Dr. Gadget's Science Machine
Device demonstrates value of wheels

By Joe Laszlo

Mililani Mauka fifth-graders Brittnie Aguilar and Tiana Atuaia experiment with the wheeled cart. Deborah Booker The Honolulu Advertise

Aloha! I'm sure hardly a day has gone by when you've not used a wheel. Do you ride to school on a car or bus? Maybe you ride a bike. Do you have a wheeled book pack? Wheels are everywhere. Why do you use them?

Let's explore the uses and movement of a wheel by making a wheeled gadget. Follow these easy instructions:

• Get a 5-by-8-inch piece of cardboard, plus a soda straw, a bamboo skewer, four similar plastic bottle caps, and white glue or an electric glue gun. If you use corrugated cardboard, make sure the corrugations are going lengthwise.
• Measure half an inch in on the long sides and draw a line. Fold the two sides down. Starting at each corner, measure 1 inch back and 1/2 inch up, then make a hole large enough to allow the soda straw to fit through.
• Cut the straw into two pieces 4à inches long. Thread it through the holes in the sides. These are the bearings. Cut the bamboo skewer into two pieces 5à inches long. These are the axles (Figure 1).
• Make a hole in the center of each plastic bottle cap wheel.

To find the center, place one of the caps on a piece of paper and trace a circle around it. Cut out the circle. Fold the paper in half twice, then unfold it. The two fold lines cross in the center. Make a small hole here. Now place the circle onto each of the caps and mark the center.

• Find a nail that is the same diameter as the bamboo skewer and carefully make a hole in each bottle cap from the outside (Figure 2). Test fit all of the wheels onto the axles.
• To assemble the cart, put one wheel onto an axle, cap top in. Put the axle into the straw bearing and put the other wheel on the other end of the axle with the cap top in. Repeat with the second axle. Use white glue or an electric glue gun to secure the wheels. The cart is done!

Now let's try this experiment:

• Find something heavy, such as a block of wood. Attach a rubber band to it and pull it along a table top using the rubber band. Did the rubber band stretch far before the block moved? Why?

Make a small hole in the front edge of your cart. Attach the rubber band. Put the block of wood on the cart and pull it with the rubber band. Did it stretch as far as it did pulling the block without the cart? It shouldn't have, but why? If you said there is less friction, you are correct. The block of wood dragging along the table top has more sliding friction than the rolling friction when using wheeled cart. The wheels you use reduce friction and make work easier.

I'll have another cart gadget for you in a couple of weeks. Until then ... a hui hou!

"Dr. Gadget's Science Machine" is written by Joe Laszlo, a retired science teacher and winner of a Presidential Award for Excellence in Science Teaching. Write to Dr. Gadget c/o The Honolulu Advertiser, P.O. Box 3110, Honolulu, HI 96802, or fax 535-8170. You may send e-mail to ohana@honoluluadvertiser.com if you're 13 or older.