demonstrates that the ruler remains at rest in the orientation
from which it is released. On the basis of this observation students
conclude that the angular acceleration of the ruler is
zero and then infer that the net torque on the ruler (about the
pivot) is also zero (according to~snet ¼ I~a). Next, after noting
that the center-of-mass is not accelerating, students infer that
the net force on the ruler must also be zero (according to
~Fnet ¼ m~acm). The tutorial then defines an extended free-body
diagram as one that shows the forces exerted on an object and
the locations at which they are exerted. Students draw such
a diagram for the ruler after it is released then check their diagram
for consistency with their earlier statements about the
net torque and net force on the ruler