Question

Mr. Ratchett, an elderly American, was found murdered in his train compartment on the Orient Express at 7 AM. When his body was discovered, the famous detective Hercule Poirot noted that Ratchett had a body temperature of 28 degrees. The body had cooled to a temperature of 27 degrees one hour later. If the normal temperature of a human being is 37 degrees and the air temperature in the train is 22 degrees.
Required:
Estimate the time of Ratchett's death using Newton's Law of Cooling.

Answer 1

Answer: Mr. Ratchett died approximately at 2AM.

Step-by-step explanation: Newton's Law of Cooling shows the cooling rate between a body and the environment, by stating that the rate is proportional to the difference in temperatures between them.

Mathematically, it is represented as:

$\frac{dQ}{dt}=\alpha.A.(T_{s}-T)$

This differential equation solved, gives the solution:

$T(t)=T_{s}+(T_{0}-T_{s})e^{-kt}$

where

$T_{s}$ is temperature of the environment

$T_{0}$ is the initial temperature of the body

k is a parameter dependent of heat transfer coefficient, heat capacity and area of the body

t is time required to change the temperature

For the murder on the Orient Express, first determine parameter k.

In 1 hour, Mr. Ratchett's body decrease 1°:

$27=22+(28-22)e^{-1k}$

$5=6e^{-k}$

$e^{-k}=\frac{5}{6}$

$ln(e^{-k})=ln(0.834)$

k = 0.1823

Using the parameter, find estimate time:

$28=22+(37-22)e^{-0.1823t}$

$6=15e^{-0.1823t}$

$ln(e^{-0.1823t})=ln(0.4)$

0.1823t = 0.9163

$t=\frac{0.9163}{0.1823}$

t ≈ 5 hours

The body was found at 7AM. So, it took approximately 5 hours to cool down to 28° at that time. Therefore, Mr. Ratchett died at 2AM.