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Calculating Kinetic Energy in English Units

  1. Jan 22, 2016 #1
    1. The problem statement, all variables and given/known data
    An object has a mass of 10 lbm.

    b. Find the change in kinetic energy ft·lbf when its velocity decreases from 50 mph
    to 15 mph.

    2. Relevant equations
    1MPH=1.4667 ft/s
    3. The attempt at a solution
    When calculating this, does it need to be in slugs, lbm, or lbf?
    I'm not too familiar with english units.
  2. jcsd
  3. Jan 22, 2016 #2


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    Science Advisor
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    Gold Member

    the lbm unit is seldom used anymore, but an object that weighs 10 pounds (10lbf) on Earth has a mass of 10 lbm. But when using the formulas for the value of m, you must use the mass unit of slugs. The value of the slug can be found by dividing the objects weight (in pounds) on earth by 32.17 (the acceleration of gravity). This will give the correct units (ft and lbf) for the values you are seeking, without using a conversion factor in your equations.
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