Parlor Tricks

1
Parlor Tricks

• by
• Robert Nemiroff
• Michigan Tech

2
Physics X About This Course

• Officially "Extraordinary Concepts in Physics"
• Being taught for credit at Michigan Tech
• Light on math, heavy on concepts
• Anyone anywhere is welcome
• No textbook required
• Wikipedia, web links, and lectures only
• Find all the lectures with Google at
• "Starship Asterisk" then "Physics X" 
• http//bb.nightskylive.net/asterisk/viewforum.php?
  f39

3
Newton's Cradle

• A set of balls clacks back and forth showing some
  kind of physics, but nobody is sure what.

4
Newton's Cradle

• Newton's Cradle shows
• conservation of momentum,
• conservation of energy, and
• a shock wave moving through the balls.
•  
• HOWEVER, the propagating shock wave disallows
  some clacking variations that also conserve both
  momentum and energy.

5
Rattlebacks

• Can an object spun in one direction stop spinning
  and then start spinning in the other direction?
•  
• 1. Yes, that seems OK to me.2. No, that does not
  conserve angular momentum.3. We're sorry, the
  number you have reached is not in service.

6
Rattlebacks

• 1. Yes, that seems OK to me.
• This occurs for a common type of "stone" called a
  rattleback.  It does not break any physical laws,
  including conservation of angular momentum.
•  
• You can order these over the Intertubes (tm).
•  
• Angular momentum is transferred between the table
  and the rattleback.

7
Rattlebacks

• Below is a YouTube video of a rattleback in
  action

8
Drinking Bird

• A toy that appears to be a perpetual motion
  machine.  A glass "bird" stands upright for a
  while, then tips over dipping its beak in a glass
  of water, then straightens up again.  Here is a
  movie

9
Drinking Bird

• In actuality , the drinking bird is a heat
  engine.  It is not a perpetual motion machine
  since it would stop as soon as the head stops
  being cooled by the water.
• The mechanism is actually pretty complex, but it
  hinges a liquid that evaporates quickly at (near)
  room temperature.  Since evaporation is a cooling
  process, and condensation is a heating process,
  temperatures shift slightly during the birds
  cycle.  The water -- even at room temperature,
  will cool the bird and add energy to the system.

10
Dropping a Magnet Through a Charged Disk

• You have a circular string on which electrically
  charged balls are affixed all around.  You drop a
  magnet through this string.  What happens?
• 1.  The string and balls begin to rotate in one
  direction, and the magnet rotates in the opposite
  direction.
• 2.  The string and balls begin to rotate in one
  direction, but the magnet does not rotate at all.
• 3.  The string and balls begin to rotate in one
  direction, and the magnet rotates in the same
  direction.
• 4.  Uncle Tesla calls and asks where you got this
  idea.

11
Dropping a Magnet Through a Charged Disk

•  
• Please don't be upset if you didn't get this
  right.
• You might take this opportunity to guess again. 
• Ready?

12
Dropping a Magnet Through a Charged Disk

•  
• 2.  The string and balls begin to rotate in one
  direction, but the magnet does not rotate at all.
• This situation is discussed in Feynman's book
  "Character of Physical Law" (page 78).  

13
Dropping a Magnet Through a Charged Disk

•  
• So how does this conserve angular momentum?
• 1.  It doesn't -- this is not a closed system.
• 2.  Considering the charge, the balls themselves
  actually carry no angular momentum.
• 3.  The electromagnetic field generated carries
  the opposite angular momentum to the rotating
  balls.
• 4.  Who cares?   Peculiar situations like this
  are irrelevant.

14
Dropping a Magnet Through a Charged Disk

•  
• 3.  The electromagnetic field generated carries
  the opposite angular momentum to the rotating
  balls.
• Fields can carry angular momentum.