Build plans / Balancing Nails Challenge
Balancing Nails Challenge
A classic puzzle: stand one nail upright in a block and balance a dozen nails on its head. The secret is an interlocking arrangement that hangs most of its weight below the support. Bruce's extension stacks a second layer to balance 23 nails, and there are tiny-nail and pencil versions for extra challenge.
Written from Bruce's original workshop handout. Bruce's final review of this plan is in progress.
What it shows
Twelve big nails balanced on one nail head looks impossible. Then Bruce doubles it.
Key ideas: center of gravity, balance, stability, torque
Build it
Make the stand
Drill a hole in a wooden block or disk and set one nail upright in it, head up. This is the support.
Lay the first nail
Lay one nail flat on the table. This will be the bottom spine.
Add ten crossing nails
Lay ten nails across the spine, alternating directions so their heads hook over the spine on alternate sides and their shanks slope down to the table.
Lock with the top nail
Lay one more nail on top, parallel to the spine, in the notch formed where the crossing nails rise up. That makes 12.
Lift and balance
Pinch the ends of the top and bottom nails together and lift the whole bundle. The crossing nails swing down into a hanging V. Rest the middle of the bottom spine on the head of the support nail.
Go for 23
Build the 12-nail bundle on the table. Lay a second set of ten nails across the top nail, again alternating directions, and finish with one more nail on top.
Lift the double bundle
Squeeze the top nail and the bottom spine together to hold everything as you lift, then set the bottom spine on the support nail. That is 23 nails on one.
How it works
An object balanced on a point is stable when its center of gravity is directly below the support. If it tips a little, gravity's pull on the low center of gravity swings it back, like a hanging weight.
When you lift the bundle, the crossing nails hang down on both sides of the spine, so most of their weight ends up below the head of the support nail. The top nail presses down on the crossing nails and locks them against the spine, so the whole bundle acts as one rigid object.
In the 23-nail version the second layer sits higher, but it is locked into the same rigid bundle, and the hanging nails keep the combined center of gravity low enough to stay balanced.
Using it
- Hand out the nails and the stand and let students try before showing the arrangement.
- Once they succeed, challenge them to balance more nails.
- Ask students to sketch where they think the center of gravity is.
- Try the smallest nails you can find; Bruce found 1 in nails the hardest of all.
- Try pencils. There is an easy trick that makes them work; see the video.
Variations
- Tiny-nail version with 1 in nails on a small block.
- Pencil version.
- Balancing toys that use the same low-center-of-gravity idea.
Troubleshooting
More build plans
Bar Pendulum vs. Simple Pendulum
A swinging meterstick keeps the same beat whether it hangs from its end or from a point a third of the way down. Why?
Block Stacking Challenges
How few blocks does it take to make the top block hang completely past the edge of the bottom one?
Centrifugal Bottles
Spin two jars of water and everyone expects the floats to fly outward. They move inward instead.
© Bruce Yeany · Homemade Science with Bruce Yeany · youtube.com/@YeanyScience
Free for personal and classroom use. Please don't resell or republish. Build and use at your own risk; follow the safety notes and supervise children.