is defined in terms of the slug (British unit of mass), foot (ft), and second (s) as 1lb=1 slug ft/s^2.
    Part B
    Find the value of 15.0 {N} in pounds. Use the conversions 1 slug=14.59kg and 1 ft}=0.3048m}.
    Express your answer in pounds to three significant figures.
    I figured out 1.028 slugs=kg. but thats about it
  2. jcsd
  3. Sep 6, 2012 #2
    It's impossible to resolve a difficulty if you don't mention what your difficulty is.

    Two things to know for unit conversion though:

    1). Units cancel just like variables of an algebraic expression. For example if I multiplied x/y and y together the lone y and the y on bottom of the fraction would cancel to 1 and I would be left with 1x or simply x.

    2). You can multiply anything by 1 without changing it. So if I said x=y I could then multiply anything by x/y without changing its value because x/y would simply = 1. Notice because x = y it doesn't matter which is on top. I can multiply by x/y or y/x because both fractions are simply 1.

    These are the ideas behind any unit conversion you do. As I said above, if you show an attempted answer, then people can help you with a specific difficulty you're having.
  4. Sep 6, 2012 #3
    I eliminated the kg and change it into slugs, so now i have 1.028*.3048ft/s^2. I don't know what to do with the s^2 now? I change the equation from 15N=15kg* m/s^2. I don't know what to do with the ft
  5. Sep 6, 2012 #4
    Hmm, are you sure you converted it into slugs? You're on the right track, but in your answer you give the units ft/s^2 but I don't see slugs?

    Q1: What are the desired units for your final answer? You're going from kg*m / s^2 to _____ ?

    Q2: Can you write a fraction in the form (something) / (something) that equals 1 from the information given in the problem?
  6. Sep 6, 2012 #5
    Yes, I got the slugs correct I know for a fact, I need to find lb only.
    1lb=1 slug ft/s^2
  7. Sep 6, 2012 #6
    I guess I'm trying to see the process you used. Sometimes it's possible to use an intuitive method and happen upon the right answer. That's great, but if you're not sure why it worked, or not sure how to use it if given a different problem then the exercise loses its purpose even if you did get the right answer.

    I was wanting you to think of it in terms of fractions and multiplying them, but if you're doing it a different way that's fine.

    As for lb, it tells you pounds is slug*ft / s^2 So as long as your units are that, you're done.
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