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: [Phys-l] heat/energy

Anthony Lapinski wrote:
I'm looking for some simple activities for a heat investigation/lab.
Specifically, when two objects collide, I want to show how kinetic energy
(loss) changes to work, sound, and heat. I thought about just hammering a
nail. They can clearly see the nail going into the wood and hear the
noise. I tried this (driving the nail about 1.5 cm), but could not really
sense a temperature change in the nail and hammer head. I then tried
pounding a lead brick with a sledgehammer, but both felt cool to the
touch. Maybe this has to be done for a longer time?

Does anyone have a demo/activity that "easily" shows a temperature rise
when two objects impact?
Interesting observations.
a hammer is rather like an explosive charge; not much energy, but a fast rate.

A 1 lb hammer dropping 16 inches provides at least 1 lb X (1 kg /2.2lb) X 9.8 N/kg X 16 in X 1m /39.4 in newton-meters or joules - thats 1.8 joules, like a 60 watt lamp shining on a nail for a second (taking a guess at the thermal coupling coefficient) but applied so quickly that it can reach the plastic yield of the mild steel in the nail - say 25,000 psi or 25000 lb/in^2 X 1kg/2.2lb X 9.8 N/kg X (39.4 in)^2 /m^2 = 17.3 MN/m^2 or 17.3 MPa

[I hope the teacher who illustrated his high school unit conversion methods here lately, will take his reward, late in the day though it is, from me...]

An activity that prolongs the energy transfer time, increases the energy coupling coefficent, and provides multiple sensors at the heating site goes like this. (It was formerly a common-place observation, when children needed to do chores in the cold of Winter....)
Apply the flat of one dry hand to the other, with a force of about 40 N or more, oscillate the friction surface at 4 Hz or more over a reciprocating
stroke of 0.2 m and detect the effect of dissipating this work.

10 seconds at 64 N.m = 640 J at 64 watts.
Bare feet on a carpet surface have much the same result.

Brian W