A beam resting on two pivots has a length of L = 6.00 m
and mass M = 90.0 kg. The pivot under the left end exerts a
normal force n1 on the beam, and the second pivot placed a
distance l= 4.00 m from the left end exerts a normal force n2.
A woman of mass m = 55.0 kg steps onto the left end of the
beam and begins walking to the right as in Figure P8.22. The
goal is to find the woman’s position when the beam begins to
tip. (a) Sketch a free-body diagram, labeling the gravitational
and normal forces acting on the beam and placing the woman
x meters to the right of the first pivot, which is the origin.
(b) Where is the woman when the normal force n1 is the greatest?
(c) What is n1 when the beam is about to tip? (d) Use the
force equation of equilibrium to find the value of n2 when
the beam is about to tip. (e) Using the result of part (c) and the
torque equilibrium equation, with torques computed around
the second pivot point, find the woman’s position when the
beam is about to tip. (f) Check the answer to part (e) by computing
torques around the first pivot point. Except for possible
slight differences due to rounding, is the answer the same?
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