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The purpose of this lab is to use Maple to study applications of
exponential and logarithmic functions. These are used to model many
types of growth and decay.
The simple model for growth is exponential growth, where
it is assumed that
is proportional to . That is,
Separating the variables and integrating (see section 4.4 of the text),
In the case of exponential growth, we can drop the absolute value
signs around , because will always be a positive quantity.
Solving for , we obtain
which we may write in the form , where is an
arbitrary positive constant.
In a sample of a radioactive material, the
rate at which atoms decay is proportional to the amount of material present.
where is a constant. This is the same equation as in exponential growth,
except that replaces . The solution is
where is a positive constant. Physically, is the amount of
material present at .
Radioactivity is often expressed in terms of an element's half-life.
For example, the half-life of carbon-14 is 5730 years. This statement means
that for any given sample of
, after 5730 years, half of it
will have undergone decay.
So, if the half-life is of an element Z is years, it must be
, so that and
What is usually called Newton's law of cooling is a simple model for
the change in temperature of an object that is in contact with an
environment at a different temperature. It says that the rate of
change of the temperature of an object is proportional to the
difference between the object's temperature and the temperature of the
environment. Mathematically, this can be expressed as the differential
where is the constant of proportionality and
the temperature of the environment. Using a technique called
separation of variables it isn't hard to derive the solution
where is the temperature of the object at .
Information can be thought of as of a physical quantity which can be measured. According to the Gallup Institute, information news diffuses through a fixed adult population of size at a rate of time proportional to the number of people who have not heard the news.
If is the number of people who have heard the news after t days, then
The initial condition yields the solution
- Suppose that the population of a certain bacteria can be modeled by an exponential function. In a particular experiment, the number of bacteria was at . Four hours later, the bacteria was . Find the value of the growth constant and use it to predict the number of bacteria that would have been present after hours.
- Exponential growth can be used to model the growth of a certain kind of investments. Suppose that the value of an investment satisfies the differential equation
where is the interest rate. If the interest rate is per year and you start with an investment of $12,000, how many years does it take to double? How many years does it take to quadruple? Approximately how many years would it take to save a million dollars?
- A 5 lb. roast having initial temperature of
is put into a pre-heated oven at
. How much longer will it take for the roast to be medium rare (a temperature of
if the roast is
after 75 minutes.
- Suppose that 75% of a freshman class of 500 students on a campus heard about the market crash on Wall Street 3 days after it happened. How long will it take for 99% of the freshman to hear the news?
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Dina J. Solitro-Rassias