## Tuesday 29 July 2014

### Science: The levitating Ping Pong Ball

This experiment got my girls ever so excited, they were all desperate to have another go.  It is because this experiment is like magic, making it great for entertaining friends and family and learning a bit about Bernoulli's principle at the same time.

For this project you will need:

• A ping pong ball
• A straw
• An empty, clean yoghurt pot (best if the wrapper is removed first)
• A little bit of modelling clay
• Scissors
• A knife (optional)

Please make sure you have a responsible adult nearby when using the scissors and knife.

How to do it...

STEP ONE

Carefully, make a hole in the bottom of your yoghurt pot - Not too big, just enough to make room to push your straw through.

STEP TWO

Push your straw through the hole you have just made. It should hang half way into the pot.

STEP THREE

Use some of your modelling clay to seal any gaps around the straw and the hole you have made.

STEP FOUR

Hover the pot you have made, call it your levitating machine if you like, over the ping pong ball.  Make sure you don't knock your straw or move it in a way that will create gaps that have been previously sealed by the modelling clay.  Now blow into your straw as HARD as you can.  DO NOT SUCK ON THE STRAW!

What happened?

You should have seen the ping pong ball rise into your pot like it was being sucked up.  But how can that be when you are blowing?

HOW IS THIS HAPPENING?

It's all about air pressure!  This is a great experiment to use to demonstrate Daniel Bernoulli's Principle, which states that as the speed of a moving fluid (liquid or gas) increases, the pressure within the fluid (liquid or gas) decreases.

What we are doing is blowing the air out of the cup, this reduces the air pressure.

How does blowing the air reduce the air pressure? Well gas, unlike liquid, will always try to expand (spread out) and fill the whole area it is in unlike liquid.  When we blow we are moving the air molecules and pushing them out of the yoghurt pot.  This reduces the air pressure in the pot, much the same way as in the Kissing Balloons experiment.

This means that the air pressure outside of the pot is now higher than the air pressure inside the pot so the higher air pressure pushes back up wanting to replace the air in the cup that we blew out.  Because of the Ping Pong's low density it gets pushed up by the high air pressure up into the cup making it look like it's being vacuumed up even though you are blowing down on to it.

Something else you can do...

If you want to do this as a trick, you could bet your friends that you can make a ping pong ball levitate by blowing on it.  Then surprise them when you demonstrate that it is in fact, true!