# How do we use exponential growth and decay in real life?

##### 1 Answer

One of the "home grown" examples is a gradual diminishing of a temperature of a hot body down to room temperature. For instance, the kettle with boiling water cools down after the heater is turned off.

The physical law describing this process of cooling sounds like this:

*The speed of a cooling of a hot object is proportional to a difference in temperature between the cooling body and environment, assuming that the environment is large enough to absorb the heat without really changing its own temperature*.

In other words, the cooling is faster if the difference in temperatures between the object and the environment is greater and the speed of cooling diminishes to zero as the temperature of a hot object gradually diminishes to a temperature of an environment

Here is the reason why it leads to exponential decay.

Let the temperature of a hot body is a function of time

The difference between the temperature of a body and an environment is

The speed of cooling is, obviously, a derivative of a function

The physical law of changing the temperature of a cooling body mentioned above now looks like

where

Basically, what we have above is a differential equation (physics, as you know, is all about differential equations).

Its solution is

Indeed, the derivative of this function

But

Therefore,

So, the process of cooling of a kettle after the heat is off is a good example of an exponential decay.

This example prompts to a conclusion that every process with a speed of change proportional to its value exhibits the exponential dependency.

Another typical example is a population grows. Obviously, the speed of a population's grows (that is, the increment of the number of species), generally speaking, should be proportional to the number of species.