A solid body cools from 80 degrees C to 60 degrees C in 10 minutes. The temperature of the surroundings is 40 degrees C. How long (in minutes) will it take the same body to cool from 60 degrees C to 50 degrees C?

10 min
5 min
20 min
15 min

Correct answer: 10 min

Solution

Using Newton's law of cooling in its average form: delta_T / t = k(T_avg - T₀). For the first interval: 20/10 = k*30, giving k = 1/15. For the second interval: 10/t = k*15 = 15/15 = 1, giving t = 10 min.

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