David finds out how to break a board with the force of his hand.
Segment length: 7:12
- Are your bones stronger than concrete?
- Why does it take training to learn to break a wooden board without hurting
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
- 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?
acceleration the time rate at which something speeds up or slows down or
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
- 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
- Gartrell, J.E., Jr. (1992) Methods of motion. Washington, DC: National Science
- Gordon, J.E. (1984) The new science of strong materials (2d ed.). Princeton, NJ:
- 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.
Martial arts master instructor
Use pretzel sticks as a testing ground to understand better what causes materials to
break. Eat the leftovers!
- 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
- 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.
- 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.
- 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.
- 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
- Breaking point: Test to see if the weakest point of the pretzel is really
at the center.
- Length and width test: Try pretzels of various lengths and widths to see
what size and length hold the most and least pennies.
- 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.
- Look at the ends of a broken pretzel with a magnifying glass. Does its
structure tell you anything about its bending strength?
- 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?
- 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
Activity designed by Jane Copes, Science Museum of Minnesota.
If you have a friend who studies karate, ask that person to explain to you what the
classes are like. How much repetitive training is involved? Does the sensei
(instructor) expect the students to practice often on their own? Call a local karate
or other martial arts school and ask if you can observe a class. Can you see how
karate practice trains both mind and body?
Think of some skill that has taken you a long time to learn. How did you learn it?
Were you training your mind or your body or both? Do you still need to practice it to
keep up your skill? Write down your answers and compare results with those of your
classmates. Which types of skills require frequent practice?
Karate practitioners develop thickened skin, or calluses, on their hands from
practicing strikes on pads and straw-covered posts called makiwara. Notice which
areas on your body have calluses. Do you know how you got them? Figure out what
purpose they serve at each spot.
Newton's Apple is a production of KTCA Twin Cities Public Television. Made possible by
a grant from 3M. Educational materials developed with the National Science Teachers