A linear accelerator for studying high-energy physics costs around
$5 billion. But you can make one for about 30 bucks with four strong
magnets, a wooden ruler, some plastic tape, and nine steel balls.
This easy project demonstrates the transfer of kinetic energy from one object to another. More importantly,
it also shoots a steel ball really fast at the target of your choice. When each ball strikes the magnet in
front of it, its kinetic energy is transferred to the next ball down the line. By the time the fourth ball shoots
off the ruler, it possesses almost four times the energy of the first ball, which means it’s moving faster,
too. (The speed increase is proportional to the square root of the increase in kinetic energy.) This project
takes just a few minutes to build once you have the parts, which can be ordered from
One 12" wooden
ruler with a groove
running through it
Nine 5/8" diameter
Scotch tape and
an X-Acto knife
Tape the ruler to a table or floor,
then tape the first magnet 2½"
from the end of the ruler. Tape
the other three magnets 2½"
inches apart from each other.
(Don’t let the magnets bang into
each other because they
Trim the excess tape from each magnet with
the X-Acto knife. Put two steel balls against
each of the four magnets as shown here. Put an
object (like a plastic cup) next to the right side
of the ruler. This will be your “target.”
Place a ball on the groove as shown. You might need to give it a gentle nudge. Once the ball
hits the first magnet, the ensuing chain reaction will move too fast for you to see.
If you want to capture images of the rail gun in action, you can make a cheap high-speed camera set
up like these guys did at web. mit.edu/Damonv/Public/Doombolt.
You can build a more powerful rail gun with a longer ruler and more magnets and balls, but at a
certain point, the magnets will start to shatter.
Illustrations by Mark Frauenfelder
12 Make: Volume 01