You and Your Students!
Vicki Cobb, Education World Science Editor
You can’t lift a hand from the top of a head; and a fistful of honey makes you stronger.
Have on hand for each pair of students:
Setting the Scene (Background)
Entertainment is most effective when the familiar is presented in a new light. Science is entertaining when it does the same thing. There is nothing more familiar to a child than his or her own body and its limitations. In these two activities your students can discover some surprising new strengths and weaknesses.
Students should work in pairs for both of these activities.
Act I The Bionic Arm
One student in each pair will sit for this activity. S/he will place a hand on top of her/his head with fingers spread as wide apart as possible. The other student grips the sitting student's forearm as close to the elbow as possible and tries to lift the hand off the head. The pull should be straight upward and can be as hard as possible.
The sitting student's "bionic" arm can't be moved! Indeed, the effort will lift the student off the chair before the hand comes off the head. (Note: If the lift comes near the wrist, the hand can be lifted.)
Act II Honey Bee Strong
For this activity, one student in the pair makes a fist around the plastic-wrapped sugar (see Required Props) in his or her favored hand. S/he extends the arm to the side at shoulder height. The partner student stands facing the sugar holder and places one hand on the outstretched forearm near the elbow while the other hand rests on the shoulder of the sugar-free arm. The sugar-holder squeezes the sugar while the partner tries to push down the arm. What happens?
Repeat the activity using the plastic-wrapped honey instead of the sugar.
Amazingly, it is much easier to push down the arm when the sugar is held than when the honey is held.
Behind the Scenes
In the "bionic arm" trick, the arm is a simple machine called a lever. The elbow is one end of the lever; the hand on the head is the other end. The hand on the head is also the resistance and the elbow is the turning point, or fulcrum. The force required to lift the hand is greatest next to the elbow and least next to the wrist. In this kind of lever (a third-class lever, to be technical, where the force is between the fulcrum and the resistance), the force required to lift the hand from the head close to the elbow would be three or four times larger than the force required to lift the hand near the wrist.
A certain amount of strength is required to keep an arm outstretched when a downward force is applied. There is a lot of space between sugar crystals, so the sugar package can be compressed when squeezed. Some of the squeezer's force is diverted into compressing the sugar and is not available to hold up the arm. Honey, like most liquids, is not compressible. More strength is available to keep up the arm. In this case, sweetness has nothing to do with success!
Your students might like to see how strong they are when they are not squeezing anything.
Article By Vicki Cobb
Copyright © 2005 Education World