Question

Two painters are working on a platform hanging off the side of a building. The platform is 10 meters

long and supported by two cables. The painters are represented by vector forces in the free-body drawing

below. The weights of the objects are: Platform 184 N, Sue 130 N, Jack 220 N. Calculate the force in each

cable when the painters are in the position shown below(Case 1).

To start, locate the pivot point where Cable 1 attached to the platform. Next add the torques produced by each

force about the pivot point together and set equal to zero (?? =0 or better ? F·r?=0 ). Remember that

clockwise (CW) torques are negative Hold your finger on the pivot point so that your paper can rotate but not

slide. Then use you other hand to move the force in the direction of its arrow. If your paper rotates CW the

torque should have a negative sign If it rotates CCW use a plus sign. Not

As Case 2, find out how far Sue can walk towards Jack before there is trouble (platform starts to tip).

Also calculate the force in the cables at this critical point. Hint – When Cable 2 is loose, the force in it is zero.

In solving Case 2, make a free body diagrams (FBD) showing the external forces and the direction of a

positive torque and the pivot point you selected use same pivot point as in Case 1. Show you equation work in

an orderly fashion and enter your results on the bottom of this sheet. Your lab grade will depend on the

accuracy of your work and the clarity of your solution.

After your group has completed the calculations you will test a model to determine if your numbers are

correct. You will turn this lab in before you check your solution on the scale modele that in calculating torques both the

force and distance are always positive. The +/- in front of each torque comes from the direction of impending

rotation.

Case 1

CALCULATED

MEASURED

Cable 1

________

________

Cable 2

________

________

Case 2

Cable 1

________

________

Cable 2

________

________

Sue’s Position

(Measure from

left end of

platform)

________

________

Answer #1

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(a) Consider two parallel wires running 2 cm apart. Each wire
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1. Equilibrium Lab Example.
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