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String and Wood (Posted on 2005-08-08) Difficulty: 3 of 5
You have a block of wood 1 by 2 by 7 units. One of the 2 by 7 faces has 2 nails inserted, the heads and part of the shaft of each nail protruding. If the coordinates of the corners of this face are: (0,0), (0,2), (7,2), and (7,0), then the nails are located at (1,1) and (6,1).

Assume the coefficient of friction between string and wood is zero, and that the diameter of the nails is negligible.

(1) The ends of a non-elastic string of length 13 units are attached, one end to each of the 2 nails. The string is taut. How is this possible?

(2) What about a second piece of taut string, approximately 8.544 units long, also with ends attached to the 2 nails?

(3) What about a third piece of string approximately 22.0880 units long?

  Submitted by Larry    
Rating: 3.0000 (1 votes)
Solution: (Hide)
Consider the block resting on a table. The Top and Bottom sides measure 2 by 7, and the Top side has the 2 nails inserted. The North and South sides each measure 1 by 7. The East and West sides each measure 1 by 2. Consider the trajectory of the string as running from the Red Nail, at (1,1) along the various faces and ending at the Blue Nail (6,1). Both nails are on the Top face.

(1) The 13 unit string “spirals” around the block twice, running from faces: Red nail, N, Bottom, S, TOP, N, Bottom, S, Blue Nail

(2) The 8.544 unit string (square root of 73) runs from: Red nail, N, Bottom, E, Blue Nail

(3) The 22.0880 unit string ((2*square root of 73) + 5) runs from: Red nail, N, Bottom, E, Top (passing by the Blue nail), S, Bottom, W, Red nail (where it makes a sharp bend, around the protruding nail), then to Blue nail. I said the string was taut, not necessarily a straight line.

Comments: ( You must be logged in to post comments.)
  Subject Author Date
re: Part 2 RevisitedCharley2005-08-10 03:16:14
Alternate Part 1Charley2005-08-10 02:29:33
Lemming Nailed it.Larry2005-08-08 19:48:47
SolutionPart 3Leming2005-08-08 16:07:53
Some ThoughtsPart 2Leming2005-08-08 15:41:58
Some ThoughtsPart 1Leming2005-08-08 15:28:59
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