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: Pb



At 03:00 PM 3/9/01 -0500, Michael Edmiston wrote:
In the "short view" it seems the KE of the Pb changed from zero to something
(by gravity) then from something to zero (by the floor). These would seem
equal and opposite making work-floor equal minus work-gravity.

I wouldn't have said that.

The difficulty here hinges on the words "by the floor".
-- Yes, the floor played a critical role, especially by transferring
momentum.
-- But that does not mean that any significant energy was transferred to
or from the floor.

It is not at all true that W_floor is related in any significant way to
W_gravity. W_floor is zero for all practical purposes. W_gravity was
turned into KE, and the KE was converted to thermal energy (and maybe some
potential energy), by processes internal to the chunk of lead.

In the "long view," if we try to perform bookkeeping on the energy including
thermal energy, wouldn't we have to say some of the KE of the falling Pb
eventually ended up as thermal energy in the floor in addition to thermal
energy in the Pb?

It might, and we can consider that case if we want, but it doesn't add much.

the "missing energy" is in the floor as a
result of the work the Pb did on the floor?

No, for the reasons given above.