Buffon’s needle
The plane is ruled by parallel lines inches apart and a -inch long needle is dropped at random on theplane. What is the probability that it hits parallel lines?
Solution.
The first issue is to find some appropriate probability space . For this,
- •
distance from the center of the needle to the nearest line
- •
the angle that the needle makes with the horizontal ranging from to .
These fully determine the position of the needle. Let us next take the
- 1.
The probability space is
- 2.
The probability of an event is denoted by is equal to
Now we denote by the event that the needle hits a horizontal line. It is easily seen that this happenswhen . Consequently and then we get
In general case, when the length of needle is and the distance of parallel lines is provided that , the probability we want is . This is obvious just taking the -point from one edge instead of the center of the needle.