You and Your Students!
Vicki Cobb, Education World Science Editor
A coin and a stamp are tied in a dropping race.
(curriculum area, e.g., Human Body, Measurement) Physical science
Setting the Scene (Background)
Science is about mystery and not knowing answers. How about starting the school year by mystifying your students? Let them know that they don’t know everything -- that you still have a few tricks up your sleeve! Give them a problem to solve -- a challenge that will give them a chance to think out of the box -- and then let them live with it for a while… Your problem: to resist the temptation to give them the solution too soon. This puzzle is a guaranteed hook.
Hold a coin in one hand and the stamp in the other as you pose this challenge of a dropping race. Then let the kids have access to the materials as they try to figure it out. You might also want to have kids think in groups. Let the suspense build for a day as they try and figure it out.
The challenge is to drop a stamp and a 50-cent piece at the same time from the same height and have them both reach the floor at the same time. Those who try are doomed to fail unless they are capable of out-of-the-box thinking.
Let students know that wetting the glue on the back of the stamp and sticking it to the coin is not an acceptable solution.
Most people will respond by holding the stamp in one hand and the coin in the other. The coin will always reach the ground first. The trick is to place the stamp on top of the coin, making sure that no corners of the stamp stick out over the edge of the coin. Also, press the stamp down so there is as much contact between the stamp and the surface of the coin as possible. The idea is to keep air from moving under the stamp and lifting it off the coin. Hold the coin and stamp horizontally and drop them together. TaDa! They arrive on the floor together.
Be sure to practice ahead of time in private so you have confidence that this will work when you do it with students present.
Behind the Scenes
Every object moving through the air sets up a disturbance called turbulence. If there is enough turbulence, the flight of the object is unsteady. Air also exerts friction on objects moving through it. That force is called air resistance, or drag. The amount of drag on an object and the steadiness of its flight depend on the shape of the object and its speed. Streamlining and smoother surfaces reduce drag and turbulence.
A postage stamp flutters slowly to the ground because drag and turbulence act against the force of gravity. But the weight of the coin counteracts them so the coin is hardly slowed down. When you put the stamp closely on the top of the coin, the coin "runs interference" for the stamp and there is little effect of aerodynamics on the stamp.
Of course, if you dropped the coin and the stamp from each hand on the airless moon, they would both hit the ground at the same time!
I would love to hear the kinds of reactions you got from your kids. Did anyone come up with the solution? Were you surprised? Did doing this early in the school year give you any early insights into your students? Any feedback from the classroom is welcome. Send your comments to firstname.lastname@example.org. Type Show-Biz Science in the SUBJECT line of your email.
Article By Vicki Cobb
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