Activity: Buffon's Needle

How to estimate Pi by dropping a match.

A few hundred years ago people enjoyed betting on coins tossed on to the floor: would the coin cross a line or not?

coin 1750

A man (Georges-Louis Leclerc, the Count of Buffon) started thinking about this and worked out the probability.

It is called "Buffon's Needle" in his honor.

Now it is your turn to have a go!

You will need:

buffons needle

A match, with the head cut off.
It must be less than 50 mm.

(You can use a needle, but be careful!)

buffons needle grid

A sheet of paper with lines 50 mm apart.

Steps

The exact height from which you drop the match is not important, but don't drop it so close to the paper that you are cheating!

If the match rolls completely off the paper, then do not count that turn.

100 Times

Now we will drop the match 100 times, but first ...

... what percentage do you think will land A, or B?

Make a guess (estimate) before you begin the experiment:

Your Guess for "A" (%):  
Your Guess for "B" (%):  

OK let's begin.

Drop the match 100 times and record A (does not touch a grid line) or B (touches or crosses a grid line) using Tally Marks:

match lands Tally Frequency Percentage
A
(no touch)
     
B
(crosses)
     
  Totals: 100 100%

Now draw a Bar Graph to illustrate your results. You can create one at Data Graphs (Bar, Line and Pie).

Now Let's Estimate Pi

Buffon used the results from his experiment with a needle to estimate the value of π (Pi). He worked out this formula:

π2Lxp

Where

We can do it too!

Example: Sam had a match of length 31 mm, and a 40 mm line spacing and 49 of 100 drops crossed the line

So Sam had:

Substituting these values into the formula, Sam got:

π2 × 3140 × 0.49  ≈ 3.16

Now it's your turn. Fill in the following table using your own results:

Length of match "L" (mm):  
Line Spacing "x" (mm):  
   
p (the proportion of needles crossing a line):  

And do the calculation:

π  ≈   2L xp   ≈   2 × _____ _____ × _____   ≈  _____

Did you do any better?

It won't be exact (because it is a random thing) but it may be close.

Changing The Subject

The next part of this activity is to "change the subject" of the formula to work out the perfect value of "p" (the proportion of times the match crosses the line):

Start with:π ≈ 2L/xp
multiply both sides by p:πp2L/x
divide both sides by π:p 2L/πx

And we get:

p  ≈   2L πx

Example: Alex had a match of length 36 mm, and a 50 mm line spacing.

So Alex had:

Substituting these values into the formula, Alex got:

p  ≈   2 × 36 π × 50   ≈  0.46...

So Alex should expect the match to cross the line (case B) 46 times out of 100

Fill in the following table using your own results:

Length of match "L" (mm):  
Line Spacing "x" (mm):  
   
Estimate for p (≈ 2L/πx):  

How close were you?

Different Size of Match

Try repeating the experiment using a different sized match (but not larger then the line spacing!)

What You Have Done

You have (hopefully) had fun running an experiment.

You have had some experience with calculations.

And you have seen the relationship between theory and reality.