Pencil (must be sharpened, but if possible only to a very dull point—see photos below)
Two ceramic donut refrigerator magnets (the hole must be big enough for the pencil to slide through without touching)
Two squares of matboard with 1-inch (2.5-centimeter) sides
Single-hole paper punch (the size of the hole must be a little smaller than the diameter of the pencil)
Optional: masking tape; soft material (coat, sweatshirt, or similar, for the device to fall on, so it doesn’t break as you work)
Using the hole punch, punch a hole in the middle of both matboard squares.
Slide one of the squares onto the pointed end of the pencil. Work it on gradually until it’s about 1.5 inches (4 cm) from the eraser end of the pencil. Make sure the square fits tightly without bending or creasing significantly and that it doesn’t easily slide around. (If the hole is too big, try wrapping some masking tape around the pencil to create a tighter fit.)
Slide the two magnets onto the pencil so that they are on opposite sides of the square and are oriented so that they are attracted to each other, not repelled (click to enlarge the photo below). They should make a sandwich with the matboard in the middle.
Slide the second square onto the pencil so that it is about 2 inches (5 cm) from the tip of the pencil (see below).
Pull the bottom magnet down until it rests on the bottom square (see
To Do and Notice
Once the magnets and squares are in position, hold the device by the eraser end of the pencil so that it’s hanging vertically at approximately shoulder level. Then release the whole assembly so it drops to the floor, and listen very carefully as it drops. (If possible, drop the assembly onto a coat, sweatshirt, or other soft material, rather than directly onto a hard floor. This will reduce wear and tear on the components.)
As the assembly falls, you should hear a clicking sound as the magnets snap together. This will likely occur fairly shortly after release, before the assembly has fallen very far. If this doesn't happen, adjust the gap between the magnets until it does.
What’s Going On?
These two magnets behave the way they do because of two competing influences: the force of gravity acting on them, plus the magnetic force they exert on each other. The gravitational force on both magnets acts downward. The magnetic force, which creates a mutual attraction between the two magnets, acts downward on the top magnet and upward on the bottom magnet.
When you drop the whole device, the magnetic and gravitational forces on the top magnet are both downward. But on the bottom magnet, the gravitational force is downward and the magnetic force is upward. As a result, the top magnet (along with the whole assembly it’s pushing on) falls with an acceleration slightly larger than normal gravitational acceleration, and the bottom magnet (which is free to lift off its square) falls with an acceleration slightly smaller than normal gravitational acceleration. The greater acceleration of the top magnet and assembly closes the distance between it and the bottom magnet as they fall. As the distance between the magnets decreases, the magnetic force increases exponentially, rapidly becoming large enough to snap the magnets together.
Try adjusting the gap between the magnets to see if you can get them to snap together either just after being dropped, or just before hitting the floor. See what changes if the magnets are oriented so that they repel.
Hold the assembly horizontally, with the magnets oriented so that they attract, but separated enough so they don't snap together. Drop the assembly. Repeat with the magnets oriented so they repel.