Rest one of the plastic balls in the palm of your hand. Now slowly move your hand into the air stream. Watch the ball to see if it becomes stable in the air stream. Repeat with another of the plastic balls.
Some of the balls have pieces of velcro attached to them. You can stick two of these balls together and repeat with the pair. How stable is the pair in the air stream?
Air that is not moving pushes in all directions with equal force. Thus air is pushing down on the top of a table top and up on the underside of the table. Moving air however pushes to the side with less pressure; if a fan pushes air across the top of a table the downward pressure on the table is reduced.
When the plastic ball is resting on your hand, air is pushing in on all sides of the ball with the same force. As you start moving your hand (and the ball) into the air stream, there is reduced pressure on that side of the ball. Move the ball far enough into the air stream and there is such a difference in pressure that the still air on the back side of the ball pushes it off your hand and into the air stream.
An airplane wing is essentially flat on the underside, but is humped or convex on the upper side. Air moving across the top of the wing has a further distance to travel than air moving across the bottom of the wing. Therefore air moving across the top of the wing must travel faster, so the air pressure on top of the wing is less than the air pressure on the bottom of the wing.
Airplanes fly as a result of the Bernoulli Principle. Perfume atomizers also demonstrate the principle: squeezing the bulb forces air to rush across the pickup tube, reducing air pressure. Surrounding (greater) air pressure forces the perfume up the pickup tube.
You can demonstrate the Bernoulli Principle in a couple ways. A hair dryer and ping pong ball work very well. You can even tilt the air stream slightly to one side and keep the ping pong ball aloft. Tear off a strip of newspaper about 2 inches wide and five inches long. Hold one end of the strip against your lower lip. Gravity will cause the far end of the strip to curve towards the floor. Now blow across the top of the strip. The far end will rise because of the reduced air pressure on top of the strip.
What is holding up the ball when it is hovering in the air stream? What do you think would happen if a given ball were made heavier?
The upward air is pushing on the bottom of the ball. This push is opposing gravity and thus making the ball hover. If a given ball were made heavier gravity would exert a heavier pull on it and the ball would hover at a lower height.
What do you think would happen if a given ball were kept the same weight but made smaller?
When a ball is made smaller there is less surface area for the air stream to push on. If the weight of the ball remains constant, it is expected that the ball would hover at a lower height.
When the ball leaves your hand and moves into the air stream, it is a sudden movement. What prevents the ball from passing through the air stream completely? Why does the ball remain in the air stream more or less indefinitely?
Differential air pressure causes the ball to move from your hand into the air stream. When the ball is within the air stream, air is moving vertically on all sides of the ball. When the ball is off center in the air stream, air is moving faster on the inner side of the ball than on the outer side, again creating differential air pressures. Therefore, Bernoulli's Principle tends to always move the ball to the center of the column of moving air.
Can you design an experiment to measure how changing the weight of a plastic ball, keeping its size constant, affect the hovering height?
Many plastic balls have openings to add air. Some require a needle, some like beach balls have a pop up valve one can simply open and blow into.
Using either type of ball, one could incrementally add a small amount of water to the ball and measure the position at which it hovers, then add more water and repeat. Of course, the column of air would have to remain unchanged.
Using the pieces of velcro on the plastic balls, stick two together and observe their action in the air stream. Does the pair achieve a stable position in the air stream? Why or why not?
The pair does not achieve stability. It continues to rotate in all directions. The bottom surface area on which the column of air is pushing is constantly changing, so the vertical position of the pair is changing. And because the pair continues to rotate in all directions, the sideways pressure is constantly changing.
How could you measure and graph the upward pressure of the air stream across the column of air? How could you measure and graph the sideways pressure across the column of air?
Create a three dimension grid of fine wire or fishing line around the air column to provide reference points. Fill a bucket with water. Insert one end of a piece of flexible clear plastic tubing in the bucket. Hold the other end of the tube in the air column at different points with the end of the tube pointing down. The upward air will push air in the tube below the level of the water in the bucket (pushing against ambient air pressure, which is pushing down on the surface of the water). Measure and record how far air in the tube is pushed at each point.
To measure the sideways pressure, hold the end of the tube horizontally in the air stream and measure how high water is drawn up the tube above the level of the water in the bucket.
The data in each situation can be graphed in two dimensions, distance across the column or up the column versus pressure.
challenge question 7 needed
challenge question 7 answer
Bernoulli Principle, pressure
further information listing needed
This exhibit is described in the Exploratorium Cookbook series.
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