Backyard Roller Coaster Physics

Have you ever seen a roller coaster loop its passengers all the way around in a circle? Why don’t the people fall out of the roller coaster? It’s not just the safety straps keeping them in their seats; there’s a force locking them in.  See how you can recreate this motion with water as your passenger!

What You Need

  • A light bucket with a sturdy handle
  • A rope (2-3 feet long)
  • Water

What to Do

Tightly tie the rope around the handle of the bucket — knot it well so that it won’t come undone. Fill the bucket about half-way full with water (but not too much — you don’t want the bucket to be too heavy for you to lift!). Make sure you’re outside or in a space where it’s okay to get wet — just in case you lose some water along the way.

Hold the rope in your hand (so that the bucket is around the same level as your knees when you hold it by your side) and spin it around in a vertical circle, passing over your head at a swift speed. As the bucket passes over your head, it should be facing down, but if you’re going fast enough, you should be able to avoid getting a shower. Instead, the water should remain in the bucket, just like the roller coaster passengers remain in their seats when they loop upside-down on the rides!

What's Going On?

Centripetal Force

Even though the water is — for an instant — right over your head, it doesn’t fall down on you the same way it would if you just turned a bucket full of water upside-down. When you simply turn a bucket upside-down, we know that the water falls out of the bucket because gravity is pulling it towards the earth, but the bucket doesn't fall because you are holding it above the earth - exerting a force opposite to gravity.

In our case, gravity is still pulling the water towards the earth, but it is also pulling the bucket towards the earth, so they’re moving together. Above your head, the bucket and water are moving forward, and they’d like to keep going that way, but the string you are using to swing the bucket yanks the water and bucket back into the circular path, exerting what we call a centripetal force.

The water and the bucket are both being pulled by the force of gravity to move downwards but are redirected into a circular path by the centripetal force of the rope! Because of this, the water stays neatly in the bucket, just like the passengers stay in their seats on the roller coaster!

Apply It!

What happens when you swing the bucket slowly? (You’d better be outside to test that one!) How slow can you go before the water starts to fall faster than the bucket?

- Aliya Merali