I was flipping through the "Flying Circus of Physics" by Jearl
Walker today and came across a problem that bothered me. The problem
basically states that you've just poured a cup of hot coffee and you have
about 5 minutes before class. If you want your coffee as hot as
possible, would it be better to put the cream in now or right before you
get to class?
The answer they gave was that it was better to put the cream in
right before you got to class. To me, that doesn't seem right. I don't
think it would matter. If you put the cream in right away, the
temperature would drop until it reaches equilibrium, then the coffee
would slowly drop as it loses its heat energy to the surrounding air. On
the other hand, if you put the cream in later, the coffee was already
slowly dropping in temperature and then by adding the cream in later, the
temperature would drop to reach equilibrium.
I visualize this as the time it takes or the energy used of a
person running 1 mile and then walking 0.5 miles or on the other hand the
person walks 0.5 miles and then runs 1 mile. I'm not sure if this
scenario helps in understanding my viewpoint, but it's the best I could
think of.
Could there be something about the cream? Was the cream sitting
out and therefore warming up? Was the cream at the same temperature
whether it was added right away or later?
Can someone help me out in understanding this problem? Or is it
possible that Jearl Walker was wrong? :-D