I have gotten quite behind in reading my email, so excuse me if I
repeat something that has already been said. It seems to me that there
is a basic confusion in the use of terms in the original posting that
needs to be addressed. As I understand it, energy is not an agent - it
can't do anything. Energy is, rather, a function of the state of a
system that appears to have great physical significance. If parts of a
system interact, energy can be transferred from one part to another; ie.
the interactions result in changes in the states of the subsystems such
that the energy associated with one decreases while that of another
increases. When the interaction is a force acting on a macroscopic
object, we say that work is done. It is always the force that does the
work; ie. results in the transfer of energy from one subsystem to
another. It is incorrect to say that energy is "the ability to do
work", although I have used that as an initial, working definition in
some non-science courses, before refining the concept. It is true that
an OBJECT that has energy has the ability to do work - ie. to transfer
some of that energy to another object through its interactions. In
classical mechanics, the energy of a (sub)system can be separated into
the energy of the center of mass and the energy associated with
internal degrees of freedom. For objects like the block and plate that
are being discussed, The kinetic and potential energy are those of the
center(s) of mass, and that of the internal degrees of freedom is
called the internal energy (or thermal energy, etc.) of the objects.
Transfers of energy that are mediated by forces on a macroscopic scale
are called work, while those that are mediated by forces between
individual atoms are called heating/cooling. The "force" of friction
can be confusing, because it results in an average force on the CM,
which does work (which results in a change in kinetic energy), but it
results from forces between individual atoms that excite internal
degrees of freedom. So, in the case of friction, it is easy to confuse
work and heat. I hope I haven't rambled too much. I think my main
point is that we need to carefully distinguish between mathematical
quantities that are functions of state, and the agents that produce
changes in those quantities.
Al
Dr. Albert H. Clark
Physics Dept.
North Carolina Central Univ.
Durham, NC 27707