Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: MOMENT OF INERTIA



On Sun, 20 Dec 1998, Brian McInnes wrote:

A few points on this thread which started on moment of
inertia and has moved on to work and contact forces.

Your points are well expressed and valid. But I would comment on the last
one, number 5.

(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.

Whether or not work must be taken into account and, for that matter,
whether or not heat transfer must be taken into account depends upon your
choice of system. Both work and heat measure transfers of energy from one
system to another. By appropriate choice of system, one can limit
consideration to only energy transformations, as you point out. However,
there are problems where it is useful to so define the system that energy
transfers are also present and, therefore, working and heating must be
considered.

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.

Careful. We need a more precise definition of the system here. Also, you
refer to energy transformation, not energy transfer. If your contact
force is the force of the wall on the person, then there is an outside
force acting and there is an energy transfer by working, not an anergy
transformation.

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.

Again, what is the system? If only the box, then friction is working. We
have energy transfer and energy transformation.

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.

Ditto with regard to the gravitational-kinetic transformation. But how
does the contact force come into play? It is perpendicular to the motion.
It can't be affecting the motion.

Mervin Koehlinger
Physics Instructor
Concordia Lutheran High School
Fort Wayne, Indiana