Learning Outcome

• Solve application problems involving kinetic energy, volume, and free-fall

Kinetic Energy

One way to measure the amount of energy that a moving object (such as a car or roller coaster) possesses is by finding its kinetic energy. The kinetic energy ($E_{k}$, measured in Joules) of an object depends on the object’s mass (m, measured in kg) and velocity (v, measured in meters per second) and can be written as $v=\sqrt{\frac{2{{E}_{k}}}{m}}$.

Example

What is the kinetic energy of an object with a mass of $1,000$ kilograms that is traveling at $30$ meters per second?

Here is another example of finding the kinetic energy of an object in motion.

Volume

Harvester ants found in the southwest of the U.S. create a vast interlocking network of tunnels for their nests. As a result of all this excavation, a very common above-ground hallmark of a harvester ant nest is a conical mound of small gravel or sand [1]

The radius of  a cone whose height is is equal to twice its radius is given as $r=\sqrt[3]{\frac{3V}{2\pi }}$.

Example

A mound of gravel is in the shape of a cone with the height equal to twice the radius. Calculate the volume of such a mound of gravel whose radius is $3.63$ ft. Use $\pi =3.14$.

Here is another example of finding volume given the radius of a cone.

Free-Fall

When you drop an object from a height, the only force acting on it is gravity (and some air friction) and it is said to be in free-fall. We can use math to describe the height of an object in free fall after a given time because we know how to quantify the force of Earth pulling on us – the force of gravity.

An object dropped from a height of 600 feet has a height, h, in feet after t seconds have elapsed, such that $h=600 - 16{t}^{2}$. In our next example we will find the time at which the object is at a given height by first solving for t.

Example

Find the time it takes to reach a height of 400 feet by first finding an expression for t.

Analysis of the Solution

We have made a point of restricting the radicand of radical expressions to non-negative numbers. In the previous example, we divided by a negative number then took the square root to solve for t.  In this example, is it possible to get a negative number in the radicand? In other words, for what values for height would we have an issue where we may be taking the square root of a negative number? We can use algebra to answer this question.

Let us translate our question into an inequality. Again, for what values of h would we get a negative quantity under the radical?

The radicand is $\frac{h-600}{-16}$, so if we set up an inequality we can solve it for h:

$\begin{array}{ccc}\frac{h-600}{-16}\lt0\\-16\cdot\frac{h-600}{-16}\lt0\cdot{-16}\\h-600\gt0\\h\gt600\end{array}$

We can interpret this as “when the height is greater than $600$ ft. the radicand will be negative and therefore not a real number.” If you re-read the question, you will see that heights greater than $600$ do not even make sense, because the object starts at a height of 600 feet and is falling toward the ground, so height is decreasing. Understanding what domain our variables have is important in application problems so we can get answers that make sense.

1. Taber, Stephen Welton. The World of the Harvester Ants. College Station: Texas A & M University Press, $1998$.