Demos: 1J-16 Walk the Plank

A long plank is placed on the lecture table with one end protruding beyond the edge of the table. The instructor (or student) can walk to the end of the plank without the board tipping. A nice illustration of the concept of center of gravity.

Directions: Different marks are placed on the board for a variety of possible weights. For example, if you weigh 180 lbs, a mark for that weight (or close) will be on the board. Place that mark exactly on the edge of the table. The distance from that mark to the center of gravity of the board is X. The distance from the mark to the center of the instructor’s feet is x. By taking torques about the edge of the table, we have



One can solve for either M or m, if the other quantity is known.

Suggestions for Presentation: Ask if anyone would like to “walk the plank.” (Or ask if they would like to see you do it. For safety reasons, it is better if you do it.) Will the board tip if you walk to the end? What will determine that? (If set properly, you can get to the end of the board and have it just start to tip on the opposite end. Adds to the drama.

Applications: Stability of overhanging objects.

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