The coffee cup on a string experiment | Hey Ray

PITTSBURGH (KDKA) - I think we should do some science that gets dangerous with my coffee cup! 

Let me show you my setup.

I tied my coffee cup to a string that was about 3 feet long. On the other end of the string, I tied some washers. 

Then, I draped the cup over the edge of a dowel rod. 

When I let go of the end with the washers, what would you expect to happen to the coffee cup?    

Most people think the coffee cup will fall to the ground and break.  However, when I let go of the string, the cup falls but the string wraps around the wooden dowel rod, catching the cup before it hits the ground.

So what is happening?

When you let go of the string, the cup begins to drop straight down.  At the same, the washers are trying to fall straight down.

As the cup falls, it pulls the string forward, sweeping the end with the washers like a pendulum, creating an arc.  

That angular momentum, or rotational energy, causes the washers and string to swing upward, wrapping around the dowel rod.

This creates friction, causing the cup to stop falling before it is smashed to pieces.  According to Harvard University, the conservation of angular momentum and the exponential increase in friction is what allows me to be able to drink coffee another day!   

This is something that happens every time. You can also use a pencil for this experiment, but make sure it is perfectly horizontal!  You should also perfect this experiment over something soft, so you don't break your cup.

Don't ask me how I know!