y
rolling carriages bring in the similar ideas where impulses rather
than forces alone can be measured.

There follow, then, the laboratory experiments of the Atwood machine
and the force table, where quantitative results are demanded. It is
desirable to have these experiments at least worked by the class in
unison. Whatever may be the exigencies of numbers and apparatus
equipment that prevent it later, these introductions should be given
to and discussed by all together. In the nature of things,
fortunately, this is possible. A single Atwood machine will give
traces for all in a short time under the guidance of the instructor.
The force table experiment is nine-tenths calculation, and
verifications may be made for a large number in a short time.
Searching problems and discussion are instigated at once, and the
notion of rotational equilibrium and force moments brought in. Because
of the very great difficulty seeming to attach to force resolutions,
demonstration experiments and problems using a bridge structure, such
as the Harvard experimental truss, will amply repay the time invested.
Another experiment here, which makes analysis of the practice of
weighing, is possible, although there will be divergence almost at
once due to the personality of the instructor and the equipment by
which he finds himself limited. The early introduction of moments is
important, however, because it seems as if a great amount of
unnecessary confusion on this topic is continually cropping out later.
At this point, if limitations of apparatus present a difficulty, a
group of more or less independent experiments may be started. Ideas of
energy may be illustrated in the determination of the efficiency and
the horse power of simple machines, such as water motors, pulleys, and
even small gas or steam engines.

In discussion of power one should not forget that in practical
problems one meets power as force times velocity rather more
frequently than as rate of doing work, and this aspect should be
emphasized in the experiments. Conservation of energy is brought out
in these same experiments with reference to the efficiencies involved.
In sharp contrast here the principle of conservation of momentum may
be brought in by ballistic pendulum experiments involving elastic and
inelastic impacts. Most students are unfamiliar with the application
of these ideas to the determination of projectile velocities, and this
forms an interesting lecture demonst