BITSAT 2011 Paper

By Newton's law of cooling,

θ1−θ2

t

= - k [

θ1+θ2

2

−θ0] ... (1)
A sphere cools from 62°C to 50°C in 10 min.

62−50

10

= - k [

62+50

2

−θ0] ... (2)
Now, sphere cools from 50°C to 42°C in next 10 min.

50−42

10

= - k [

50+42

2

−θ0] ... (3)
Dividing eqn. (2) by (3) we get,

56−θ0

46−θ0

or 0.4θ0 = 10.4
Hence θ0 = 26°C