# Torque of a bar on a crane

1. Feb 25, 2013

1. The problem statement, all variables and given/known data

You are a member of a medical forensics team
investigating a fatal accident at a freight
yard. A new crane for lifting cargo broke
while lifting a package and the police suspect
sabotage. The crane consists of 45.0 kg, 15 ft
lightweight aluminum bar supported at its
base by a hinge that allows the bar to pivot
vertically. A support cable runs from the
other end of the bar to the ground. When the
hinge failed, the bar was at an angle of 45° to
the ground and the support cable was at an
angle of 30° to the ground. The yard owner
says it is impossible for this to be an accident
since the hinge is rated to support a force of
three times the weight of the 225 kg package
that was being lifted. The package was lifted
straight up from the end of the bar at a
constant speed.

2. Relevant equations

Torque = Force x Length

Weight = Mass x Gravity

ƩFx = 0

ƩFy = 0

ƩT = 0

T = Torque

3. The attempt at a solution

I'm having trouble setting up the picture for this problem. I understand how to solve for forces and am still getting comfortable with torque but I'm having a difficult time drawing out the picture for this problem as I felt like the information given was a bit vague. Any help is appreciated!

2. Feb 25, 2013

### haruspex

Draw a 45 degree right angled triangle, with the horizontal base, AB and the vertical, AC, being the two equal sides. The hypotenuse, BC, represents the bar, and the vertical is the load cable.
Extend the base AB out beyond B to a point P s.t. angle APC is 30 degrees. PC is the support cable.