# Bernoulli's Slalom

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Using some household materials and some basic physics, you can have a fun game that is playable for hours.

Note: Ball may not glow as shown in the picture below.

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## Step 1: Gather Materials

For this project, all you need is string, clothes hangers, a hair dryer, and a ping-pong ball. You may also need an extension cord or tape.

## Step 2: Build the Course

Tie a length of string to the clothes hanger and then hang it from the ceiling. As long as the clothes hanger is dangling freely in the air, it is okay. Repeat this with the rest of the coat hangers until you have something looking kind of like a mix between croquet and slalom gates.

## Step 3: Levitate a Ball

Plug in the hair dryer and turn it on its highest speed. Hold it so that the air blows straight up. Then hold the ping-pong ball towards the top of the stream of air and let it go. It should be suspended in air becuase of Bernoulli's Principle. Bernoulli's Principle states that as the speed of a fluid (in this case air) increases, the pressure decreases. The momentum of the air coming out of the hair dryer provides the lift for the ball. The tendency of the ball is to be pushed out of the stream of air, but because the air stream is moving faster than the air around it, it creates low pressure, and sucks the ball back into the air stream.

Practice with keeping the ball up and moving the hair dryer around. You may want to plug the hair dryer into an extension cord so that you can be more mobile.

## Step 4: Race

You have probably guessed it by now, but you are going to be trying to move the ping-pong ball through all of the clothes hangers. You can make a fun game out of it and see how fast you can do the course. Get a friend to do it too, and then you can have races. Change the course around and see what things make it more challenging.

This is a really fun way to demonstrate physics!

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## 15 Discussions

I am afraid that your description of how the fluid mechanics behind this game includes a number of inaccuracies. For a start, in the turbulent jet above the hair dryer, the law of Bernoulli does not even apply. Secondly, even if there was no turbulence, the pressure in the jet would not be lower that that of the surrounding air. If it was, the jet would contract real quick after the nozzle. The fan adds energy to the air in the jet, so we cannot (easily) use Bernoulli across the edge of the jet. ...and other things. To cut a long story short, small balls (small in comparison with the jet diameter) are dragged into the jet because the surrounding air is dragged into the jet. Large balls are attracted because of the Coanda effect. Still, the game works, and that's all we are interested in

2 replies

Thumbs is correct. This Bernoulli Principle misunderstanding has been parroted by so many for so long that the real science has been burried in so much of this bad science noise that no one can get the true science.

P.S. small balls experience Coanda ad well. The curved path of the air creates the pressure fifference in both cases.

won't the hangers spin due to the air? though it will make more tough.

so, what do you do with the hangers? you shoot the ball inside??

chill out, dude... i remember doing this in Physics class in 10th grade... i never thought of making a game out of it!!! you sir, are a wonderful man.

I have seen this in the windows of electical stores, when vacuum-cleaners were still an exciting buy I guess...

I seem to remember doing this is school with just a straw and a pingpong ball. you can do it with one of those flexy straw. can't do it for very long, but it's still fun. less energy consumption too..

Another great idea for my Sunday school group! Pete

I've done this on a couple of children's TV shows here in the UK. The best variation we came up with used a garden leaf blower and a small mandarin orange or satsuma. Levitating soft fruit is way more fun than you'd think. Just... think carefully before you try it indoors.

I used to have a vacuum that did this. there was a ping-pong sized ball that rose up into the air when you turned the vacuum on. it was a strong vacuum, so there was still plenty of exhaust air to make it like a hovercraft also. it had no wheels, it was orange & white, and yeah it hovered and had a ping pong ball to tell you it was running - as if the intense noise wasn't enough.

use the 'no heat' setting if you have one to save power! the heater part uses 1500W, the blower part maybe 50W.

Sweet! I'm going to try this out with my boys small group.