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Homework Help: Total Kinetic Energy

  1. Nov 11, 2008 #1
    1. The problem statement, all variables and given/known data

    Biomedical measurements show that the arms and hands together typically make up 13.0 % of a person's mass, while the legs and feet together account for 37.0 %. For a rough (but reasonable) calculation, we can model the arms and legs as thin uniform bars pivoting about the shoulder and hip, respectively. Let us consider a 72.0 kg person having arms 65.0 cm long and legs 90.0 cm long. The person is running at 12.0 km/h, with his arms and legs each swinging through 30 decrees }. Assume that the arms and legs are kept straight.

    What is the total kinetic energy due to both his forward motion and his rotation?

    2. Relevant equations

    0.5 m v^2 =KE

    3. The attempt at a solution
    This is for online hw. The problem is that no matter how i do the sig figs for this problem, i still get it wrong.

    After converting the 12 km/h the proper sig figs indicated a m/s value of 3 m/s. by plugging that into the equation i get 72kg*9m/s *0.5=300j but if i do it like this 72 kg*3.333^2 *0.5=400 but the 300 j answer is wrong but the 400 j answer is wrong but due to sig figs or rounding. Can somebody help me and tell me what is wrong?
  2. jcsd
  3. Nov 12, 2008 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    There are two problems I see here.

    1. You haven't included the kinetic energy due to rotation of the arms and legs.

    2. "12 km/h" has two sig figs, so the conversion to m/s should also have two sig figs, i.e. 3.3 m/s, not 3 m/s. And the final answer would have two sig figs, since the mass "72 kg" also has two sig figs.
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