Physics Torque (1)

Two window washers, Bob and Joe, are on a 3.00 m long, 405 N scaffold supported by two cables attached to its ends. Bob weighs 780 N and stands 1.00 m from the left end, as shown in the figure below. Two meters from the left end is the 500 N washing equipment. Joe is 0.500 m from the right end and weighs 900 N. Given that the scaffold is in rotational and translational equilibrium, what are the forces on each cable? 

Left cable?

Right cable?


The torque of the tension in the left cable T1 must equal the torque of the Bob weight (W1) plus the torque of the washing equipment (W3) plus the torque of Joe (W4) plus the torque of the weight of the scaffold(W2) , all with respect to the right end of the scaffold (rotational equilibrium).

$T1*3 =W1*(3-1) +W2*3/2 +W3*(3-2) +W4*0.5$

$3*T1 =780*2+405*1.5+500*1+900*0.5$

$3*T1 =3117.5$

$T1 =1039.17 N$

On the y (or vertical axis) the forces must cancel each other. Let T2 be the force on the right cable.

$T1 + T2 = W1+W2+W3+W4$

$T2 =780+405+500+900-1039.17 =1545.83 N$