This is an Instructable on how to demonstrate that circular motion requires the application of a constant force directed toward the center of the circle, and the vectors that accompany that motion.  First we create the apparatus to demonstrate the principle, then, we will discuss how it is to be used.

## Step 1:

Supplies:
A rubber ball (a high bouncing ball works too, get a large one)
A thick string, 3 or 4 feet long
A size E crochet hook
A pair of pliers
Scissors
Candle
Matches

## Step 2:

Using the counter top, apply force to the base of the crochet hook forcing the tip  through the center of the rubber ball and out the other end.

## Step 3:

Grab the thick string, with the end of the hook.  Use the pliers to pull the hook back through the ball taking the thick string with it.

## Step 4:

When the hook is out of the ball and the loop is out, stop.  The rubber (or high bouncing ball) will close in around the string and hold it in place.

## Step 5:

If you used nylon string, after you cut off the 3/4 foot length, use the lit candle to melt the end.  If you don’t want to do that, tie a slip knot in the end.

## Step 6:

Now you have the apparatus completed.

## Step 7: A Demo for Circular Motion

During your lesson on Circular Motion, walk around the room, swinging the ball in a circle above the top of your head, (practice so you don't need to use a lot of wrist motion), as you describe the circular motion and the fact that circular motion requires the application of a constant force directed toward the center of the circle. Get the students to acknowledge that your hand at the center of the circle is causing the ball to travel in a circle. Believe me when I say that, every student will follow you with their eyes their whole body as, you walk around the room, swinging the ball. When you get to the part where you talk about force vectors and circular motion, keep swinging the ball in the circle, and ask every student to tell you, what they think will happen to the ball when you let go. Get an answer from every student. Then let go of the ball. (It would help if you practice with the ball, to you can let go at a predetermined point. You want it to go where you want it to, not through a window or hit a student). Those who guessed (or knew) that it would travel in a straight line will beam. Those who thought it would curve, or shrugged their shoulders with an “I don’t know”, will never forget that it traveled in a straight line. Now would be a good time to go to the white board and draw circles and vector lines with accompanying equations. This is a fun, easy demo. Enjoy!

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Bio: In a valiant attempt to keep myself from dying of boredom, I create.
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