We’re going to experiment with Newton’s Third law by blowing up balloons and letting them rocket, race, and zoom all over the place. When you first blow up a balloon, you’re pressurizing the inside of the balloon by adding more air (from your lungs) into the balloon. Because the balloon is made of stretchy rubber (like a rubber band), the balloon wants to snap back into the smallest shape possible as soon as it gets the chance (which usually happens when the air escapes through the nozzle area). And you know what happens next – the air inside the balloon flows in one direction while the balloon zips off in the other.

Question: why does the balloon race all over the room? The answer is because of something called ‘thrust vectoring’, which means you can change the course of the balloon by angling the nozzle around. Think of the kick you’d feel if you tried to angle around a fire hose operating at full blast. That kick is what propels balloons and fighter aircraft into their aerobatic tricks.

We’re going to perform several experiments here, each time watching what’s happening so you get the feel for the Third Law. You will need to find:

• balloons
• string
• wood skewer
• two straws
• four caps (like the tops of milk jugs, film canisters, or anything else round and plastic about the size of a quarter)
• wooden clothespin
• a piece of stiff cardboard (or four popsicle sticks)
• hot glue gun

First, let’s experiment with the balloon. Here’s what you can do:

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1. Blow up the balloon (don’t tie it)

2. Let it go.

3. Wheeeee!

4. Tie one end of the string to a chair.

5. Blow up the balloon (don’t tie it).

6. Tape a straw to it so that one end of the straw is at the front of the balloon and the other is at the nozzle of the balloon.

7. Thread the string through the straw and pull the string tight across your room.

8. Let go. With a little bit of work (unless you got it the first time) you should be able to get the balloon to shoot about ten feet along the string.

This is a great demonstration of Newton’s Third Law – the air inside the balloon shoots one direction, and the balloon rockets in the opposite direction. It’s also a good opportunity to bring up some science history. Many folks used to believe that it would be impossible for something to go to the moon because once something got into space there would be no air for the rocket engine to push against and so the rocket could not “push” itself forward.

In other words, those folks would have said that a balloon shoots along the string because the air coming out of the balloon pushes against the air in the room. The balloon gets pushed forward. You now know that that’s silly! What makes the balloon move forward is the mere action of the air moving backward. Every action has an equal and opposite reaction.

Multi-Stage Balloon Rocket

You can create a multi-stage balloon rocket by adding a second balloon to the first just like you see here in the video:

Tie a length of string through the room, having at least twenty feet of clear length.  Thread two straws onto the string before securing the end.  Punch the bottoms out of two foam coffee cups and tape parallel to the threaded straws.

Blow up balloons while they are inside the cups, so they extend out either end.  When blowing up the second balloon, sandwich the untied end of the first inflated balloon between the second inflated balloon surface and inside the cup.  Hold the second balloon’s end with a clothespin and release!

Balloon Racecar

Now let’s use this information to create a balloon-powered racecar. You’ll need the rest of the items outlined above to build your racecar. NOTE: in the video, we’re using the popsicle sticks, but you can easily substitute in a sheet of stiff cardboard for the popsicle sticks. (Either one works great!)

 
Download Student Worksheet & Exercises

You now have a great grasp of Newton’s three laws and with it you understand a good deal about the way matter moves about on Earth and in space. Take a look around. Everything that moves or is moved follows Newton’s Laws.

In the next unit, we will get into Newton’s Third Law a little deeper when we discuss momentum and conservation of momentum by whacking things together *HARD*. But more on this later…

Exercises

1. What is Newton’s Third Law of Motion?
2. Why does the balloon stop along the string?

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Advanced students: Download your Balloon Racer Lab here.

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