You and Your Students!
Vicki Cobb, Education World Science Editor
Break a pencil in half by giving it a “karate chop” with a folded dollar bill.
Setting the Scene (Background)
I’ve been working on a biography of Houdini and it got me thinking about how science and magic relate. Often science seems like magic -- so why not use it to create a real illusion (now that’s an oxymoron!)? To perform this demonstration correctly you’ll have to do what most magicians do: practice out of sight from your audience; give the demo a big buildup -- to make it seem impossible; and perform unhesitatingly and with supreme confidence. That shouldn’t be difficult. The science is on your side.
Do this as a demonstration. When you have thoroughly snowed your students, reveal the trick. They’ll love doing it themselves, and they will have a greater understanding of Newton’s Second Law of Motion. (So will you.)
Announce that you can break a pencil with a dollar bill. In an open manner, fold the dollar bill in half lengthwise. Ask a student to hold the pencil securely at either end with a firm grip
Hold the folded bill at one end between your thumb and the first joint of your index finger. Stand so that your thumb faces your audience and your index finger is concealed behind the dollar.
To dramatize the feat, raise and lower the bill over the pencil twice while counting, "Onetwo" As you bring your hand down on the count of three, straighten your index finger so that you can deliver a karate chop to the pencil. Be sure to move your hand as swiftly as possible and don't hesitate before impact. Be sure to curl your index finger again after the blow has been delivered so your audience won't guess how you broke the pencil.
Performance tip: Before doing this demo in front of students, practice karate chopping pencils with your finger without the bill until you are confident. Then practice with a bill so you can get the timing of extending and withdrawing your finger.
Invite others to try to duplicate your feat with another pencil.
Behind the Scenes
A force has two parts to it. One part is the mass (as measured by weight here on earth) of the object delivering the force. The other part is the change in speed of the object after it delivers the force. (A change in speed is called acceleration. Positive acceleration is speeding up; negative acceleration is slowing down.) It's easy to see how a large object like a sledgehammer can deliver a larger force than a household hammer. But a small object, like a bullet, can shatter a brick because it is traveling so fast.
A karate chop is a force delivered by the hand, which doesn't have a lot of mass. But people with training in karate can use the hand to deliver a blow so swiftly that the force on impact can be great enough to break a brick or block of wood. When the struck object breaks, the hand is not injured since it didn't receive the force of the blow. But if the struck object doesn't break, the hand does receive the painful force of the blow. So the trick to this trick is to keep your hand moving as quickly as possible. She who hesitates is lost. Houdini, move over!
Incidentally, Newton's Second Law of Motion is that a force is the product of mass and acceleration. Mathematically speaking, force = mass x acceleration or f = ma.
Article By Vicki Cobb
Copyright © 2005 Education World