Equilibrium of a Rigid Body
Figure 1: Equipment for the “Equilibrium of a Rigid Body” experiment showing a meter stick, which acts as
our rigid body, with the knife edges (which also are the mass positioners), knife edge support, masses and
The objective of this experiment is to investigate and understand the conditions for the
equilibrium of a rigid body.
In particular, we wish to understand that the equilibrium of an
extended object depends not only on the magnitudes and directions of the forces being applied,
but also where on the object these forces act (see Figures Figure 1 and Figure 2).
One meter stick, 3 knife edges/mass positioners, 1 knife edge support stand, set of
masses, 2 mass hangers, 1 body of unknown mass.
An object in equilibrium has no net influences to cause it to move, either in translation (linear
motion) or rotation. In other words, the object experiences no net force acting on it and no net
Thus, the two conditions for a rigid body to be in equilibrium are:
the resultant of all the forces acting on the body must equal zero.
the resultant torque acting on a body (computed about any axis) must equal zero.
As discussed in your textbook, a torque may be identified with the ability of a force to produce
rotation. That is, an object may be fixed in position, but free to rotate about some axis.
go-round, for example, is fixed in position so it can not move across a playground, but is free to
rotate about an axis through its center. A force acting on the merry-go-round will cause it to
rotate, so long as the line of action of the force does not pass through the merry-go-round’s axis