A 40 kg, 5.0 m-long beam is supported, but not attached to, the two posts in the

Question

A 40 kg, 5.0 m-long beam is supported, but not attached to, the two posts in the figure. A 20 kg boy starts walking along the beam. a) How close can he get to the right end of the beam without falling over? Use the following link to see the picture. http://session.masteringphysics.com/problemAsset/1070547/3/12.P64.jpg

b) When the boy is standing at the point where the beam is about to start falling over, what is the normal force on each support?

c) What is the instantaneous angular acceleration of the beam when the boy is standing at the right edge of the beam? Assume the beam is a thin, uniform rod. Its moment of inertia is Icm =   

Explanation / Answer

Assume uniform beam ====> CG at 5.0/2 = 2.5 m from left end

===> CG to right post is 3 m - 2.5 m = 0.5 M

CCW Moment about right post = 0.5 * 40 kg * 9.8 = 196 N - m

CW Moment about right post = 196 N-m

196 = (20 * 9.8) * d

196 d = 196

d = 1 m to the right of the right post

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