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# Designing Balloon Cars

Media Type:
Video

Running Time: 2m 03s
Size: 2.9 MB

or

Source: ZOOM

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At first glance, balloons and rocket engines have very little in common. However, when attached to, or part of, vehicles of the appropriate size, they both rely on an important physics principle to produce forward motion. In this video segment adapted from ZOOM, balloon car "engineers" put their designs to the test.

Background Essay

Although none of us travel around behind the wheel of a balloon car, all vehicles, including balloon cars, rely on one of the most important laws of physics for their forward motion. Newton's third law of motion states that every action has an equal and opposite reaction. This means, for example, that when you push against a wall, the wall, as inanimate as it is, pushes back on you with an equal amount of force. If you're doubtful of a wall's ability to push you, try leaning against one while standing on a skateboard.

Balloon cars use the principle of Newton's third law in the same way that rocket- and jet-propelled vehicles do. Before it is inflated, a balloon exerts no force on the relatively few molecules of air it contains. As it is inflated, however, more and more air molecules crowd into it, increasing the balloon's internal pressure and causing it to expand. As the rubber of the balloon stretches, it applies an increasing amount of force on the air inside. When the balloon is released, the air escaping from the balloon pushes against the air just outside the balloon. As the third law predicts, the outside air pushes back on the escaping air, propelling the balloon car forward.

Just as all vehicles rely on Newton's third law to propel them forward, the forward motion they create (or harness, in the case of wind-propelled vehicles) must counteract the forces that resist forward motion, namely friction and drag. Although these forces cannot be eliminated, at least not on Earth, intelligent vehicle designs can reduce them considerably. Wheels, for example, are probably the simplest way to reduce friction on land. The more easily and smoothly they roll, the more of a vehicle's force will be applied to forward motion and the faster and farther it will travel. Likewise, the more streamline a vehicle, the more easily it will cut through air or water and the more efficient it will be. Engineers who design cars, boats, trains, and planes go to great lengths to create vehicles that maximize the forward-acting force they produce and minimize the forces that act against this forward motion.

Discussion Questions

• One student states that a balloon car can't be designed for both speed and distance. Why do you think a fast car cannot cover as much distance as a slow car?
• What was the problem these students were trying to solve? How did they use data (measurements) from their tests?
• Explain why some friction is necessary for wheeled vehicles to move.
• What were some of the design considerations that students had to keep in mind? Why did a juice carton make a good chassis for these balloon cars?
• Everyone had the same design challenge, yet their solutions (their cars) were not exactly the same. Why do you think this was so?

• Transcript

(footsteps) (two knocks) (creaks) (people exhaling hard)

BOY 1: We had to design a balloon car... out of orange juice boxes. The wheels are made of spools... And we're using balloons.

BOY 2: And you inflate the balloon...

BOY 1: And the force of the air coming out of the balloon was to make the car go, so what we really had to do is design a good car... That would go far...

BOY 3: For the longest time.

EVERYONE: (all exhaling loudly).

BOY 1: You have to have the right size opening at the end, because if you let the air out too fast, it'll just go fast for a second, and then it will stop. But if you let the air out too slowly, you won't get enough force. I tried with this long, skinny balloon, and what I realized is that the balloon was starting to flop over and get in the way of the wheels.

MAN: Go!

(boys chuckle)

BOY 1: So I had to design this to keep the balloon straight.

BOY 4: I put the wheels farther apart so it wouldn't, like, be able to turn like this more.

MAN: Go!

BOY 5: Caramba!

MAN: Ah! Okay.

BOY 6: There's always a trade-off for how fast it can go and how far it can go.

MAN: Let's go. Let's get a bunch of them going there.

BOY: Doh!

MAN: Nice!

BOY: It worked! All right! Oh, nice! Yay! (laughter)

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