Forces and Motion

Mesmerizing CD Top

Turn an old CD into a mind-bending tabletop spinner. Spin, stare, and watch the world change.

Adult supervision required

From the Junk Drawer:

[] Old CD

[] Paper

[] Markers

[] Scissors

[] Superglue or hot glue

[] 2 flat glass beads

Step 1: Trace around a CD on a piece of paper. Use a marker to create swirls going out from the center, as shown. Then cut around the outside of the CD line.

Step 2: You need adult permission or help for this step. Lay a piece of scrap paper on your work surface to catch any glue that drips. Use hot glue or superglue to affix the round piece of paper to the CD. Then glue one glass bead to each side of the hole in the center of the CD, with the flat sides facing the hole. These glass beads are commonly found in craft stores and are used as decorations for flower vases, tabletop water fountains, and candle sets. Let the glue dry completely.

Step 3: If you color in the swirls, it will make them wider, which will make the illusion better.

Step 4: Pinch the top glass bead with your fingers and spin it. Practice until you can get it to spin like a top while sitting in place.

Step 5: Now spin the top and lean directly over it. Stare at the center for about 30 seconds as it spins, then look up at a plain wall. It should make the wall spin a little bit. Keep practicing until you master it.

Try staring at the top for different amounts of time. Crazy! Do it again and look at other things. Amaze your friends as you warp the world around you. You can make other tops and try different patterns and colors.

The Science Behind It

The spiral on the top of the CD "trains" your eyes to follow the swirl. This effect is called persistence of vision. Your brain holds onto an image for a fraction of a second. Since that image is a spinning spiral, when you look at a wall or something else, it will spiral for a few seconds. It might take a few tries to get good at it. (Even if you don't always get the swirl effect, you still made a fun top.)

Grocery List Tug-of-War

Use a piece of paper to learn about inertia.

From the Junk Drawer:

[] Several strips of paper

[] Scissors

[] Coins

[] Tape

Step 1: For this experiment, you need several strips of paper. A long grocery list notepad will work, or you can take a sheet of letter size paper and cut it into four strips. Paper that is already written on is perfect — you're recycling.

In each strip, make two cuts that almost go through the paper, as shown. To do this, fold each strip of paper, but do not crease the center. This will insure the cuts are equal in length.

Step 2: Pick up a single strip, holding one end in each hand. Your goal is to try to tear both ends off at the same time so that you are left with just the center part. First, pull slowly and watch what happens. Then repeat with another strip and try pulling faster. Can you do it?

Step 3: Now tape four or five coins to the center section of a cut strip.

Step 4: Quickly pull on the ends of the strip and watch what happens this time. The trick is to pull fast and hard.

The Science Behind It

Inertia is the tendency of an object to resist changes in motion. A heavy object is hard to start moving, but a heavy object is also harder to stop once it is moving. Just think of a semitruck on the interstate and how long it takes to come to a stop.

When you try this experiment with paper alone, the center piece doesn't have much inertia — it's light. When you tug it, one end tears before the other, leaving two pieces together. But when you tape coins to the center section, you give that section extra inertia. This time, when you tug quickly, the center section wants to stay in place more than before (it has more inertia). The ends tear off, leaving you with just the center.

Jar Spin

Defy gravity as you take a ball for a spin.

From the Junk Drawer:

[] Clear, large-mouth plastic jar (mayonnaise or peanut butter jars work well) with label removed

[] Small bouncy ball

Step 1: Place a clear plastic jar mouth-down over a small bouncy ball. (Don't do this on a nice wooden table.) Ping-pong balls also work well.

Step 2: Grasp the bottom of the jar. While keeping the jar's mouth on the table, swirl the jar quickly in small circles, causing the ball to roll in circles as well. Move the jar faster, until you can pick up both the jar and the ball.

The Science Behind It

As you swirl the jar in a circle, the ball wants to move in a straight line. Of course, it can't, because the jar is in the way. The jar supplies a centripetal (center-seeking) force on the ball. The ball pushes back against this centripetal force, which creates friction between the inside of the jar and the ball. When the speed of the ball is great enough, the friction will keep the it "glued" to the inside wall as you lift the jar.

Floating Coin

Magically lift a coin with science.

From the Junk Drawer:

[] Crisp dollar bill (or piece of paper)

[] Coin

Step 1: Fold a dollar bill in half so it forms a V, as shown. You can also do this with a stiff piece of paper. Place the coin over the V.

Step 2: Slowly pull apart the two ends of the dollar bill. The coin should stay in place and balance on the paper. Now slowly lift the dollar bill, picking up the coin with it. With practice and a steady hand, you can pick up the coin even when the bill is completely straight. Practice this trick, then amaze your parents, friends, and teachers.

The Science Behind It

All objects have a center of mass. If the center of mass is over a support, the object will stay balanced. When you balance on one foot, you have to move your center of mass over that foot. Try it; as you lift one foot up, your body will naturally move your center of mass over the other foot. Similarly, when you slowly pull the ends of the dollar bill, the center of mass for the coin will move directly over the dollar bill. As long as the center of mass stays over the supporting object, you can pick up the coin.

Crash Test Dummy

Learn Newton's laws with a dummy to help.

From the Junk Drawer:

[] Toy car

[] Small action figure

[] Rubber band

[] Large marshmallow

Step 1: Set a crash test dummy — a plastic action figure — on a toy car. Push the toy car into a wall. What happens to the dummy?

Step 2: Put the crash test dummy back on the car. This time, use a rubber band to attach the dummy to the roof. Make sure the rubber band does not interfere with the wheels. Push the toy car into a wall again. What happens?

Step 3: Put the action figure on the car one last time. Use a rubber band...