The Pinball Machine

pinball machine

The picture to the right shows a type of pinball machine that you can build yourself. You will need 10 nails, 5 small cups, a wooden board and a pinball (marble). Nail the nails part way into the board in the triangular pattern shown, with one nail in the top row, two in the second row, three in the third row and so on, and with enough space for the pinball to fit between the nails.

To operate the machine release the pinball so that it hits the top nail dead center. The pinball will be deflected either left or right with equal probability by the first nail. It will then continue falling and hit one of the nails in the second row and be deflected either left or right around that nail with equal probability, and so on.

The result is that the pinball follows a random path, deflecting off one pin in each of the four rows of pins, and ending up in one of the cups at the bottom. The various possible paths are shown by the gray lines and one particular path is shown by the red line. Click here to see an animation of a similar machine called the Galton Box.

Let's describe the path shown in red using the notation "LRLL" meaning "deflection to the left around the first pin, then deflection right around the pin in the second row, then deflection left around the third and fourth pins".

Question: How many different paths are there through the pinball machine and what are they?

Answer: There are 16 different paths through the machine. The list of paths is:


Click here for an explanation.

Question: How many paths are there that end up in any given bin?

To answer this question we have to sort the list of the 16 different paths according to how many R's each path contains:

Number of R's = 0: { LLLL }
Number of R's = 1: { LLLR   LLRL   LRLL   RLLL }
Number of R's = 2: { LLRR   LRLR   LRRL   RLLR   RLRL   RRLL }
Number of R's = 3: { LRRR   RLRR   RRLR   RRRL }
Number of R's = 4: { RRRR }
paths per bin If the number of R's = 0 then the pinball ends up in the first (leftmost) bin, (1 path leads here)

If the number of R's = 1 then the pinball ends up in the second bin, (4 paths lead here)

If the number of R's = 2 then the pinball ends up in the third bin, (6 paths lead here)

If the number of R's = 3 then the pinball ends up in the fourth bin, (4 paths lead here)

If the number of R's = 4 then the pinball ends up in the last (rightmost) bin, (1 path leads here).

The result is as shown in the picture to the right.

Question: What is the probability that the pinball will end up in any given bin?

There are 16 paths and each path is equally likely so each path has a 1 in 16 chance of being followed. Thus the probabilities are:
  • probability = 1/16 to end up in first (leftmost) bin
  • probability = 4/16 to end up in second bin
  • probability = 6/16 to end up in third bin
  • probability = 4/16 to end up in fourth bin
  • probability = 1/16 to end up in fifth (rightmost) bin

A histogram of these values is shown to the right. If we drop many pinballs through the machine and let them pile up in the bins then over the long run they will be distributed as shown in the histogram to the right. This is an example of the binomial distribution which is studied in probability. The bell curve or normal distribution is based on this distribution.

pascal's triangle

Pascal's Triangle

The object shown to the right is known as Pascal's Triangle. (Blaise Pascal, its discoverer, was born in France and died in 1662 at the age of 39. The Pascal programming language is named after him.)

Pascal's triangle is very useful for analysing the pinball machine. Pascal's triangle also pops up in a variety of other seemingly unrelated areas, as we shall soon see.

First we mention that Pascal's triangle continues on forever and we have only shown the first 5 rows. Can you see the pattern and guess what the next row of numbers is?

pascal's triangle animation

The next row is shown in the animation to the right. The pattern for each row of the triangle is:

  • the left and right ends always equal 1
  • each internal element is the sum of the two elements above left and above right, like this:
    next element

If we superimpose Pascal's triangle on top of the pinball machine then we can see a connection between the two: Each number of Pascal's triangle is equal to the number of distinct paths that a pinball can take to arrive at that point in the pinball machine. (The very way the pinball machine is constructed guarantees this: Any point on the left edge can only be reached by 1 path and the same for any point on the right edge. Any interior point can only be reached from the above-left point or the above-right point, so adding their path numbers together gives the number of paths leading to the interior point.)

paths in pascal's triangle

The problem with this method of constructing Pascal's triangle is that to get the numbers for any given row we must construct all the rows above it.

But there is another way to get all the numbers in Pascal's triangle at once. To do it we must think of the various paths through the pinball machine as arrangements of L's and R's. The mathematics of arrangements involves factorials, permutations and combinations. Click here to learn about those.

If you clicked the link above then you learned about combinations and the combination function:

Definition: A combination is a selection of k items from a set that has n members, such that the order of selection does not matter (unlike permutations where the order does matter).

Definition: The combination function nCk counts the number of possible combinations of k items selected from a set of n members.


Typical combination question: Suppose that we have 4 people, call them A, B, C and D, and we want to select 2 of them at random to work together on a project. So we put the 4 names in a hat and randomly select 2 names. We don't care about the selection order – selecting A, then B is the same as selecting B, then A. How many different outcomes are there and what are they?

Answer: This is a combination problem because the selection order does not matter. The set consists of n=4 members and we are selecting k=2 of them. The combination function gives

So the number of possible combinations or teams is 6. They are:

{A and B}, {A and C}, {A and D}, {B and C}, {B and D}, {C and D}.

To see how combinations can describe paths in the pinball machine consider how a pinball can arrive at the point circled in gray. It has to be deflected by 4 pins and it has to go right 2 times. This is just like the combination question above if we put slips of paper with the numbers 1 to 4 in a hat and randomly select 2 slips. Let these slips tell us where we will deflect to the right. For example if we select the numbers 1 and 4 this means we will follow the path RLLR. From the above question we know that there are 4C2=6 combinations so this entry in Pascal's triangle should also be 6.

Thus Pascal's triangle is essentially a listing of all the possible values of the combination function!

combination triangle

The Binomial Theorem: It is perhaps surprising that the combination function can be used to expand an expression like (a+b)4. Here is the expansion:

The second row shows how the combination function is involved. It appears here because for example the term a3b comes from multiplying one b from inside one of the brackets and three a's from inside the other three brackets. There are 4C1=4 ways of selecting which bracket the b should come from.

The blue arrows show one of these ways:

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