Experiment 6
Equilibrium

Objectives
To show that the sums of forces and torques on a static body are zero.

Introduction
The sums of external forces and torques equal zero for bodies in equilibrium.  If the applied forces and their points of application are measured for a given stationary body, the calculations of the appropriate sums should yield zero.  We will take a horizontal stick as a stationary body to which forces will be applied.

Procedure
Obtain a meter stick, two spring balances and several weights.  Find the center of mass of the meter stick and record it.  Check the accuracy of the balances by hanging a known weight on them and then suspend the meter stick by hanging it from the balances (which, of course, are supported by a system of rods and clamps).  The points of attachment of the balances to the stick are arbitrary.  Now suspend three weights of your choice from the stick at such positions that the meter stick is horizontal and the balances are well but not fully extended.  Record all the forces and their points of application in the form of a sketch of your apparatus.  Be sure to include the weight of your meter stick as a force.

Calculations
Calculate the sum of all forces.  When the uncertainties in the forces are considered, is the sum consistent with zero?

Choose an axis and calculate the sum of all torques.  Is the result consistent with a null result?

Choose a second axis and recalculate the sum of all torques.  Is this result consistent with a null result?