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# 6.2: A Modeling Example: Counting Ping Pong Balls

Difficulty Level: At Grade Created by: CK-12

Suppose you have a cylinder of height h\begin{align*}h\end{align*} with base diameter b\begin{align*}b\end{align*} (perhaps an empty pretzel jar), and you wish to know how many ping-pong balls of diameter d\begin{align*}d\end{align*} have been placed inside the cylinder. How could you determine this? This problem, along with the strategy for computing the lower bound on the number of ping-pong balls, is adapted from Starfield (1994).

A lower bound for this problem is found as follows. Define the following variables:

• NL\begin{align*}N_L-\end{align*}Lower bound on the number of balls that fit into the cylinder.
• Vcyl\begin{align*}V_{cyl}-\end{align*}The volume of the cylinder.
• Vcube\begin{align*}V_{cube}-\end{align*}The volume of a cube that encloses a single ball.

VcylVcube=hπ(b2)2=d3\begin{align*}V_{cyl} & = h\pi(\tfrac{b}{2})^2 \\ V_{cube} & = d^3\end{align*}

The lower bound NL\begin{align*}N_L\end{align*} is found by dividing the volume of the cylinder by the volume of the cube enclosing a single ball:

NL=VcylVcube\begin{align*}N_L = \tfrac{V_{cyl}}{V_{cube}}\end{align*}

Exercise 11

You are given the following values:

• d=1.54in\begin{align*}d = 1.54\;\mathrm{in}\end{align*}
• b=8in\begin{align*}b = 8\;\mathrm{in}\end{align*}
• h=14in\begin{align*}h = 14 \;\mathrm{in}\end{align*}

Type commands at the command line prompt to compute NL\begin{align*}N_L\end{align*}.

Exercise 12

Create an m-file to solve Exercise 11.

To complicate your problem, suppose that you have not been given values for d\begin{align*}d\end{align*}, b\begin{align*}b\end{align*}, and h\begin{align*}h\end{align*}. Instead you are required to estimate the number of ping pong balls for many different possible combinations of these variables (perhaps 50\begin{align*}50\end{align*} or more combinations). How can you automate this computation?

One way to automate the computation of NL\begin{align*}N_L\end{align*} for many different combinations of parameter values is to use a for loop. The following exercises ask you to develop several different ways that for loops can be used to automate these computations.

Exercise 13

Add a for loop to your m-file from Exercise 12 to compute NL\begin{align*}N_L\end{align*} for b=8in\begin{align*}b = 8 \;\mathrm{in}\end{align*}, h=14in\begin{align*}h = 14 \;\mathrm{in}\end{align*}, and values of d\begin{align*}d\end{align*} ranging from 1.0in\begin{align*}1.0 \;\mathrm{in}\end{align*} to 2.0in\begin{align*}2.0 \;\mathrm{in}\end{align*}.

Exercise 14

Modify your m-file from Exercise 13 to plot NL\begin{align*}N_L\end{align*} as a function of d\begin{align*}d\end{align*} for b=8in\begin{align*}b = 8 \;\mathrm{in}\end{align*} and h=14in\begin{align*}h = 14 \;\mathrm{in}\end{align*}.

Exercise 15

Modify your m-file from Exercise 13 to compute NL\begin{align*}N_L\end{align*} for d=1.54in\begin{align*}d = 1.54 \;\mathrm{in}\end{align*} and various values of b\begin{align*}b\end{align*} and h\begin{align*}h\end{align*}.

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