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Homework Help: Simple center of mass Q

  1. Nov 8, 2005 #1
    I don't understand how to approach this question, I'm lost in where energy gets involved. Any help would be appreciated :smile:

    A 110kg person is travelling at 3 m/s on a frictionless surface toward a wall, then stops themselves by pushing against the wall. During the push, the person's center of mass moves 0.3 m toward the wall. What is the change of the person's kinetic energy, and what is the average force exerted on the wall?
  2. jcsd
  3. Nov 8, 2005 #2
    answer these questions and see how you can connect them to each other
    what is the
    initial velocity?
    final velocity?
    Given the above two can you find the change in kinetic energy?

    Also what is the distance covered while stopping?
    What is the formula for force?
    Givne the first three quantities, V2, V1 and D, can you find the force? Hint: what is the formula for force? What you need in that formula.
  4. Nov 8, 2005 #3
    So the change in kinetic energy would be

    Kf - Ki = chng in K

    where Kf = 0 and Ki = 1/2 (110kg)(3m/s) = 165J

    chng in K = 165 J

    and for the second part I'm still confused on what formula to use, would the distance covered during the stop be x2 in:

    Xcom = [m(x1) + m(x2)] / m ?

    I don't think it is :grumpy:
  5. Nov 9, 2005 #4


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    Homework Helper

    KE = (1/2)mv²
  6. Nov 9, 2005 #5
    Yeah, I caught that error. How to get the average force still boggles me though :yuck:
  7. Nov 9, 2005 #6

    Doc Al

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    Staff: Mentor

    This problem has nothing to do with calculating the center of mass. It's an application of the "Work-Energy theorem". (Look it up.)

    If you haven't covered that theorem, you can always find the average acceleration (using kinematics) and apply Newton's 2nd law to get the average force.
  8. Nov 9, 2005 #7
    So center of mass isn't needed at all? I just assumed it would be because the question came out of the center of mass chapter in my textbook.
  9. Nov 9, 2005 #8

    Doc Al

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    Staff: Mentor

    All you need to know is the displacement of the center of mass...and that's given.
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