Re: How would you measure the temperature of a tennis ball?

Dear Sarah:

I am sorry it took me this long to answer your question, but I was away 
from my office and I did not have the chance of checking my email.
Regarding your question, it is a tricky one.
If you are interested in measuring the temperature of a tennis ball which 
is sitting in some fixed place doing nothing, the answer is very simple: 
The ball must have the same temperature as its surroundings. This is 
called the Zero Law of Thermodynamics (this funny name is due to the fact 
that it was discovered afeter the First and Second Laws). In order to 
measure such temperature you could use a regular mercury thermometer or 
some other device like a bimetallic thermometer, depending upon how 
precise you need your measurement. Check the following website for some 
really nice explanations: http://www.scitoys.com/.
In the other hand, if you are interested in measuring the temperature of a 
ball which is traveling as a consequence of a direct hit, the measurement 
is much more difficult. Since the ball is heated by the hit, and it is not 
in thermal equilibrium with the surroundings, you need to measure its 
temperature directly, which means you need to “attach” a thermometer to 
it. You can not use a mercury thermometer or any other contact 
thermometer. The only solution is using a radiation thermometer (which is 
sometimes callked a pyrometer). This instrument is very much as the 
thermometer used for measuring your temperature by inserting it in your 
ear. It is based upon Planck´s Law of radiation. In order to make a 
noncontact measurement, you need to point this device to the ball, in the 
same way as the device used for measuring the velocity og the ball in a 
baseball game. (This last device and the thermometer works based upon 
different principles)
Unfortunatelly, the measuremet using such thermometer is dificult because 
you need to know that the atmpsphere is not ionterfering, that there is 
not hot bodies arround and that you are focusing in the ball and not in 
the air close to it. Besides that, diferent surfaces will give you 
different answers even if they are at the same temperature, because they 
emmit diffrent amounts of energy. This is call the body emmisivity, and, 
if you do not know this number, your measurement could be very flawed.

Check the following sites:
 http://www.kapili.com/topiclist
.html
 http://www.physic
s4kids.com/files/thermo_main.html
 http://www.infopleas
e.com/ce6/sci/A0848442.html


I hope this helps

Jaime