Have you ever had someone give you directions, but then that person kept switching around while giving them?  Turn here!  Wait, no.  Turn here!  Oh, we missed it, turn back around!  That can be frustrating, but that is what actually happens when something moves in a circle.  It keeps changing directions, little by little, so that it always ends up back where it started.  What would be the best way to show this?  Hmm.  Have you ever been inside of a soccer ball?  No?  Well, better late than never.

Before we shrink you down and stick you inside the ball, let's watch it move from the outside first.  Go ahead and kick it across the grass.  KICK!  Watch as the soccer ball speeds up right after you kick it and then slows down until it stops.  Acceleration is when something goes faster, slower, OR changes direction.  If a thing keeps rolling at the same speed, it is not accelerating.  Okay, now are you ready to get inside that soccer ball?  We have got to get you loose first.  How else will you squeeze inside?

This will give it some acceleration.

Go ahead and kick the soccer ball in a circle around the grass.  We call it circular motion when something moves in a circle.  Can you kick it in such a way that you only kick it one time but still make it go in a circle?  Of course not!  You have to kick it a little to the side, run, kick it a little to the side, run, and kick it to the side again.  In order for something to move in a circle, it has to keep changing directions.  If we were to draw a line for each time you kicked the soccer ball, the shape on the grass would look less like a circle and more like a shape with many, many sides.  All right, looks like you are loosened up now.

Remind me to never try this kind of circular motion again.

Ready to climb inside the ball?  Of course you are!  I will take off one of these black shapes on the side of the ball, and you can crawl inside.  I know it's dark and a little cramped in there, but trust me, this is the best way to learn about circles . . . I think.  When I kick the soccer ball, feel the side of the ball push against your body as it spins around in circles.  If it weren't for the push of the side of the ball, you'd just fly out in one direction.  The outside of the ball pushes you in toward the center of the ball to keep you spinning.  Centripetal acceleration is the force that moves the parts of something toward the middle of the circle.  This is really what makes the ball move in a circle.  It's also what makes you feel squished against the seat when you are on a roller coaster and it turns.  The seat is pushing you toward the middle of the turn, so you don't fly off into the air. . .

Tell me again what is keeping us in our seats when we're upside down?

I did not warn you about this next part, but now I am going to kick the soccer ball into outer space.  Do you want to stay inside or come out?  Stay in?  Great.  KICK!  WHOOOOOOOOOOSSSSHHHHHHHHH.  The soccer ball exits the Earth and starts moving around the planet in the same place where there are satellites and space junk.  Now you are moving in a MUCH bigger circle.  Orbit is when something moves in a curved path around a planet or star.  Earth moves around the sun.  The moon moves around us.  Right now you are orbiting around the Earth.  You may not be the first soccer ball in space.  But I will bet you are the first kid inside of a ball in space.

In space, inside a soccer ball, you move in a lot of different kinds of circles.  You curve around the Earth.  You feel a pull to the middle as the ball spins in place.  Circular motion means always changing the way you are going.  It means turning little by little so that it comes right back around to where it began.  Speaking of which, how do we get you back to where you began?  If you see any astronauts up there, have them kick you back this way.


How Stuff Works.  "Centripetal Force"  Discovery, 2012.  <http://science.howstuffworks.com/centripetal-force-info.htm>