Projects

A paintball hits a cardboard target, played back at 1/67 of its actual speed.

Katherine Nash Scafide, who is studying nursing with a specialty in forensics, recruited me to help her measure the impact force of a paintball. Her work focuses on the bruising, but for my part we didn’t use any human targets. We wanted to know how much firing distance mattered — how differently a shot from 15 feet and a shot from 30 feet might arrive at the target. (Since this was just an exercise to gather background information, she generously granted me permission to share it.)

Her husband took shots at a target next to my lab’s high-speed camera. We recorded shots from several distances, and I analyzed the videos to measure the paintball’s incoming velocity.

In most cases, the paintball completely smashed onto the target without backward splashing or flying bits. (The video above was an exception, a neat-looking one.) The impact occurred during one or two frames of high-speed video, meaning 0.0005 or 0.001 seconds. From this and the paintball’s mass (3.2 grams) we can get an estimate of impact force. Fired at 15 feet, the average force during impact is in the range of 250-500 Newtons, or 45-90 pounds. Ouch.

With a trip to the toy store and an hour of snapping and unsnapping LEGOs, I came up with a little gearbox connecting a motor to a stirring rod. The gears give this light-weight motor the mechanical advantage that it needs to churn thickening ice cream. For each turn of the rod, the motor spins 15 times.

The assembly — shown with some spare pieces attached as stirring implements — fits snugly into a large empty yogurt container with notches cut in the sides.

Ice cream is milk : cream : sugar in the proportion 2 : 2 : 1 with a pinch of salt and flavoring. Just mix them and agitate the mixture while it freezes. (Other, more complicated recipes involve heating the milk and using eggs too. No need for more complication here.)

I let it chill in peace for about an hour. At 7:30, when I could see ice crystals starting to form, I switched on the motor, headed out the door for the evening, and hoped for the best. Around midnight, I came home to a delicious success!

Thanks to Jeff Potter for sharing the idea of LEGO ice cream makers in his book. With the help of the enthusiastic people at Shananigans Toy Shop, I decided on LEGO Power Functions (#8293) for a motor and LEGO Crazy Action Contraptions (Klutz Press) for a starter set of bricks. It was just the right number of pieces to give me room to experiment and create something sturdy enough. Try making one yourself!

I wrote a Beats-per-Minute calculator. Tap any key to a beat and it will display the tempo. It’s a step more refined than other similar tools because it shows correct precision and it changes color once it has an accurate reading. Try it!

This is a screenshort. Click it open the BPM calculator.

You can turn a piece of paper into a sort of puzzle piece in just a few quick folds.  With 6 pieces, you can make a cube.  With 12 pieces or 30 pieces, you can make pointy stars.  (Mathematicians call them stellated icosahedra.)  Making the pieces is simple, and assembling them into these shapes is easier than it looks.

Check out more possibilities and video instructions →

I considered two questions about the board game Risk.

1. What are the odds of winning a particular dice roll? (like 3 dice vs. 2 dice, 3 dice vs. 1 die, etc.)
2. What are the odds of conquering a territory? (for example, 20 men invading 17 men)

Anyone who has played risk has an intuitive sense of the answers to the first question. Odds of winning a 3 dice vs. 2 dice battle are about 50/50. The invading army gets the advantage of an extra die, but ties go to the defender. It turns out that these advantages roughly balance each other out. (Full results.)

Question #2 is harder because battles between a large number of soldiers are complicated. It all comes down to who has to roll with a reduced number of dice. For example, if a large army is cut down early with a string of bad luck, its odds of winning go down much faster.

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With a giant glass pipe, a slice of ceramic cut from a diesel exhaust system, and a strip of toaster wire, I built an acoustical cannon: The Horn of Jericho. It is the brainchild of John Wight, a research scientist at Corning Incorporated, and it is probably unique. Read More to see better pics and video clip of the horn in action.

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