Newtons Law - Cooling, 2

Category: Chemical Engineering Math, Differential Equations, Algebra

"Published in Newark, California, USA"

The body of a murder victim was discovered at 11:00 pm. At 11:30 pm, the victim's body temperature was measured to be 94.6 °F. After 1 hour, the body temperature was 93.4 °F. The room where the body was found at a constant temperature of 70 °F. Assuming that Newton's Law of Cooling is applicable and assuming that the normal human body temperature is 37 °C, determine the time of death.

Solution:

This is a great application of Newton's Law of Cooling. Mostly the Federal Bureau of Investigation (FBI) and the police officers in United States of America are using this method to calculate the time of a death or murder. Even in other countries like Philippines, Canada, Mexico, Japan, Italy, China, and so on are using this method, too. Let's analyze the given word problem as follows

Let u = be the temperature of a dead body
      t = be the time of death
      k = constant of cooling/heating

According to Newton's Law, the time rate of change of temperature is proportional to the temperature difference. 
 

      
The value of k is negative because it is a cooling process. When k is positive, then it is a heating process. The temperature of the surrounding is always a constant which is 70 °F. Solve for the general solution of the above equation, we have
 

     
Integrate on both sides of the equation
 

 

       
Take the inverse natural logarithm on both sides of the equation
 

 

       
but e^C is still a constant. The above equation becomes
 

       
To solve for the values of k and C, we need to get the values of the limits as follows

If u₁ = 94.6 °F, then t₁ = 0 (measured at 11:30 pm)
   u₂ = 93.4 °F, then t₂ = 1.0 hr (measured at 12:30 am)

Substitute the first limit to the above equation to solve for the value of C
 

 

 

 
Therefore,
 

Substitute the second limit to the above equation to solve for the value of k

 

 
Take natural logarithm on both sides of the equation

Therefore,
 

   
Next, we need to use the normal temperature of a body (u = 37 °C = 98.6 °F) in order to calculate the time of death of a person as follows
    

   

Take natural logarithm on both sides of the equation
             

              

or

Since the value of time is negative, then we have to subtract it from the time where a dead body was first measured, which is at 11:30 pm, as follows

         11:30        ------>  11:30:00    ------->   11:29:60
         - 3:00:48              - 3:00:48                - 3:00:48
                                                                ---------------
                                                                     8:29:12

Therefore, a person is dead at 8:29:12 pm.

Square Pyramid Problems

Category: Solid Geometry, Plane Geometry

"Published in Newark, California, USA"

A vessel is in the form of an inverted regular square pyramid of altitude 9.87 in. and a base edge 6.27 in. The depth of the water it contains is 6 in. How much will the surface rise when 1 pint of water is added? (One gallon = 231 cubic inches) Find the wetted surface when the depth of the water is 9.23 in. 

Solution:

To illustrate the problem, let's draw the figure and label as follows

Photo by Math Principles in Everyday Life

The volume of the empty vessel is calculated as follows

When the depth of the water is 6 in, the volume is calculated by ratio and proportion as follows

When 1 pint of water is added, the volume in cubic inches is

Therefore, after the addition of 1 pint of water, the height of water can be calculated by ratio and proportion as follows

The difference of the height of water after the addition of 1 pint of water is

When the depth of the water is 9.23 in, the length of a square base is calculated by ratio and proportion as follows

By Pythagorean Theorem, the slant height will be equal to

Photo by Math Principles in Everyday Life

Therefore, the wetted surface of a vessel is 

Angle - Elevation Problem

Category: Trigonometry, Plane Geometry, Algebra

"Published in Newark, California, USA"

A tower of height h stands on level ground and is due north of point A and due east of point B. At A and B, the angles of elevation of the top of the tower are α and β, respectively. If the distance AB is c, show that 

Photo by Math Principles in Everyday Life

Solution:

In the given figure above, there are three right triangles involved. Let's solve for the height h in terms of α, β, and c as follows

By Pythagorean Theorem

Take the square root on both sides of the equation, we have

Therefore,