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Homework Help: Calculating work done by a force?

  1. Jan 29, 2017 #1
    1. The problem statement, all variables and given/known data
    An object initially at an elevation of 5 m relative to the Earth's surface with a velocity of 50 m/s is acted on by an applied force R and moves along a path. Its final elevation is 20 m and its velocity is 100 m/s. The acceleration of gravity is 9.81 m/s^2. Determine the work done on the object by the applied force, in kJ.
    2. Relevant equations
    Work of all non-gravity forces = Change in KE + Change in PE
    3. The attempt at a solution
    I can get the work done by R in terms of the mass using the equation
    Work of all non-gravity forces = Change in KE + Change in PE
    but the problem doesn't tell us the mass and I don't know what other equation to use to set up a system of equations to eliminate the mass.
  2. jcsd
  3. Jan 29, 2017 #2


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    Homework Helper
    Gold Member
    2017 Award

    Welcome to PF!

    You are right. Without the mass, you cannot find a numerical value for the work done by the applied force.
  4. Jan 29, 2017 #3
    I thought so! thanks for the help
  5. Jan 30, 2017 #4
    I get the impression that the conclusion is that the question is invalid. Not necessarily so however. Since the result is dependent on the unknown mass, it likely should be presumed that the question is simply asking for a formula as an answer, with the mass as a parameter in it. One would also have to specify the units of the mass in such an answer since the question is so specific in asking that the work deduced is explicitly in kj.
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