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Spinner: How Can Wheels Be Made to Rotate Faster?

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Author: Janice VanCleave

Problem

How can wheels be made to rotate faster?

Materials

  • 1-gallon paint can (one that has never been opened)
  • 6 marbles
  • 4 heavy books

Procedure

  1. Place the can on a table.
  2. Space the marbles evenly around the rim of the can.
  3. Balance a stack of books on top of the marbles.
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  5. Use you hand to push gently against one comer of the stack of books.
  6. Observe the movement of the books.

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Results

The books spin around easily on top of the marbles.

Why?

Wheels allow you to move things more easily, and ball bearings within wheels allow them to rotate faster. Ball bearings are spheres placed between a wheel and an axle. These balls reduce the friction (the resistance to motion) between the surfaces of the wheel and the axle. Without the ball bearings, the surfaces would rub together because even very slick materials have slight bumps on their surfaces. As a wheel turns, the bumps on the surface of the wheel catch on the bumps of the axle's surface, thereby slowing the wheel's rotation. The marble bearings in this experiment rotate as the book pushes against them, and only the tops of the balls touch the book. Since only a small part of the surface of the marbles touch the book, there is less friction. The motion of the marbles and the reduction of friction between the book and the marbles increases the rotation speed of the book.

Let's Explore

  1. Would increasing the number of ball bearings reduce friction? Repeat the experiment using more marbles around the rim of the can.
  2. How effective are the marble bearings? Repeat the original experiment without the marbles. Science Fair Hint: Display models with and without marble bearings. Demonstrate the difference between moving a stack of books on each model.

Show Time!

  1. Pulleys can reduce friction. Demonstrate this by placing a pencil through the hole in a thread spool. Be sure that the spool turns easily on the pencil. Tie a 6-inch (15-cm) piece of string to each end of the pencil. Suspend the pencil and spool from a table edge by taping the strings to the edge of the table, making sure the pencil is level and does not rotate. Use tape and a pen to label two paper cups with the letters A and B. Punch a pair of holes at opposite ends of the top of each cup, and attach a string to cup A, as indicated in the diagram. Set cup A on the floor, and place the string over the spool. Ask a helper to hold cup B about 12 inches (30 cm) above the floor while you tie the end of the string to the cup. Place 20 pennies in cup A Add pennies to cup B, one at a time, until it starts to move downward slowly. Repeat the experiment, but remove the spool and place the string over the pencil. Compare the number of pennies needed to raise the cup with and without the rotating spool pulley.
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  3. Construct a conveyor (a mechanical device for moving materials from one point to another). Use a paper hole punch to make six pairs of holes across from each other about 1 inch (2.5 cm) from the top of a small cardboard box. Insert six round pens (all the same size) through each pair of holes, forming bridges across the box. Move each pen around to hollow out the holes so the pens rotate freely. Cut out the top ends of the box about 2 inches (5 cm) deep to allow objects to be pulled across the pen rollers. Pull objects across the rollers and then across a surface top without rollers. Display the conveyor model, and compare the ease of moving an object with and without it.

Check It Out!

Scientists continue to seek a "perpetual motion machine." Find out what perpetual motion means. What is your opinion about the possibility of such a machine ever being designed?

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