(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

A 45 kg, 5.0 m-long beam is supported, but not attached to, the two posts in the figure . A 25 kg boy starts walking along the beam. How far can he get to the end without falling over?

2. Relevant equations

Support on the left is leftEnd, support on the right is rightEnd

Torque equations. Equilibrium.

3. The attempt at a solution

Sum of Forces and Torque must be 0 for static equilibrium.

Forces = normal Force on leftEnd + normal Force on rightEnd - weight of the boy

When the board tips, the normal force on the leftEnd is 0, so:

normal Force on rightEnd= weight of the boy = 25 * g

Sum of the torques (leftEnd is the axis):

Gravitational Torque is at the center of mass = -2.5 * 45 * g

Torque of the boy is -25 * g * (3 + d)

Toruqe of rightEnd = 25 * 9.8 (from above)

Sum them and solve for d:

25 * g * (3+d) = -2.5(45)(g) + 25 * g

25 *(3+d) = -2.5(45) + 25

3 + d = -3.5

Which can't be right. Where did I go wrong?

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# Homework Help: Torque/Rotational Motion

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