Field Trips and Simple Physics

You can turn a bus ride into a physics experiment that will even have your fellow teachers scratching their heads. Everyone knows that a weight suspended by a string and taped to the ceiling of the bus will appear to swing backward as the bus speeds up, and forward as it decelerates, due to inertia. Also take along a helium-filled balloon on a string to tie to a seat or fasten to the floor. What will it do? The opposite. You’ll have a great time getting students to hypothesize then try to explain the result.

We know that air has mass so it also has a certain inertia; when the bus moves forward the weight hanging on the string AND the air in the bus both tend to stay where they are, thus appearing to move backward. The air molecules tend to compress together toward the back of the bus, changing the air pressure in much the same way as barometric pressure changes. Tighter molecules, more pressure. The balloon, meanwhile, is trying its best to move to an area equal in density to its helium (somewhere out in the upper atmosphere, actually) so as the air pressure in the bus increases, it tends to move to an area of lesser pressure. What we really have is a bus full of air sloshing around inside, changing pressure as the bus accelerates, decelerates, or turns, with a balloon trying to get to the area of lowest pressure.

(I should mention here that we have to assume the windows to the bus are closed, otherwise the pressure changes wouldn’t take place.) I also will mention that when I first heard of this, I didn’t believe it until I consulted someone who knows much more about these things than I. Now when I drive down the street behind a vehicle carrying one of those “balloon bouquets” I pay attention to what happens to the balloon inside as they change speed or turn. Sure enough, they defy common sense, and adhere to the laws of physics instead.

Happy experimenting,
Lee Shiney, Teachnet editor

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