Hot coffee stands in a cup at 8 5 ∘ C in a 2 5 ∘ C room. A person wants it as hot as possible five minutes later and has a small amount of cream from a refrigerator at 5 ∘ C. Assuming Newton's la

Question

Hot coffee stands in a cup at 8 5 ∘ C in a 2 5 ∘ C room. A person wants it as hot as possible five minutes later and has a small amount of cream from a refrigerator at 5 ∘ C. Assuming Newton's law of cooling with the same cooling constant, when should the cream be poured?

TestHub · Accepted Answer

Both the mixing and the cooling act on the excess over room temperature. The cold cream (below room temperature) lowers the temperature either way, but if it is added first its cooling effect is reduced by the decay factor e − k t in the following five minutes. So the coffee ends up slightly warmer by pouring the cream at once. A.

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