Re: [Phys-l] heat/energy

[Brian Whatcott <betwys1@sbcglobal.net>]

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