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This chapter is from the book

Air Streams on Top of Cars, Roofs, and Mountains

Discover what happens on top when a lateral stream of air turns up.

Step By Step


  • empty pen tube or drinking straw

  • piece of paper 3 x 6 in (7.5 x 15 cm)

  • glass (preferably clear)

  • small piece of cotton or crumpled paper

  • bucket

  • inflated party balloon (smaller than the bucket)

  • fan

Bend the middle of the paper around a finger to make a bump there (your "mountain" or the top of a car). Hold the paper on a surface as shown in the figure. Place one end of the pen tube or straw close to the bump and blow through the other end. What happens to the top of the bump? Since the pressure in the "tunnel" below the bump is atmospheric pressure (there is no air flow there), what do you conclude about the pressure on the top of the bump? What about the speed of the air flow there (place a small ball of cotton or crumpled paper to "probe" it.) Does the air flow speed up as it moves up to the top? Now, place the glass with the small piece of cotton or paper inside on top of a work surface (see Experiment 30, "Make Your Own Sprayer," next). Position the output of the hair dryer in front of the side of the glass with the nozzle resting on the work surface. Move the nozzle up with the hair dryer switched on and see what happens when the air flow starts crossing the top of the glass (see figure). Now replace the glass with the bucket with the balloon inside, and replace the hair dryer with the fan. Position the fan so that the air flow it produces is below the top of the bucket. Move the fan up and see what happens. You can cover the bucket with a page of a newspaper (the "roof") to see the effect of a lateral wind on it.

Fun Facts

To circumvent an obstacle, the incoming air stream must change direction. This requires a force component perpendicular to the direction of the flow. As the stream hits the obstacle, it is slowed down and the pressure increases locally (kinetic energy is transformed into "pressure energy"). Consider the air pressure around the obstacle at points away from the spot directly hit by the incoming flow. If the pressure at these points becomes less than the atmospheric pressure, a net force is created perpendicular to the air flow, which enables it to bend. In addition, the pressure imbalance around the obstacle speeds up the air flow as it moves toward the point of lowest pressure. Pressure energy is then transformed into kinetic energy (Bernoulli's effect). Imagine a flag that is spread out in the wind with a ripple in it. The air pressure over the ripple decreases so that the air stream can bend as it crosses over the ripple. The air pressure on the two sides of the flag near the ripple becomes unequal, just as in the experiment with the piece of paper. As the ripple moves randomly all over the flag, the flag flutters in the wind.

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