Activity 3.3: Billiard Paths
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Your Challenge: See if you can come up with a way to predict:
  • what the number of touches will be for any given size billiard table 
  • which corner the ball will end up in. 


 

Let's look at determining the number of touches first.
 
W
L
Touches
5
7
 12
8
3
 11
5
4
 9
10
13
 23
 5
 9
 14
 6
 9
 5 (surprise!)
     

Why 5 and not 15? Look at the path:

6 by 9 table

Here are two other paths that also have 5 "touches"


4 by 6 table


2 by 3 table

Note that for the smallest of the three tables the "adding the length and width rule" once again works. The tables have the same path because they are geomtetrically similar which means that their sides are in poroportion to each other. Another way of thinking about this is that the ratios of the dimensions are all equal. So if the dimensions are relatively prime, the path will traverse every square and the number of touches will be the sum. Otherwise, find the the smallest rectangle that will have the same path. The sum of its dimensions will give you the sum.

Part B - finding which corner the ball ends up in also has to do with reduced form rectangles. Similar paths will always end in the same corner. so the investigation should be done with paths of relatively prime dimensions.
 
W
L
Touches
Corner?
5
7
 12
Top Right (UR)
8
3
 11
Top Left (TL)
5
4
 9
Bottom Right (BR)
10
13
 23
Top Left (TL)
 5
 9
 14
Top Right (UR)
 6
 9
 
Top Left (TL)*
 7
 10
17 
Bottom Right (BR)

Note that the two odd, relatively prime dimensions both end up in the upper right or opposite corner. If the width is even and the length is odd and they are relatively prime, the width "dominates" the path and it ends up in the Top Left corner. If the opposite is true the, then the length dominates and the ball ends up in the Bottom Right corner. Does this always work?

If the dimensions are even, which corner will welcome the ball? (It could wind up in any of the three corners. Two even numbers are NOT relatively prime.)

Why can't the ball return to the Bottom Left corner?

Does switching length and width numbers change the final destination? (For example, does the ball end up in the same corner for a 7 by 5 table as a 5 by 7 table? (NO!))

*reduce to 2 by 3.
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