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Impossible Question

  1. Sep 22, 2009 #1
    From the following defining equations, determine the dimensions of the names physical quantities. Use L to represent the dimension length (distance), T to represent time and M to represent mass.

    a) force, F: F=ma, (where m is mass and a is acceleration)
    b) pressure, p: p=F/A, (where F is a force (see previous question) and A is an area)

    -I don't need the answer it's just that I can't even attempt to solve the question as I have no idea what the question is asking for.

    a) F=M*d(L/T)/dT ?
    b) p=M*d(L/T)/dT/L^2 ?

    ..that is all that I could come up with.

    Thank you for your time.
  2. jcsd
  3. Sep 22, 2009 #2


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    Staff: Mentor

    Welcome to the PF. You are close... For a), just clean up what you have, and leave out the d symbols. The units of a change in lenght are still length. Does that help?
  4. Sep 22, 2009 #3
    OK yea I get it..

    a) F = M(L^3)(T^2)
    b) p = ML/T^2
  5. Sep 22, 2009 #4


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    Staff: Mentor

    Much closer. But you messed up a division in a) (I didn't check b).

    Hint -- The unit of force is a Newton. Look up what the sub-units are that make up a N.
  6. Sep 23, 2009 #5
    Well I checked it a couple times..seems correct to me.
  7. Sep 23, 2009 #6
    Try plugging in actual units. M = kg, L = m, T = s

    For a) you're saying F = M(L^3)(T^2)

    which means, N(Newton) = kg * m3 * s2, and that is close, but not right.

    Edit: Also, b) is wrong. Again, p = ML/T^2 is close, but not right.

    You're saying, p = kg * m / s2
    Last edited: Sep 23, 2009
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