Originally posted by sonhouse
So you have an ice chest for your picnic and its 0.5 meter ^3. (1/8 cubic meter). So the problem is this: You can fill it completely with ice which at zero degrees C has X amount of cooling power, heat absorbtion, whatever you want to call it, and its easy to see if its completely full of ice you have no room for food but if its empty you have plenty of roo ...[text shortened]... numum amount of ice? You can assume the food to be the same density and heat capacity as water.
You've neglected to tell us the ambient conditions. If I were at the North Pole I doubt I'd need any ice in the box; I'd be surrounded by the freezing stuff. Do you want a function of outside temperature? Does this picnic take place in the shade of a tree or in direct sunlight?
And what does "R12" mean?
Also if the food starts of at 12 deg C, how quickly do you need to get it to 7 before it goes off? Is the food something like strawberries with a high surface area to mass ratio or more like watermelon (low)?
Just Googled for R12 - according to Home Depot it's equivelent to 2" of Polyurethene/Polyisocyanurate