Bernoulli says "You can’t blow ping pong balls for toffee!"

Try this little experiment I guarantee you will have hours of fun with it either alone or like me if you use it at a Kids Science Party.

What you will need

  1. One plastic drinks bottle
  2. One very sharp knife
  3. One very big adult to do the cutting (This is not an optional extra)
  4. One ping pong ball
  5. Antiseptic wipes (if you are going to share the experiment)

How to do it

Get your big adult to cut the end of the bottle at the neck so that the total length is about 5 cm or 2 inches. This should give you roughly 2.5 cm (1inch) for the top and neck section and 2.5 cm (1 inch) for the bowl.

What to do?

Place the ping pong ball in the palm of your hand. Test how easy it is to blow it. Now place the ball on a table or on the floor. Try to see how far you can blow it now. Easy isn’t it. The ping pong ball is very light and very easy to blow away.

Ok then, now for the next part. Place the ball in the bowl of the bottle top and blow the ball out of the top by blowing up through the bottle top. Go on, try again! What is happening? No matter how you blow whether hard, soft or medium, the ball just won’t move. In fact, the harder you blow, the more it seems to be stuck in the bowl.

What is Happening?

Bernoulli worked out that in a stream of faster moving fluid (that is gas or liquid) the pressure is less than in a slower moving fluid.

You could think of it like this. In the fast stream the volume of material, that is the total number of molecules, are stretched because they have further to go and must move faster to get there. The air that goes around the side of the ping pong ball has to go faster because it has to go around the curvature of the ball. And we know that because it has to go faster the pressure is lower.

Now consider the top of the ball. The air there does not have to move so the pressure is normal. Not only is it normal, but it is higher than the pressure surrounding the ball. The ball can’t then move from an area of low pressure to an area of higher pressure and the higher pressure is effectively pressing it back into the bowl of the bottle.

Share the Experiment ... not your Germs!

I am sure that your friends would also like to have a go at this experiment. In fact you would be very strange if you did not challenge them to blow the ball away. However, a word of caution. Whilst it would be good to share your experiment it would be better not to share your germs as well. Wipe the neck of the bottle with your antiseptics wipes as the bottle top is passed from person to person.

There are lots more experiments that one can do with Bernoulli’s principals so keep coming back to find them!

 

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