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Determine the value of pressure in pascals

  1. Jan 6, 2009 #1
    1. The problem statement, all variables and given/known data
    I don't understand how to convert from lb/in^2 to Pa (Pascal)=1 N/m^2
    Question: A pressure transducer measures 300 lb/in^2. Determine the value of pressure in pascals.
    2. Relevant equations



    3. The attempt at a solution
    I know that you can just input it into google, but could somebody show me step by step on the conversion.

    I tried: 300lb/in^2 * in^2/??
     
  2. jcsd
  3. Jan 6, 2009 #2

    CompuChip

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    Re: Conversion

    First, let's think about the problem a bit. Obviously, an inch is a measurement of length, as is a meter. What does a pound (lb) measure? What about Newton (N)?

    Also, you can do an intermediate step already: how many meters is one inch? Then how many meters squared is one inch squared? Suppose now that you have 300 lb on one inch^2, how many lb (!) are there on one meter^2? (Do you need to multiply or divide? :))
     
  4. Jan 6, 2009 #3
    Re: Conversion

    We can convert pounds weight into kilograms weight and convert that into Newtons.

    We can convert an inch into a fraction of a metre and by squaring it convert it into square metres.

    Then dividing the value in Newtons by the value in square metres we get our result in Pascals.

    Being able to convert units is one of the basic skills. For example, you could be asked by some wicked teacher to express a momentum of a thermal neutron is mile-tons per fortnight.
     
  5. Jan 6, 2009 #4
    Re: Conversion

    i got to here and now stuck:
    300 lb/in^2 x .453 kg/lb = 135.9 kg/in^2

    135.9 kg/in^2 x in^2/.000645m^2 = 210697.6 kg/m^2

    where do I go from here. I know N=kg*m/s^2. but where do the seconds come into play?
     
  6. Jan 6, 2009 #5
    Re: Conversion

    First off, don't forget it's a pressure transducer. It's not measuring mass per unit area in lb/in2 but force per unit area in lb/in2.

    One pound of force is equal to the force experienced by 1 lb due to gravity. You've converted from pounds to kilograms, but what force is experienced by that mass in kilograms due to gravity?

    Knowing the most common interpretation of Newton's second law, relating the force acting upon an object and its mass, and finding a suitable constant of acceleration due to gravity (on Earth), you should then not only get the correct answer but see where your 'seconds come into play'.
     
    Last edited: Jan 6, 2009
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