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Homework Help: Rotational kinetic energy with no friction

  1. Apr 10, 2012 #1
    1. The problem statement, all variables and given/known data

    Engineers are designing a system by which a falling mass m imparts kinetic energy to a rotating uniform drum to which it is attached by thin, very light wire wrapped around the rim of the drum. There is no appreciable friction in the axle of the drum, and everything starts from rest. This system is being tested on earth, but it is to be used on Mars, where the acceleration due to gravity is 3.71 m/s^2. In the earth tests, when m is set to 17.0 kg and allowed to fall through 6.00 m, it gives 200.0 J of kinetic energy to the drum.

    a. If the system is operated on Mars, through what distance would the 17.0-kg mass have to fall to give the same amount of kinetic energy to the drum?
    - I figured out this answer to be 15.8 m. I just can't seem to figure out this next part:

    b. How fast would the 17.0-kg mass be moving on Mars just as the drum gained 200.0 J of kinetic energy?

    2. Relevant equations

    Potential energy = Kinetic rotational + kinetic linear
    mgh = rotational + translation kinetic energy

    3. The attempt at a solution

    Kinetic energy = potential energy
    mgh= .200
    For Mars, replace 9.8 with 3.71.

    Can't seem how to put everything together, haven't been able to come up with the right answer according to mastering physics.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Apr 10, 2012 #2


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    Staff: Mentor

    Think about conservation of energy. Where is the energy coming from? If the drum ends up with 200 J of it, where did the rest go?
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