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Painter (another Equilibirum problem)

  1. Nov 23, 2009 #1
    1. The problem statement, all variables and given/known data
    A physics student of mass mm = 95 kg gets a summer job painting houses. On his first project, he builds a platform using large pine board with a mass of mb = 50 kg The board has an overall length of L = 8.5 meters and set on two 1 meter tall saw horses so that l = 2.25 meters overhangs each side as shown above.


    Later that day, after thinking about how cool rotational dynamics really is, the student decides to conduct an experiment. He removes one of the supports and places the other one 1/3 of the way from the left edge. Standing at the end of the board, he has his girl friend place paint cans, each of mass mc = 1.59 kg, on the opposite end. How many cans will the girl have to place on the board to provide the best balance? (You may neglect the small length of the board that both the man and the cans occupy. Assume both are points at the ends of the board.)

    https://wug-s.physics.uiuc.edu/cgi/courses/shell/common/showme.pl?cc/DuPage/phys2111/fall/homework/Ch-12-Equilibrium/painter/painter_3.jpg [Broken]


    2. Relevant equations

    torque = rFsin(theta)


    3. The attempt at a solution
    torque(A) = (1/3)mass(man)g - (2/3)*g*mass(cans) - (1/2)*g*mass(board) = 0
    torque(A) = (1/3)*95*9.81 - (2/3)*9.81*m(cans) - (1/2)*9.81*50 = 0

    I think for the mass of the cans I got 9.94 (finding the number of cans is easy), and I dont think its correct. I think I'm missing something, any ideas?
     
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Nov 23, 2009 #2

    rl.bhat

    User Avatar
    Homework Helper

    Check your calculation.
    You can rewrite the second equation by canceling 9.81. It becomes
    1/3*95 - 1/2*50 = 2/3*m
    Now solve for m.
     
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