1)

A body cools from 50.0ºC to 49.9ºC in 5s. How long will it take to cool from 40.0ºC to 39.9ºC? Assume the temperature of surroundings to be 30.0ºC and Newton's law of cooling to be valid.


A) 2.5 s

B) 10 s

C) 20 s

D) 5 s

Answer:

Option B

Explanation:

$\frac{50-49.9}{5} = K\left(\frac{50+49.9}{2}-30\right)$ ....(i)

$\frac{40-39.9}{t} = K\left(\frac{40+39.9}{2}-30\right)$ .....(ii)

From equations (i) and (ii), we get t ≈ 10 s