You and Your Students!
Vicki Cobb, Education World Science Editor
Stop keys from falling to the floor in an amazing feat.
Physics, Motion, Simple Machines
Setting the Scene (Background)
To get students engaged in an activity, few things work quite as well as asking them to predict an outcome. In this demonstration -- a tug of war between three paper clips and a set of keys -- you will produce an unpredictable result. Your students will only understand the result after thinking it through.
Do this activity first as a demonstration. After that, many of your students will want to try it themselves. Doing the activity first as a demo drives home my point that science is about sharing an experience. Anyone can repeat the experiment if they have any doubts about it.
Tie the keys to one end of the string; then tie the paper clips to the other end. Hold the pencil by one end so that it extends horizontally in front of you. Grasp the paper clips in your other hand. Drape the string over the pencil so that the keys are hanging down. The string between the pencil and the clips is also held horizontal. Two-thirds of the string should be between the clips and the pencil. [See illustration.]
At this point in the demonstration, tell your students that you are going to let go of the paper clips. What do they think is going to happen? Since the keys are [clearly] heavier than the paper clips, will the keys pull the clips down to the floor? Will the keys hit the floor? Discuss the possibilities with your students.
Now let go of the paper clips. Naturally, the keys drop. But amazingly, they don't reach the ground! The paper clips spin around the pencil and wind up the string. Six wraps are enough to break the fall of the keys. You've stopped the drop cold!
Your students might be under the misconception that heavier items fall faster than lighter ones. Untie both ends of the string. Hold the keys in one hand and the clips in another. Open both hands simultaneously. Listen to learn which object first hits the ground. This dropping race is always going to be a tie.
Behind the Scenes
The instant you release the paper clips, gravity acts simultaneously on both the keys and the clips and they both start to fall. Because the keys are heavier, they fall with a greater force than the clips; the keys win the tug-of-war on opposing ends of the string. In this activity, the pencil acts as a fulcrum. It changes the direction of the string. As a result, the length of the string between the clips and the pencil is shortened; that gives a sideways force to the clips, which combines with the downward force of gravity. These combined forces cause the clips to rotate around the pencil. The shorter the string gets, the faster the clips rotate. After a series of wraps, another force -- friction -- kicks in. Friction is a force that resists motion between two surfaces -- in this case, the pencil and the string. Six wraps is usually enough friction to top the fall of the keys.
See more of these show-stopping activities in You Gotta Try This! Absolutely Irresistible Science by Vicki Cobb and Kathy Darling. For younger kids interested in why things fall, look for Vicki's brand new "Science Play" book, I Fall Down.
Article By Vicki Cobb
Copyright © 2004 Education World