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Defy Gravity! Balancing Balls on Air

Resource for Grades K-8

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Media Type:
Video

Running Time: 3m 19s
Size: 9.9 MB

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Source: ZOOM

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Defy Gravity! Balancing Balls on Air (Document)

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Background Essay

Liquids and gases are both considered fluids for certain purposes of analysis. Fluids are substances whose molecules, unlike those of solids, do not occupy fixed positions at normal temperatures because the forces that bind them together are relatively weak. From experiments conducted in the eighteenth century, the Swiss mathematician Daniel Bernoulli determined that for a fluid in motion, pressure and velocity are inversely related: pressure is greatest when speed is lowest, and vice versa.

By applying Bernoulli's principle, aeronautic engineers have been able to design airplane wings that can help put 400-ton passenger jets in flight and keep them there. The top surface of the wing is curved and the lower surface is flat, and as a result, the air rushing over the wing, which has a longer distance to travel, has a greater velocity than the air passing under the wing. Because pressure is greatest where velocity is least, the pressure pushing up on the wing from below is greater than that pushing down from above. The difference in pressure provides a net upward force, called lift, on the wing.

We can see Bernoulli's principle in action in this demonstration adapted from ZOOM. The air blown up from the hair dryer is of sufficient velocity and volume to counter the downward force of gravity acting on the balls. Because of the fairly random molecular movement of fluids, the balls don't stay still. But unless the airflow is interrupted, they also don't fly away. What keeps the balls floating within the air column, constantly adjusting back to the center, can be explained by Bernoulli's principle.

If a ball begins to drift to one side of the air column, it quickly returns to the center. This is because the air around the center column of fast-moving air is moving more slowly and therefore has a greater pressure. When the ball drifts outside the center of the air column and into a region where the air is moving more slowly, the greater pressure in that region pushes the ball back into the fast-moving air column. Even when the cast members try to intentionally direct the airflow to one side or the other, the ball always returns to the fast-moving center of the air column.

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Discussion Questions

What force pushes the ball up? What force pulls the ball down?

What force keeps the ball in the air column over the hair dryer?

If you watch closely, does the ball seem to try to move sideways and get pushed or pulled back? Why doesn't the ball just slip sideways and fall off the air that is holding it up?

How can you make the ball fall sideways without touching it?

When two balls are balanced in the same stream of air, why do you think they continuously change position and float at different distances from the hair dryer?

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Transcript

BUZZ: Did you know that you can balance balls in a stream of moving air? Well, Kayla S. of Iowa City, Iowa, told us how to do it. All you need is a hair-dryer and a small, lightweight ball. Like a Ping Pong ball.

KALEIGH: Just turn your hair-dryer on. Be sure to turn it on cool and face it towards the ceiling. Put a small ball in a stream of moving air and watch. It just floats there. You can even try moving your hair-dryer from left to right and the ball still stays in the stream of moving air.

BUZZ: You're probably wondering why the air doesn't just blow the ball away. You know that the air from the hair-dryer is blowing on the ball and pushing it up. But did you know that gravity from the Earth is also pulling the ball down? That's why the ball doesn't just float up into space.

KALEIGH: So do you know what's keeping the ball from falling off from side to side? It's got to be the fast air coming from the blow-dryer. I'll show you why. If I try to balance this ruler on my fingers my hand will be holding it up and gravity will be pulling down on it. But there's nothing to keep the ruler from falling from side to side, so it just falls. But the ball doesn't fall because the air from the blow-dryer is moving very fast around the edges of the ball. The air pushes on the sides of the ball and keeps the ball from falling. The reason I know that there's air on the sides of the ball is because I can feel it.

BUZZ: It's so cool. Okay.

KALEIGH: Do you want to try two?

BUZZ: Sure. Two ball.

KALEIGH: Ready. Oh, wow!

BUZZ: Cool!

KALEIGH: You see it change like that?!

BUZZ: Awesome! They're changing position. Whoa, one fell, awesome, cool.

KALEIGH: Oh, do you want...

BUZZ: Want to try with a different...

KALEIGH: Sure.

BUZZ: Okay, let's try it with this ball. This ball looks cool. Whoa!

KALEIGH: Do you want me to catch it and put it back and try to get it?