Instructor Outline:
Static Equilibrium
UM Physics Demo Lab 07/2013
Lab length: 80 minutes
Lab objective: To teach the concepts of vector addition, the net force/torque as the
vector sum of multiple forces/torques, torque as vectors, right hand rule for torque
vector directions, unit conversion, mechanical advantage and center of gravity.
Materials
8 sets of 3 spring scales (Ohaus 20N/2000g)
8 2’ x 2’ perforated boards
8 sets of 3 tool hooks for boards
8 one inch diameter split rings
8 sets of ten ½ inch cut steel washers (of equal mass)
24 Clear plastic rulers
90 Xeroxed coordinate grid sheets
8 yardsticks
8 steel gussets (angle brackets)
3 boxes sharp pencils
**Demonstrations:
 1A40.30 Magnetic Vectors
 1D40.10 Dumbbell with Light
 1J20.60 Wine Bottle Balance
Introduction: 10 minutes-Lecture
The concepts of tip-to tail vector addition and subtraction are introduced.
Exploration stage: 30 minutes – Group lab-work
The students pull on the ring with three scales and hold the scales down with the tool
hooks to hold the forces steady. A pre-printed coordinate grid is then slid under the ring
and the force directions marked with a pencil and the force magnitudes recorded. The
three force vectors are then represented as arrows with 1 cm corresponding to 1 N of
force. The students then trace the vectors onto a second coordinate sheet and
pictorially add the vectors tip to tail to get a result close to zero
Analysis stage: 10 minutes – Lecture
The results of the Exploration are discussed and the force condition for static equilibrium
is stated.
Exploration stage: 20 minutes – Group lab-work
The students now balance yardsticks with various weights at different distances
and deduce that the product of the weight and distance must be equal to achieve
balance.
Analysis and summary stage: 10 minutes – Lecture
The concept of torque as a vector and the right-hand rule (RHR) for assigning the vector
direction are introduced. The condition for rotational equilibrium is then stated and
discussed along with the mechanical advantage afforded by a long lever arm. The
concepts of mechanical advantage and center of gravity are introduced and a final
summary presented.
Property of LS&A Physics Department Demonstration Lab
Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109
Concepts developed:
1. Vector addition and subtraction.
2. Torque as a vector and the RHR for assigning direction.
3. Net force/net torque as the vector sum of individual forces/torques.
4. Condition for linear and rotational static equilibrium.
5. Mechanical advantage.
6. Center of mass and center of gravity.
Property of LS&A Physics Department Demonstration Lab
Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109