1Q40.50 - Action-Reaction Demo - Pocket Watch - Tail Wags the Dog

See first paragraph of procedure.
See second paragraph of procedure.
Code Number:
Demo Title:
Action-Reaction Demo - Pocket Watch - Tail Wags the Dog
Action & Reaction
Area of Study:
Wind up pocket watch or stop watch with attached small mirror, Laser or laser pointer, thread suspension if desired.

The pocket watch should have the mirror already attached at the appropriate point.  Balance the watch on the watch glass so that it sits level. Shine the laser onto the mirror so that it reflects from a point nearest the center of the watch.  The reflected laser spot should be directed at a screen across the lecture room.  As the watch ticks, the movement of the balancing wheel will make the watch move in the opposite direction.  This will be apparent as the laser spot will move up to 4 inches with each "tick".

The watch that can be hung by the strings will be more sensitive to movements than the one balanced on the watch glass.  However, it also takes an appreciable amount of time to settle down if the table is moved or bumped. 

  • David Kagan, "Detecting the Hard Drive in an iPod", TPT, Vol. 43, # 8, Nov. 2005, p. 551.
  • Jeffrey M. Wetherhold, "Modeling a Spacecraft's Malfunction", TPT, Vol. 37, #4, Apr. 1999, p. 196.
  • Gerge Barnes, "Fun with a "Power" - Return Measuring Tape", TPT, Vol. 23, # 9, Dec. 1985, p. 551.
  • R. W. Pohl, "(2)", Physical Principles of Mechanics and Acoustics, p. 258.
  • Charles Vivian, "Watch Your Pulse Work", Science Experiments & Amusements For Children, p. 95.
  • Martin Gardner, "Pulse Detector", Entertaining Science Experiments with Everyday Objects, p. 25.
  • "A Mysterious Watch", The Boy Scientist, p. 226.
  • Alistair Lightfoot, "Tap-L Discussions", 1/12/05.
  • Julius Sumner Miller, Q161 & A161, Millergrams II – Some More Enchanting Questions for Enquiring Minds, p. 36 & 94.

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