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# A body cools down from 65°C to 60°C in 5 minutes. It will cool down from 60°C to 55°C inA. 5 minutesB. less than 5 minutesC. more than 5 minutesD. less than or more than 5 minutes depending on whether its mass is more than or less than 1 kg.

According to Newton’s Law of Cooling:

Here is the rate of fall of temperature is the constant depending on nature of surface and surrounding conditions, A is the area of cross-section of the body, T is the average Temperature of the body and T0 is the surrounding temperature.
It can also be written in Celsius scale as

Where θ and θ0 are the temperature of the body and surrounding in Celsius scale respectively , k is a constant.
First condition:
Temperature falls from 65° C to 60° C.
Which means 5°C falls in 5 minutes,

θ =
62.5 ° C
Substituting we get,

Second Condition:
Temperature further falls from 60 °C to 55 °C.
dθ= 5 °C.
θ =

Substituting value of -k,dθ and θ in the equation we get,

Here we can conclude that time required for the body to go from 60 ° C to 55° C will be,

Thus, option (c) is the correct option.

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