Question

Newton's law of cooling states that the temperature of an object changes at a rate proportional to the difference between its temperature and that of its surroundings. Suppose that the temperature of a cup of coffee obeys Newton's law of cooling. If the coffee has a temperature of 185 degrees Fahrenheit when freshly poured, and 3 minutes later has cooled to 167 degrees in a room at 60 degrees, determine when the coffee reaches a temperature of 137 degrees.

Answer 1

Answer:

9.4 secs

Step-by-step explanation:

From Newton's law of cooling;

T(t) = Ts + Do e^-kt

Where;

D0= initial temperature difference

Ts= Temperature of the surroundings

t= time

K = positive constant

Do = 185 - 60 = 125 degrees

167 = 60 + 125 e^-3k

167 - 60 = 125 e^-3k

107/125 = e^-3k

ln(e^-3k) = ln(107/125)

-3k = -0.1555

k = 0.1555/3

k = 0.0518

Substituting the value of k to find the time taken to reach 137 degrees

T(t) = Ts + Do e^-kt

137 = 60 + 125 e^-(0.0518t)

137 - 60 = 125 e^-(0.0518t)

77 = 125 e^-(0.0518t)

e^-(0.0518t)= 77/125

ln [e^-(0.0518t)] = ln(77/125)

-0.0518t = -0.4845

t = 0.4845/0.0518

t = 9.4 secs