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Combined: What is a Compound Machine?

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Author: Janice VanCleave

Problem

What is a compound machine?

Materials

• scissors
• large piece of cardboard, 12 inches × 12 inches (30 cm × 30 cm)
• 18 3-ounce (90-ml) paper cups
• box, at least 12 inches × 24 inches × 12 inches (30 cm × 60 cm × 30 cm)
• dowel rod, ¾ inch (2 cm) in diameter and 1 yard (1 m) long string
• paper clip
• large pushpin

Procedure

1. Cut two circles, each with a 6-inch (15-cm) diameter, from the cardboard.
2. Form gear teeth for each cardboard circle by taping two sets of nine 3- ounce (90-ml) cups together as shown in the diagram. Do not leave gaps between the cups.
3. Form a circle with each of the two sets of cups by taping the end cups together.
4. Place a cardboard circle inside each ring of cups, with the bottoms of the cups facing out.
5. Use tape to secure the cups to the paper circle.
6. Ask an adult to make a hole at each end of the box. The holes should be centered about 3 inches (7.5 cm) from the top of the box and slightly larger than the dowel rod's diameter.
7. Make a hole in the center of one of the cardboard circles. The hole should be just large enough for the dowel rod to slip through tightly.
8. Cut out a 12-inch (30-cm) × 6-inch (l5-cm) section from the top of one end of the box.
9. Hold one gear inside the cutout in the box, while your adult helper pushes the dowel rod through one of the holes in the end of the box, through the hole in the gear, and then out the second hole in the end of the box.
10. Tape a 12-inch (30-cm) string to the end of the dowel rod nearest the gear.
11. Attach the paper clip hook to the free end of the string.
12. Lay the second gear on top of the box, with its teeth meshed with the teeth of the first gear.
13. Stick the pushpin through the center of the second gear and into the box.
14. Hold the pin with one hand to secure the gear, and slowly turn it clockwise with your free hand.
15. Observe the movement of the gears, dowel rod, and string.

Results

The horizontal gear turns clockwise, pushing the vertical gear toward the front of the box. The dowel rod turns with the small gear; thus, the string winds around the rod, raising the paper-clip hook.

Why?

A machine is any tool used to change either the magnitude, the direction, or the speed of a force. All compound machines, no matter how complex, are combinations of two or more simple machines. The six simple machines are: lever, wheel and axle, inclined plane, screw, wedge, and pulley. Combining simple machines allows you to do work that one simple machine can not do. The compound machine in this experiment used gears and a wheel and axle to lift a load. The teeth of the gears mesh (fit together); thus, turning one gear causes the second gear to turn.

Let's Explore

1. When gears are of equal size and have the same number of gear teeth, they both turn at the same speed. Would increasing the number of teeth on one of the gears change the speed of the smaller gear? Repeat the experiment using a 12-inch (30-cm) diameter cardboard circle and 14 5-ounce (150-ml) paper cups.
2. Would increasing the radius of the axle of the wheel and axle machine affect the results? The gear and dowel rod act as a wheel and axle machine. Repeat the original experiment using a dowel rod with a larger diameter. Measure the height that the hook rises for every turn of the dowel rod. Science Fair Hint: Every time the dowel rod makes one complete turn, the paper clip hook is raised a height equal to that of the rod's circumference (distance around the rod). Compare the number of turns needed by each axle to raise the hook to the top.

Show Time!

Build a bubble machine to demonstrate the combining of simple machines to make a compound machine. Push a spool of thread onto the end of a dowel rod. The spool should fit snugly on the rod. Punch holes through the centers of two boxes. Push the end of the dowel rod through the holes in the first box, through the center of a Styrofoam™ wheel, and through the holes in the second box. Use flexible wire to form looped bubble wands and insert the ends of the wire loops around the edge of the wheel. Position a pan of soap solution, made with one part liquid dish soap and eight parts water, under the wheel so that the bubble wands dip into the liquid as the wheel turns. Slowly pull the thread while a helper blows through the loops to make bubbles.