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Driving Under Pressure

You’re driving along on your way to deliver some helium balloons to a birthday party. Suddenly you hit the brakes. As your car rapidly decelerates, you feel yourself pushed up against the wheel. Why?

That one’s probably not too tough. Although you may think you are sitting still when driving, your body is moving forward at the same speed as the car itself. When you hit the brake the car slows down but your body is still trying to go forward. You experience a sensation something like being pressed against the front of the car.

The next question is a little trickier. Which direction did the balloons move?

Think for a second. The car is going forward and abruptly stops. Which direction do the balloons go?

Surprisingly, if you’ve ever had this experience you might have noticed that the balloons move backwards, just the opposite of your body. Why?

It’s because the balloons are floating in the air that fills the car. The air itself is subject to the same acceleration as your body. When you hit the brakes, the air continues moving forward the same way you do. More air gets pushed to the front than remains in the back. So there is momentarily greater air pressure at the front of the car. The floating balloons register this by being pushed backwards.

You can try this with a candle inside a glass container. Push the container forward and then stop. The flame will tilt backward! The air inside has been accelerated so that it briefly crowds one end of the container, forcing the candle flame back toward the area of lower pressure.

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