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]

CONSERVATION OF ENERGY



This was posted yestrday but did not bounce to me. I suspect the message
was lost. The reposting is caused by an observation Barlow made yesterday.
He was right, the term "internal energy" should not be confused with the
term "thermal energy". Also note that the mass of the plate M is now "much
larger" than the mass of the sliding cube, m. This takes care of the
"pseudo-work" comment made by B. Surendranath (from India). Thermal energy
is associated with random motion of molecules, atoms, free electrons, etc.;
it can be calculated from experimental data as dU=c*(M+m)*dT, where c is
specific heat of iron and dT is a change in temperature.

**********************************************************************

An iron cube whose mass is m slides horizontally over a long iron plate.
The mass of the plate M is much larger than m. The plate is a rest while
the cube has an initial kinetic energy K. Friction brings the cube to
rest before its base reaches the plate's boundary. Everything is in the
vacuum and the thermal energy, dU, added to both pieces of iron is not
radiated. [In principle, dU can be calculated from the observed change of
temperature but no thermometer is available.] Which of the following is
correct and why?

1. The thermal energy change, dU, is equal to K.
2. Only part of K goes into dU, the rest is used to do work W=R*x, where
R is the force of kinetic friction and x is the distance the cube
slides before comming to rest.
3. Neither of the above is correct.

I am really confused in this situation. My frame of reference (for K and x)
is attached to the plate.
Make sure I understand you.
Ludwik Kowalski

P.S. To prevent proliferation of confusing terminology please delete the
previous version of the file, if you saved it. For the same reason
I am going to change the "subject".