You and Your Students!
Vicki Cobb, Education World Science Editor
Discover the strange moves of a balloon on your next field trip.
Setting the Scene (Background)
I’m a great believer in the “teachable moment” -- taking advantage of circumstances to make a point to your students in an especially meaningful and dramatic way. Sometimes those moments are serendipitous and can’t be planned. But here’s one you can plan the next time you take your class on a field trip. All you need is a helium-filled balloon on a string and a moving vehicle.
Observation is a very important part of scientific discovery. Before you set off on a trip in a school bus, tell your kids to observe how the balloon moves differently in the moving bus from the way they move. Before a scientist can understand why something happens, he or she must see exactly what happens. Give your kids time to observe the peculiar behavior of the balloon. At another time, you can discuss what is really happening.
Tie the string of a helium-filled balloon to the back of a seat in the front of the bus so that everyone can see it. Make sure it doesn't touch the roof. Place the small ball (optional) on the floor in the aisle of the bus.
Ask students to make the following observations.
Amazingly, the balloon moves in the opposite direction to them and the ball. When the bus turns left, the kids are pushed right and the ball rolls right, but the balloon leans left like the bus. When the bus speeds up, the kids are pushed back into their seats but the balloon leans forward.
Behind the Scenes
The first thing you should notice is that when the bus is moving at a steady speed, you don't experience any force on your body. You sit quite comfortably in your seat. It is only when the bus changes speed or direction that you experience a force on your body. When the bus speeds up you are not actually being pushed back into your seat. It only seems that you are being pushed back because the seat is moving forward faster than you are (because the seat is attached to the bus) so that it presses into your back.
In this same way, the air in the bus is given a shove forward as the bus speeds up. Since the balloon is lighter than the air, the force of the forward-moving air gives it a shove forward. Since there is very little resistance (the balloon is not in contact with any part of the bus), the balloon moves forward at a speed slightly faster than the bus.
Inertia is any resistance to a change in motion. When a bus speeds up or changes direction, your body has more inertia than a helium-filled balloon. That's why you experience the change in force or direction and the balloon seems not to.
Article By Vicki Cobb
Copyright © 2005 Education World