Question

A coffee drinker is faced with the following dilemma. She is not going to drink her hot coffee with cream for ten minutes, but wants it to still be as hot as possible. Is it better to immediately add the room-temperature cream, stir the coffee, and let it sit for ten minutes, or is it better to let the coffee sit for ten minutes and then add and stir in the cream? Which results in a higher temperature after ten minutes? Use your Temperature Probe to examine this dilemma. Explain your results in terms of the assumptions Newton made about cooling.

Answer 1

The greater the difference between the coffee's temperature and room temperature, the more quickly the coffee will cool. (This is Newton's law of cooling). So it cools more quickly when it's hot than when it's not as hot.

If you waited to put in the cream, the coffee would cool off quite a bit, and then you would cool it even more by putting in the cream.

If you put in the cream first, the cream itself will have roughly the same effect in cooling the coffee as in the first scenario, but then since the coffee is sitting out at a lower temperature, it won't cool as quickly just by sitting there. The result is that you have less cooling taking place in this situation than in the first.

So put the cream in first.