Spinning coin inside a ballon- centripetal and centrifugal forces

Today we are going to talk about forces. Yes, those from physics. For the most of young restless minds this is a lot of formulas that make no sense at all.

In fact physics studies natural world, formulas are just the way we have to express that knowledge. Everything we see in a formula we can see in nature, the opposite is not true... yet

The demonstration we show here today only uses a coin and a balloon.
Easy and simple, like most of the great things in nature. This demonstration is perfect to show you what are centripetal and centrifugal forces.

What we need:

• small coin, 1cent or 5 cents;
• balloon;

How to:

1. Place the coin inside the balloon;
2. Inflate the balloon;
3. Make a knot;
4. Move the balloon, from bottom to top or upside down, rotate the balloon in your hand. Do this until the coin gain a steady circular motion.

What happened?

First the coin goes up and down, hits the balloon wall with an uncoordinated movement, after a while the coin begins a  steady circular motion, the coin will keep motion if we keep rotate the hand.

Why?
In a circular movement, centripetal and centrifugal forces are the same value, but with opposite signals, that means each of them pushes the coin in opposite directions. Centripetal force attracts the coin to the center of rotation, on the other hand centrifugal force pushes the same coin far from this center.
The coin maintains its movement because these two forces are in equilibrium with a third one, the tangential force, which makes the coin move inside the balloon.

Some examples of this dynamic are:
A carousel, on which the bars where the vehicles are placed exert a centripetal force, while the person in the vehicle will feel a centrifugal force (1).
When describing a curve with a car, the friction of the road exerts a centripetal force on the wheels so it does not get out of the curve (1).

Another simple observation we can do with this balloon is Newton First Law of Movement, like we did with the egg

How to:

• Spin the balloon until the coin start the steady circular motion;
• Stop moving your hand, quickly place the balloon on the table and let it go.

What happened?
The balloon will keep spinning, like the egg did.

Why?
Because the coin in its interior will keep moving for a few more seconds, and like the Newton First Law of Movement says, if something is moving or stationary, it will stay that way unless something disturbs the system.

References: 
(1)força centrípeta. In Infopédia. Porto: Porto Editora, 2003-2011. Disponível em http://www.infopedia.pt/$forca-centripeta;

Et Voilá!
A coin, a balloon, a demonstration and lots of fun!

Enjoy!