# Hands-on science (no lab required): after reading "Bent on Speed", try out this hands-on experiment to learn how bikes balance.

Low racers--which are all about straightaway speed--can't make turns. But recreational recumbent bikes can--and for that they need to be able to balance. Perform this activity to see how center of gravity, or the point around which an object's weight is balanced, helps some bents whiz around corners.

PREDICT

Would you expect that a lower or higher center of gravity would lead to better balance for a biker, or even a tightrope walker?

YOU NEED Corrugated cardboard (10 cm by 10 cm, or 4 in. by 4 in.) * pencil * ruler * drinking straw (5 cm, or 2 in.) * yarn (90 cm, or 36 in.) * 3 pieces of tape (8 cm, or 3 in., each) * 1 pipe cleaner (30 cm, or 12 in.) * 2 metal washers * notebook paper (for recording your results)

PROCEDURE

1. Form groups of 3 to 4 students.

2. Place the cardboard square on your desk. Using a ruler, find the middle of the cardboard's bottom edge. Mark this middle point with a pencil. This point will be your tightrope walker's feet. (Feel free to draw a face on your walker!)

3. Tape a straw at this mark. The straw should be perpendicular to the cardboard's bottom edge.

4. Thread a piece of yarn (about 90 cm, or 36 in.) through the straw.

5. Tape or tie one end of the yarn to a desk leg. Tape or tie the other end to another desk leg so that the yarn is taut. This is your "tightrope."

6. Have one student see if the tightrope walker balances upright on the string. Another student will record the result.

7. Stick a pipe cleaner that's 30 cm (12 in.) long through the cardboard's corrugations, or grooves, near the straw. The pipe cleaners' ends should stick out an equal amount on each side of the cardboard.

8. Lower the walker's center of gravity: Bend each side of the pipe cleaner so they hang straight down.

9. Repeat Step 6.

10. Lower the walker's center of gravity even more: Bend the tip of each pipe cleaner into a U-shape. Add a metal washer to each end.

Twist the ends to secure the washers.

11. Repeat Step 6.

CONCLUSIONS

1. Which tightrope walker--with no pipe cleaner, with a pipe cleaner, or a pipe cleaner plus washers-showed the best balance?

2. Which lowered the tightrope walker's center of gravity most: the pipe cleaner by itself or with washers attached? Explain.

3. Which would you prefer: a recumbent bike with a lower or higher center of gravity? Explain.

1. The pipe cleaner with washers attached will result in the most-- balanced tightrope walker. That's because the washers pulled the walker's center of gravity lower and helped it balance.

2. The pipe cleaner with attached washers lowered the tightrope walker's center of gravity most. With more weight farther below the string, the center of gravity is lower.

3. A recumbent bike with a lower center of gravity would help keep balance around sharp turns.