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Homework Help: Determine the location of her center of mass

  1. Mar 2, 2016 #1
    1. The problem statement, all variables and given/known data
    To determine the location of her center of mass, a physics student lies on a plank of weight 100.0 N and length 2.5 m supported by two scales 2.50 m apart. The left scale reads 420.0 N, and the right scale reads 210.0 N. (Note: Don’t neglect the weight of the plank when you do this problem.) a. Find the student’s weight. [4] b. Find the distance of her center of mass from the left scale (x=0). [4]

    2. Relevant equations
    T = rF
    Wgirl = 420 + 210

    3. The attempt at a solution
    I get her weight as 630 N. But when I go to solve for the distance using center of mass, when I place the point of rotation at the left support I get her center of mass as .635 m from the left side, but using the right support, I get her center of mass as 1.47 m away from the right side or 1.03 m from the left side. What am I doing wrong to get two different center of masses.
  2. jcsd
  3. Mar 2, 2016 #2

    The girl's weight is not 630 N. Recall the problem says: "don't neglect the weight of the plank when you do this problem".
  4. Mar 2, 2016 #3


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    Science Advisor

    How does the weight of the plank figure into that calculation?

    How did you calculate either number?

    Edit: Too late.
  5. Mar 2, 2016 #4
    So I placed the point of rotation on either scale. The plank is 100 N with its cm 1.25 m from either side. I then used the other scale's reading and multipled that by 2.5 m, and subtracted the plank's torque contribution before dividing the whole thing by 630.
  6. Mar 2, 2016 #5
    Why divide by 630? You have done the calculation for the torque from the plank. Again, the girl does not weigh 630 N.
  7. Mar 2, 2016 #6
    But isn't there a torque from the plank's cm, and then an added torque of the girl's cm?

    EDIT: I now understand why you say the girl does not weigh 630 N. I now get the correct values. Thanks!
    Last edited: Mar 2, 2016
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