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Homework Help: How to find KE and PE without mass

  1. Oct 20, 2015 #1
    I am working on Computational physics homework and it asks to find the kinetic and potential energy of a simple pendulum. My only issue is that i don't know how to solve it without mass.
    It gives me :
    theta (pendulum angle)
    omega (pendulum angular Velocity)
    t (time)
    length (length of string)
    dt (time step)

    I was looking at the kinetic energy equation and maybe I need to use Inertia for where KE = (1/2) (inertia)(omega) ? but then how would i find inertia?

    Any help would be appreciated.
  2. jcsd
  3. Oct 20, 2015 #2


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    Staff: Mentor

    Hi alex steve, Welcome to Physic Forums

    Please be sure to retain and use the formatting template that is provided in the edit window when posting a question in the homework areas of the forums. This is a requirement of the forum rules.

    If no mass or moment of inertia are provided then the best you can do is provide a symbolic result, or assume an arbitrary value of mass for the problem.

    An alternative is to use "specific" values. A specific value is a per-unit-mass value. For example, one might say that the specific kinetic energy of some object is 20 Joules per kg. If the body turned out to have a mass of 1 kg then it would have a KE of 20 J. If it turned out to be 100 kg, then it would have 2000 J of KE. Specific values are handy in some areas where particular mass values aren't known or don't matter too much to the details of the problem. A case in point might be where one is equating a change in potential energy to a change in kinetic energy. Normally one would write something like ##M g Δh = 1/2 M v^2##. Note that the M's cancel on both sides. So you could just write it as ##g Δh = 1/2 v^2##.

    ##g Δh## would be the "specific potential energy, and ##v^2/2## would be the specific kinetic energy.
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