1405 10 2008c Name ______________________________________________
Rotational Equilibrium
Equipment:
Meter – Stick Balance
with Fulcrum
slotted gram mass set
(3) knife-edge clamps (one for fulcrum, two to hold masses)
Torque:
Torque = force x lever-arm
Units: The SI torque units are meter·newtons (m·N). Since the force supplied in this lab is the
weight-force which is proportional to the mass, we will use units of cm·gram. This unit can
easily be converted to m·N if necessary.
Rotational Equilibrium: An object is in rotational equilibrium when the net torque acting is
zero.
Procedural Notes:
 Perfect balance of the meter stick is not needed to obtain good results. Once it is close to
balanced you can move on. Stay on the same side of the experiment so your definition of
clockwise (CW) and counterclockwise (CCW) will not change.
 For greater accuracy, add the mass of the knife-edge clamps (16grams) and hangar
(5grams) to the values of mass used in the experiment [total of 21 grams].
Procedure:
1) Place the fulcrum at the 50cm mark of your meter stick.
2) Hang a 500gram mass from the 40cm mark.
3) Place 300grams on a mass hangar on the opposite side of the fulcrum at a location such
that the meter stick is balanced.
What is the lever-arm of the 521 gram mass
What is the lever-arm of the 321 gram mass
about the fulcrum in cm? ___________
about the fulcrum in cm? ___________
Calculate the torque created in cm-gram
Calculate the torque created in cm-gram
Torque = (____cm)(________gram)
Torque = (____cm)(________gram)
= ______________cm-gram
= ______________cm-gram
Is this torque CCW or CW?
Is this torque CCW or CW?
4) Do the torques due to these masses cancel each other out? ________ Why or why not?
5) Change your 300gram mass to 200grams (leave the 500 gram as it is), and move it to a
location such that the meter stick is still balanced.
521 gram Torque = ______________cm-gram
What is the lever-arm of the 221 gram mass
CCW
about the fulcrum in cm? ___________
Calculate the torque created in cm-gram
Torque = (____cm)(________gram)
= ______________cm-gram
Is this torque CCW or CW?
6) Circle the correct word: The 221 gram mass required a ( smaller larger ) lever-arm in
order to produce the same amount of torque as the 321 gram mass.
7) Experiment by moving the hangar from Step 3 and by putting less than 200grams on it.
8) What is the smallest mass that can be hung such that the meter stick remains in rotational
equilibrium? ___________ grams
9) What is the lever-arm of this mass? _________cm
10) How much torque does this new mass create at this location? _________________cm-gram
11) Is this torque equal to the previous torque values? ______________
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