Columns: Experimenting with Balloons

Resource for Grades 3-8

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Columns: Experimenting with Balloons (Document)

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Background Essay

Columns are among the oldest and most effective building elements when it comes to resisting the pushing, or compression, that occurs in all kinds of structures. Paired with horizontal beams, these critical forms sometimes carry the weight of an entire structure. To resist the downward force of all that weight, columns apply an equal upward force.

Balloons are never used as columns in building construction. Even so, when placed in a column-like circumstance, as in this ZOOM activity, balloons encounter the same types of forces as columns and apply their own resistant forces in return. The manner in which they apply these resistant forces, however, is very different from the way a more traditional column might respond.

There's an unmistakable difference between a fluid-filled balloon and the types of materials -- metal, concrete, stone -- most often used to build columns: Balloons stretch a lot, while traditional materials stretch very little. For example, when a car drives over a bridge, the columns supporting the bridge deck flex only slightly. The dense material in the column resists compression and remains rigid and strong. Fluid-filled balloons, in contrast, compress immediately, even under small forces. As weight is applied from above, the pressure of the fluid inside the balloon rises, applying a force to the interior walls of the balloon. The rubber of the balloon stretches in response, applying equal resistance inward.

As weight is increased, or the number of columns decreased, the downward force on each column increases also. Balloon columns respond in two ways: The pressure inside the balloon increases, and the balloon stretches more, spreading out and covering a larger area. These two responses have the effect of increasing the amount of upward force the balloon can apply until the upward force once again matches the downward force. The balloon breaks when increased internal pressure and stretching finally fracture the balloon material.

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Discussion Questions

Describe the forces at play when the balloons are supporting the weight of the boy.

Why do you think the water balloon supported the boy's weight better than air did?

Did the ZOOM cast members investigate the differences between air and water in a systematic way?

Have you noticed the flat place where a car tire touches the road? Do you think the size of that flat place depends on the air pressure in the tire? Was there more pressure in the water or in the air in these balloons?

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Transcript

SHING YING: We were able to get one paper cup to support Garrett's weight. This is because paper is very strong when you pull on it a certain way. See how strong it is when I tug on it like this? This is called tension.

GARRETT: Well, now we want to see if we can support my weight with something other than cups— balloons.

SHING YING: I think it might pop.

GARRETT: I think it might pop, too.

SHING YING: I think it'll work with ten, but maybe pop when it gets, like, nine.

GARRETT: Okay, do you have a pin? All right, ready? Okay. Whoa! It's really slippery.

CAROLINE: What's it feel like?

SHING YING: The balloons are, like, bending. Would you feel comfortable if I popped one?

GARRETT: Okay.

CAROLINE: So that's nine. So I'll pop another one.

GARRETT: Whoa.

SHING YING: So now there's eight left.

GARRETT: The balloons are changing shape.

SHING YING: Okay, ready?

CAROLINE: Okay, pop.

GARRETT: Okay, now... I felt that one. Okay, I felt that one, too.

CAROLINE: Why don't you pop one in the back?

SHING YING: Okay.

CAROLINE: So how many do we have left, six?

SHING YING: There's six.

GARRETT: Oh, I felt that one.

SHING YING: Wow! You went really down.

CAROLINE: Look at how much the balloons are stretching. They totally changed. Hold on to him really tight.

SHING YING: There's four.

GARRETT: Oh, okay, cool.

SHING YING: Now there's three.

GARRETT: Now there's three, and now I'm...

SHING YING: And they're stretching down really low.

GARRETT: They're changing shape?

CAROLINE: Yeah. Why don't you pop one in the front?

GARRETT: I felt that one.

CAROLINE: Whoa, you went down really low.

SHING YING: They're really flat!

CAROLINE: Here, why don't you pop the second to last?

GARRETT: Okay, that...Oh! Hold on. (pops)

SHING YING: It worked for, like, a second.

GARRETT: I know, it was up there and then once I got all my weight on it, it just went pop.

CAROLINE: It totally was like "phoo"!

SHING YING: I guess it just couldn't take all the pressure.

CAROLINE: You guys want to take them off and then try water balloons?

GARRETT: Oh, that would be so cool!

CAROLINE: Nine...

SHING YING: Ooh, can I get the last one?

CAROLINE: Yeah. Ten. Oh, my gosh.

GARRETT: This is going to be so much fun.

CAROLINE: It'll be awesome when we pop them.

GARRETT: Guys, what do you think...

CAROLINE: I don't know if it's going to hold you.

SHING YING: I think maybe you'll feel sturdier.

GARRETT: I don't know. You know how air was taking up the space of the balloon?

CARLINE: Now it's water.

SHING YING: We have to find out, don't we?

GARRETT: Okay, ready?

CAROLINE: This is so exciting.

SHING YING: Let's just feel it.

GARRETT: All right, ready?

CAROLINE: Okay, the big moment.

SHING YING: Yeah, they're definitely squished. Do you want me to pop one?

CAROLINE: Yeah, pop one. (squeals)

ZOOMers: Oh! So cool!

CAROLINE: Now my feet are all wet. Wow, it went down a lot.

SHING YING: Oh! I just got it in the sock!

CAROLINE: That went down a lot in the front. Let's pop one in the back.

GARRETT: Yeah, in the back. It feels really weird.

CAROLINE: So how many do we have left?

SHING YING: I think we have six. Do you want me to pop one?

CAROLINE: Yeah.

CAROLINE: Each time you pop a balloon, it goes down more.

SHING YING: Whoa, they're really squished.

CAROLINE: Okay, I'll pop one.

ZOOMers: Oh!

SHING YING: Are you still going? So that's five.

GARRETT: Oh, man, that one was, like...How many are left, you guys?

CAROLINE: I think there are three left. Wow, they're really smooshed.

SHING YING: Oh! Oh, that one's...

GARRETT: Hold on, I'll try to stand on it. Wait a minute.

CAROLINE: Don't move.

SHING YING: Okay, let's see the bottom.

CAROLINE: Pop that one in the back.

GARRETT: Oh, I can see it coming out of the back. Look it! (squeals) Oh, that one's...

SHING YING: Hey, are you still going?

GARRETT: Yeah, I'm on this...

SHING YING: We got one!

CAROLINE: It's like surfing.

GARRETT: Is that one?

SHING YING: Yeah. Wait. There's one!

GARRETT: Oh, that's so cool! It's holding me up. Oh, my God, it's so cool!

SHING YING: Oh, my gosh!

CAROLINE: You're wicked low.

GARRETT: I can't believe we had ten water balloons. This one worked better than the air did. Like, it gradually...

CAROLINE: It has more space to stretch.

GARRETT: See how fat it gets when it stretches?

SHING YING: And it just goes, like, out.

GARRETT: That is so cool. Do you remember when we tested out cups and found that one large paper coffee cup was able to support my weight? Since the balloons changed shape when I stood on them, they weren't able to hold me up as high as the cup did.

CAROLINE: If you pull on a balloon like this, it stretches, but if you pull on a piece of paper, it doesn't stretch.

SHING YING: Would your results be the same if you filled the balloon with something like helium or salty water?

ZOOMer: Think of a question you'd like to answer. Make a prediction. Then test it out.