This is a recording of a recent live teleclass I did with thousands of kids from all over the world. I’ve included it here so you can participate and learn, too!

We’re going to cover energy and motion by building roller coasters and catapults! Kids build a working catapult while they learn about the physics of projectile motion and storing elastic potential energy. Let’s discover the mysterious forces at work behind the thrill ride of the world’s most monstrous roller coasters, as we twist, turn, loop and corkscrew our way through g-forces, velocity, acceleration, and believe it or not, move through orbital mechanics, like satellites. We’ll also learn how to throw objects across the room in the name of science… called projectile motion. Are you ready for a fast and furious physics class?

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Materials:

• marbles
• 9 popsicle sticks
• 4 rubber bands
• one plastic spoon
• ping pong ball
• hot glue gun with glue sticks

# Key Concepts

Centripetal means ‘center-seeking’. It’s the force that points toward the center of the circle you’re moving on. When you swing the bucket around your head, the bottom of the bucket is making the water turn in a circle and not fly away. Your arm is pulling on the handle of the bucket, keeping it turning in a circle and not fly away. That’s centripetal force. Centrifugal force is equal and opposite to centripetal force. Centrifugal means ‘center-fleeing’, so it’s a force that’s in the opposite direction. The car pushing on you is the centripetal force.The push of your weight on the door is the REACTIVE centrifugal force, meaning that it’s only there when something’s happening. It’s not a real force that goes around pushing and pulling on its own.

# What’s Going On?

Engines used to use an automatic feedback system called a centrifugal governor to regulate the speed. For example, if you’re mowing the lawn and you hit a dry patch with no grass, the blades don’t suddenly spin wildly faster… they get adjusted automatically by a feedback system so maintains the same speed for the blades, so matter how thick or thin the grass that your cutting is. You’ll find these also in airplanes to automatically adjust the pitch (or angle) of the propeller as it moves through the air. The pilot sets the intended speed, and the airplane has a governor that helps adjust the angle the blades make with the air to maintain this speed automatically, because the air density changes with altitude. It’s really important to know how much centrifugal force people experience, whether its in cars or roller coasters! In fact roller coaster loops used to be circular, but now they use clothoid loops instead to keep passengers happy during their ride so they don’t need nearly the acceleration that they’d need for a circular loop (which means less g-force so passengers don’t black out).

Here are more roller coaster maneuvers you can try out:

Loops: Swing the track around in a complete circle and attach the outside of the track to chairs, table legs, and hard floors with tape to secure in place. Loops take a bit of speed to make it through, so have your partner hold it while you test it out before taping. Start with smaller loops and increase in size to match your entrance velocity into the loop. Loops can be used to slow a marble down if speed is a problem.

Camel-Backs: Make a hill out of track in an upside-down U-shape. Good for show, especially if you get the hill height just right so the marble comes off the track slightly, then back on without missing a beat.

Whirly-Birds: Take a loop and make it horizontal. Great around poles and posts, but just keep the bank angle steep enough and the marble speed fast enough so it doesn’t fly off track.

Corkscrew: Start with a basic loop, then spread apart the entrance and exit points. The further apart they get, the more fun it becomes. Corkscrews usually require more speed than loops of the same size.

Jump Track: A major show-off feature that requires very rigid entrance and exit points on the track. Use a lot of tape and incline the entrance (end of the track) slightly while declining the exit (beginning of new track piece).

# Troubleshooting

Marbles will fly everywhere, so make sure you have a lot of extras! If your marble is not following your track, look very carefully for the point of departure – where it flies off. For instance, when the marble flies off the track, you can step back and say:

“Hmmm… did the marble go to fast or too slow?”

“Where did it fly off?”

“Wow – I’ll bet you didn’t expect that to happen. Now what are you going to try?”

Become their biggest fan by cheering them on, encouraging them to make mistakes, and try something new (even if they aren’t sure if it will work out).

1. Does the track change position with the weight of the marble, making it fly off course? (You can make the track more rigid by taping it to a surface.)
2. Is the marble jumping over the track wall? (You can increase your bank angle – the amount of twist the track makes along its length.)
3. How can you make your marble zip through two loops at once?
4. How could you increase your marble speed?
5. Where would you put a tunnel? (Leave one piece of track uncut to use as a tunnel.)

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