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Homework Help: What exactly is the total energy ?

  1. Jul 12, 2015 #1
    1. The problem statement, all variables and given/known data
    A mass of 20kg is suspended at a high of 5 meters above the ground. It is on the moon (then acceleration = 1.635 m/ss). It falls, and we want to know the total energy halfway, at 2.5 meters above the ground.

    2. Relevant equations
    I suppose the equations I need to use are kinetic and potential energies :
    Ek = 1/2.m.v2
    Ep = m.g.h
    3. The attempt at a solution
    Once I have the values for Ek and Ep, can I proceed like this : Etotal = Ek + Ep ?
    I have seen on many physics books that without friction, the Et is constant. But what means that constant ? Is there any numeric values ? What do I have to answer at the question : what is the Etotal ?

    Thank you very much for your help,

  2. jcsd
  3. Jul 12, 2015 #2
    The constant itself could take on any numerical value depending on what units you use. Suppose, for example that you calculate the energy as say 20J in the initial configuration. The total energy will remain 20J no matter how the configuration of the system changes. So, if the gravitational potential energy is reduced by half (10J) then that 10J must have gone into some other form of energy (say, kinetic energy).

    So if the question asks for the total energy and there are no dissipative forces (like friction) the total energy will be equal to whatever the initial energy is. If the question asks for the kinetic energy or speed, you can easily calculate this based on the conservation of energy principle I described above.
  4. Jul 12, 2015 #3
    Hi brainpushups,

    Ok, it is becoming more clearer now. So it means this constant can be a different numerical value depending on the isolated system...

    Thank you for your help !
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