Randy Scalise generated this in response to a e-mail question, but we though it would be of use to include on the web page. --------------------------------------------------------------------------- Dear Mr. Willis, This is Randy Scalise from SMU Physics. I was the one who performed the cantilevered blocks demonstration. Use 7 identical blocks. The trick is to start at the top. Align each block (BBBBBBBB) with the edge of the table (TTTTTTTT). BBBBBBBBBBBB BBBBBBBBBBBB BBBBBBBBBBBB BBBBBBBBBBBB BBBBBBBBBBBB BBBBBBBBBBBB BBBBBBBBBBBB TTTTTTTTTTTT The top block can be pulled half way off the second block BBBBBBBBBBBB BBBBBBBBBBBB BBBBBBBBBBBB BBBBBBBBBBBB BBBBBBBBBBBB BBBBBBBBBBBB BBBBBBBBBBBB TTTTTTTTTTTT The top two blocks can be pulled as a unit 1/4 off the third block BBBBBBBBBBBB BBBBBBBBBBBB BBBBBBBBBBBB BBBBBBBBBBBB BBBBBBBBBBBB BBBBBBBBBBBB BBBBBBBBBBBB TTTTTTTTTTTT This is a great time to ask the class to guess the next fraction in the pattern. About half will guess "1/8" seeing the pattern as inverse powers of two. The correct answer is "1/6" and the pattern then emerges as inverse even integers. The top three blocks can be pulled as a unit 1/6 off the fourth block BBBBBBBBBBBB BBBBBBBBBBBB BBBBBBBBBBBB BBBBBBBBBBBB BBBBBBBBBBBB BBBBBBBBBBBB BBBBBBBBBBBB TTTTTTTTTTTT Continue. Move the top four blocks as a unit 1/8 off the fifth block. Move the top five blocks as a unit 1/10 off the sixth block. Move the top six blocks as a unit 1/12 off the seventh block. Move all the blocks as a unit 1/14 off the table. 1/2 + 1/4 + 1/6 + 1/8 + 1/10 + 1/12 + 1/14 = 1.29643 > 1 In theory, four blocks would be enough (1/2 + 1/4 + 1/6 + 1/8 = 1.04 > 1), but in practice at each stage you must back off from the ideal overhang to avoid collapse. Now is a good time to ask the class to calculate in theory how many blocks would be required to get the top TWO blocks off the edge of the table (the answer is 31). You can also introduce the harmonic series (1 + 1/2 + 1/3 + 1/4 + ...) at this point. Our series is one half times the harmonic series. The harmonic series diverges, that is, the sum is infinite. As a consequence, one can cantilever any number of blocks past the edge of the table, but the total number of blocks in the vertical stack gets large VERY quickly. To cantilever three blocks past the edge of the table requires a stack of 227 blocks. To cantilever four blocks past the edge of the table requires a stack of 1674 blocks. Feel free to write or call if I can help further. --Dr. Randall J. Scalise Senior Lecturer in Physics ,____________________________________________________________________. | Office: +1(214)768-2504 | Department of Physics | | Dept: +1(214)768-2495 | 104 Fondren Science Building | | FAX: +1(214)768-4095 | 3215 Daniel Avenue | | Email: scalise@phys.psu.edu | Southern Methodist University | | http://www.phys.psu.edu/~scalise | Dallas, TX 75275-0175 | `--------------------------------------------------------------------'