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Different conversion for angular momentum

  1. Mar 31, 2014 #1
    1. The problem statement, all variables and given/known data

    Express the units of angular momentum (a) using only the fundamental units kilogram, meter, and second; (b) in a form involving newtons; (c) in a form involving joules.

    3. The attempt at a solution

    a) L = r x p = r x mv = Iω

    This can be simplified to kgm2/s

    b) L = r x p = r x mv
    I express r in meter m , m in kg and v in ms-1

    what I got was m2kg/s

    I have to convert it to n.m.s
  2. jcsd
  3. Mar 31, 2014 #2
    What are the fundamental units of a Newton? What are the fundamental units of a Joule?

  4. Mar 31, 2014 #3


    1N is equivalent to the force required to give a mass of 1kg an acceleration of 1m/^2.

    1J is the work done by applying a force of 1N over a meter.
  5. Mar 31, 2014 #4
    So, again, what are the units?
  6. Mar 31, 2014 #5
    They are kg.m.s-1 and kg.m2.s-2
  7. Mar 31, 2014 #6
    For Newtons, that should be kg.m.s-2. Now the trick is combine these with your units for angular momentum to express the angular momentum in terms of N, m, and sec, or J, m, and sec.

  8. Mar 31, 2014 #7
    Let me give the brick wall another shot.
  9. Apr 1, 2014 #8
    Still finding it very difficult to solve. Any hints?
  10. Apr 1, 2014 #9
    Solve for kg in one set of units, and substitute for it into the other set of units.

  11. Apr 2, 2014 #10
    Thanks Chet.

    L = m2kg.s-1

    1N = 1kg ms-2

    ∴1kg = 1Ns2/m

    In subtituing into L:

    m2(1Ns2/m) s-1


    1J = 1Js2/m2

    In substituting into L:

    ∴ m2(1Js2/m2)s-1
    = 1J.s

    Both are consistent with the system's answer.

    It has also enabled me to solve a next different question that asked for J.s.
    Last edited: Apr 2, 2014
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