• Are your bones stronger than concrete?
• Why does it take training to learn to break a wooden board without hurting your hand?

Contents

Insights & Connections
Vocabulary
Resources
Main activity
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INSIGHTS & CONNECTIONS

Ron McNair raises his hand and brings it quickly down onto a pile of concrete bricks. As he pulls back his left hand, his right hand smashes through the concrete, sending it crumbling to the ground. An exciting picture, but what is it doing in Scientific American magazine?

That powerful karate chop might remind you of the superhuman feats found in Hollywood action movies. But the ability of karate experts to break bricks has an easy explanation in the principles of physics and physiology. In fact, karate experts view breaking bricks and boards as a demonstration of form and concentration. The true nature of karate involves a complete mental and physical discipline that goes far beyond simple hand strikes.

One key to understanding brick breaking is a basic principle of motion: The more momentum an object has, the more force it can generate. When it hit the brick, McNair's hand had reached a speed of 11 meters per second (24 miles per hour). At this speed, his hand exerted a whopping force of 3,000 newtowns--or 675 pounds--on the concrete. A slab of concrete could likely support the weight of a few people weighing a total of 675 pounds (306 kilograms). But apply that amount of force concentrated into an area as small as a fist and the concrete slab will break.

Another key to brick breaking lies literally in the palm of your hand. Feel the bone on the edge of your hand, directly below your little finger. This bone (known as the fifth metacarpal) bears the brunt of McNair's hand strike. Human bones can actually resist 40 times more stress than concrete. (Picture a piece of concrete the size of a bone, and imagine how easily it would break.) The natural engineering of the human hand also lessens the severity of the impact. The muscle, tendons, ligaments and other soft tissue in the hand provide a natural cushion, dispersing the impact energy up through the arm.

If you attempt brick breaking without proper training, you'll end up with an injured hand and possibly serious nerve damage! You must be instructed by an expert in proper technique. Proper training protects your hand because regularly striking a striking pad or post causes your skin to develop calluses, your muscles to strengthen, and your bones to thicken. Extensive training is also necessary to train your brain and muscles to bring your hand down just right--exactly as it reaches its full speed and right smack in the center, at the brick's weakest point.

• Have you ever broken a bone? What caused it to break?
• Why do you think karate practitioners yell "kiai" when they are carrying out a strike?
• Why do karate practitioners put their hands down several times on the board or brick before attempting to go through it? Why do they pull their other hand back when they strike?

VOCABULARY

force a push or pull on an object

karate literally, "empty hand" in Japanese. A martial art developed in Okinawa and brought to Japan in the early 1900s.

kiai Japanese word for the shout used in martial arts. Its literal meaning is "energy meeting."

ligaments strips of connective tissue in the body that hold bones together

metacarpals bones in the human hand that connect the wrist to the fingers and thumb

momentum the mass of an object times its velocity. An object's "bashing power."

newton unit of measurement for force. Approximately equal to the force exerted by the weight of an apple.

strain relative deformation of a brick, bone, or piece of wood when it is stressed

stress amount of force placed on an object that tends to bend or break it

tendon strips of connective tissue that attach bones and muscles to one another

RESOURCES

• Amann, G.A. & Holt, F.T. (1985, Jan) Karate demonstration. The Physics Teacher, p. 40.
• Feld, M., McNair, R. & Wilk, S. (1979, Apr) The physics of karate. Scientific American, pp. 150-158.
• Finn, M. (1988) Martial arts: A complete illustrated history. Woodstock, NY: Overlook Press.
• Funikoshi, G. (1973) Karate-do kyohan. Tokyo/New York: Kodansha International.
• Funikoshi, G. (1975) Karate-do: My way of life. Tokyo/New York: Kodansha International.
• Gartrell, J.E., Jr. (1992) Methods of motion. Washington, DC: National Science Teachers Association.
• Gordon, J.E. (1984) The new science of strong materials (2d ed.). Princeton, NJ: Princeton University.
• Microcomputer-based lab experiments in motion. Grades 6 and up. Apple IIs, 64K. Queue, Inc.: (800) 232-2224.
• Miller, D.E. (1987, Apr) Brick breakers. Atlantic Monthly, pp. 79-82.
• Stein, S. (1992) The body book. New York: Workman Publishing.

Community resources

Mechanical engineer

Orthopedist

Martial arts master instructor

MAIN ACTIVITY

Materials

• pretzel sticks of varying thicknesses
• several pieces of uncooked spaghetti
• paper cup
• 2 rolls of 50 pennies each, plus 50 loose pennies
• empty plastic film container or small squeeze bottle with top removed
• thick string or wire approximately 6 cm (2.5") long
• craft knife or scissors with points
• pair of tweezers or chopsticks
• pencil and piece of paper

1. Build a pretzel strength-testing machine. Start by cutting a large hole out of the bottom of the paper cup. Set the cup on a table, bottom side up. Rest a pretzel stick across the center of the cup.
2. Now create a weight bucket to hang on the pretzel. Take the empty plastic container and make two holes approximately 1 cm (0.4") from the top rim and directly across from each other. Thread the string or wire through the holes and tie the end at each hole. You'll want to make sure the bucket will hang on the pretzel without touching the table.
3. Start testing: With the bucket hanging on the pretzel stick, begin adding pennies. See how many pennies the pretzel can hold without breaking. Find the average number of pennies one type of pretzel stick can hold.
4. Gaining momentum: Test to see if it makes a difference if you drop the pennies in the bucket or if you place them in gently, using the tweezers or chopsticks. Record your results.
5. Breaking point: Test to see if the weakest point of the pretzel is really at the center.
6. Length and width test: Try pretzels of various lengths and widths to see what size and length hold the most and least pennies.
7. Compare with other materials: Do you think a pretzel or an uncooked piece of spaghetti is stronger when bent? Try testing uncooked spaghetti to see how it holds up in comparison to the pretzel sticks.

Questions

1. Look at the ends of a broken pretzel with a magnifying glass. Does its structure tell you anything about its bending strength?
2. What foods break by stretching but not by bending? What foods crush easily that would be hard to break apart? Do you think bending, crushing, and stretching involve different forces?
3. Can you figure out a way to spread weight out across the entire length of the pretzel? Can it hold more weight when the weight is distributed over a larger area?

Activity designed by Jane Copes, Science Museum of Minnesota.

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