encourage the audience to watch carefully whether
the stationary magnets change position. Tell them
to listen to the sound as the magnet collides. People
are not normally observant of small details unless
told what to look for. A little practice will give you the
“feel” for the best starting speed.
When I first played with this toy some years ago,
I didn’t discover this neat magic trick because I was
always being careful to avoid collisions of the magnets, thinking I might break them. So my caution
prevented me using a higher speed and discovering
how strong the illusion is when done forcefully.
How It Works
The magnets do exchange positions as they collide,
but too quickly to observe. Put a bit of colored tape
on each one to verify this if you doubt it (Figure D).
Each collision and exchange is completed without
substantially disturbing the others (this is why we
begin with the magnets separated enough that they
don’t influence each other much). In each collision,
the conservation of momentum and energy ensure
that the moving magnet stops and the previously
stationary magnet achieves the same velocity as
the one that collided with it. Then the next collision
occurs, and so on, down the line.
Illustrations by Donald Simanek; engraving from Scientific American
This also explains why a larger gap between magnets is preserved, wherever you make the gap, and
whichever direction the magnets move. I marked
the initial positions of magnets on the rod with a
pencil, and found that their final positions are often
less than 1 millimeter from the initial positions.
The similarity between this magnet demonstration
and the classic toy Newton’s Cradle is striking. In
this toy, 5 suspended metal balls are initially barely
touching. When 1 ball is pulled back and released,
it collides and comes to rest, and 1 ball at the other
end is propelled away at the same speed as the initial
moving ball (Figure E). If 2 balls are pulled back and
released, then 2 balls at the other end are propelled
away. It works for any number of balls.
How does the system know how many balls to
eject? Conservation of momentum and energy are
responsible, as well as the fact that the balls have
spherical shape, and equal mass and size.
Just because the behavior of my Penetrating
Magnets Illusion is something like the classic
Newton’s Cradle doesn’t mean the two are alike.
Newton’s Cradle doesn’t have friction, and its balls
do not exchange positions. The magnets do.
B
C
D
E
Fig. A: This version of the Penetrating Magnets
Illusion uses a plastic curtain rod. The magnets are
colored for visibility. Fig. B: The left magnet seems
to pass right through the others. Fig. C: The irregular spacing is preserved. Fig. D: Colored magnets
show what really happened. Fig. E: An engraving
of Newton’s Cradle from a 19th century issue of
Scientific American.
Donald Simanek is emeritus professor of physics at Lock
Haven University of Pennsylvania. He writes about science,
pseudoscience, and humor at www.lhup.edu/~dsimanek.
Make: 167
