Worcester Polytechnic Institute
PH-1111
Experiment #7, Torque
ReadMeFirst.Exp7.Torque.Part1
Procedure:
In the following experiment, you will be setting up and calculating the
torque for the following 6 equilibrium situations for the popsicle stick – hang 200
g masses from each of two strings on the same side of the large pivot hole (with
one string making an angle of more than 60° with the long axis of the stick) ,
increase both masses to 400 g, decrease one mass back to 200 g while leaving the
other at 400 g and reposition both pulleys so that the string angle of the 200 g
mass is within 1 degree of its previous angle, and now repeat these three
arrangements, now using one string (your choice of which strings to use) on each
side of the large pivot hole.
 Using the mm scale on the plexiglass square, measure the distances from the
MIDDLE of the large hole in the popsicle stick to the middle of EACH of the
other FOUR smaller holes in the stick TO THE NEAREST MM.
 Make sure that there is a piece of white paper centered on the pin of the force
table, so that you will be able to easily discern the black strings attached to the
stick as you look down at the white surface. Please note that you should NOT
make any pencil or pen markings on that white paper surface – all of your
measurements can be make directly with the protractor and mm scale
WITHOUT any supplemental markings! The TA will describe how!
 Place the popsicle stick on the fixed pin in the center of the force table – the large
hole in the stick fits easily over that central pin. Pick two strings on the same
side of the large hole, pass each string over its own pulley, and hang a total of
200 g from each string (as instructed by the TA). Adjust the position of each
pulley so that at least one string makes an angle of 60° or more with the long
axis of the stick. The most important adjustment in this process is to make
sure that each pulley is directly aligned with the string that passes over it –
this will require loosening the tightening screw that holds the pulley in place,
swiveling the pulley to align it with the string, and retightening the screw.
You may have to align and realign each pulley multiple times during the
course of this experiment.
 Note that the popsicle stick will automatically seek an equilibrium orientation
relative to the pivot and the two strings applying a torque about the pivot. In
order to coax the stick to the best equilibrium position, you should push the
stick away from equilibrium with your finger, and then release it so that it
snaps back toward equilibrium.
 Now you will make the measurements needed to calculate the torque applied by
each string on the extended object (the popsicle stick) relative to the pivot
point. First you will measure the string angle with a protractor to the nearest
½°, and then you will measure the “perpendicular lever arm” with the scale on

the plexiglass square to the nearest ½ mm. To be sure that you have good
measurements with which to calculate torque, you must check all of your
measurements by comparing “r sinθ” with “rperpendicular” = “perpendicular lever
arm.” If they agree to within 1 mm, then you can be confident that you have
good data with which to calculate torque values. Your TA will explain
(briefly) how all of this is done.
As specified above at the very beginning, you are to set up a total of 6 torque
equilibrium situations, 3 using two strings on the same side of the large pivot
hole, and then 3 more using one string on each side of the large pivot (your
choice of which strings, but then use the same pair each time!). Your task,
then, is to use the measured values of these 6 situations to calculate the net
torque acting on the stick in each case AND showing that each pair of torques
sum to a net torque value that is no more than 8% of the average of the two
torque magnitudes. If, as will likely be the case, you are unable to finish all of
these calculations during the lab hour, then complete those calculations on the
Worksheet and hand them in at your next Conference Meeting!