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A few points on this thread which started on moment of
inertia and has moved on to work and contact forces.
(5) Bob's examples are interesting but a question I would
like to ask is what is the point of calculating work at all?
Surely the important item is not whether work is done or how
much is done but what are the changes in energy and what
mechanisms are involved in those changes.
For Brian's astronaut launching from wherever in a
space-ship or a person pushing off a wall, there is an
energy transfer from chemical energy of the muscles to
kinetic energy of the center of mass. The mechanism is the
normal contact force between the body and the surface.
For the box sliding to rest on a rough horizontal surface,
there is an energy transfer from kinetic energy of the box
to thermal energy of the system of box and surface. The
mechanism is the frictional force between the box and the
rough surface.
For the hoop rolling down the incline (which is close to
where this thread started) there is a transfer of energy
from the gravitational potential energy of the system of
hoop and earth to translational and rotational kinetic
energy of the hoop. The mechanisms are the gravitational
force between the hoop and the earth and the contact force
between the incline surface and the hoop.