Posted by bob on Tuesday, January 20, 2009 at 3:53pm.
Sum moments about any point. I will choose the Right end.
-.3*75g -1*20g+Fleft*2=0
That gives you Fleft end.
Now, sum forces vertically
Fleft+Fright-20g-75g=0
solve for fright.
This is a sum of torque problem.
T(left) = F(board)*Distance from left + F(person)*Distance left + F(right)*Distance from left = 0
T(right) = F(board)*dist from right + F(person)*dist from right + F(left)*distance from right.
In reality, we are summing clockwise torques with counter clockwise torques and since there is no motion ( we are in equilibrium) setting this equal to 0.
This also works with the diving board problems, or the stacked books problem (after establishing the center of gravity for the books).
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