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Homework Help: Pressure 101 - haven'

  1. Jun 11, 2010 #1
    Ok, so I was at lowes and I was trying to figure out the gauge pressure of a certain air compressor that used psi measurements. I don't have an iPhone or anything that can pull up internet converters so I figured I would have to do the conversion dimensionally. This posed a problem though as the pounds to kilograms converter on my phone doesn't recognize pounds as a force, but as a mass. I was wondering if there was some sort of calculus involved with dimensionally converting psi to kPa.



    1 lb = [tex].4536 kg_{in earths gravity}[/tex]



    with respect to time, pressure is the second derivative of kg/meter
     
    Last edited: Jun 11, 2010
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  3. Jun 11, 2010 #2

    PhanthomJay

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    an object with a mass of 0.4536 kg has a weight of 1 pound on Planet Earth (that's about 4.45 Newtons).
    No need for calculus......1 psi is 1 pound/square inch...convert pounds to kN (1 pound is 0.00445 kN) and convert square inches to square meters , and you end up with 1 psi is 6.9 kPa, or 1 kPa is 0.145 psi, approximately.
     
  4. Jun 11, 2010 #3
    So I guess what you are saying is there is no way of converting pounds directly into newtons by dimensional analysis without a definition, in newtons, of what a pound is, or without multiplying the conversion from lbs to kg by the gravitational acceleration constant?
     
  5. Jun 11, 2010 #4

    PhanthomJay

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    That's right, you need a definition. It's all in the definition of the unit and the conversion factors between units. That is true even within the chosen system of measure itself. For example, a kN is the force required to accelerate 1000 kg of mass at a rate of 1 meter per second per second. But you first need to define the kilogram and meter and second per an international standard.
     
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