Slinky Physics

 

Overview

When the Slinky's inventor Richard James, a naval engineer, and his wife decided to demonstrate their new toy at Gimbels Department Store in Philadelphia in the early 1940s, they feared that no one would buy it because it was so simple. They were so worried they gave a close friend a dollar to buy one. An hour and a half after the first demonstration, they had sold a total of 400 Slinkys! The Slinky, whose design has been modified only once to crimp the ends for safety, remains the same today as it did at Gimbels. Not only is the Slinky an excellent toy, its action also demonstrates a variety of physical forces and principles. The Slinky, like all objects, tends to resist change in its motion. Because of this inertia , if it were placed at the top of the stairs it would stay at rest without moving at all. At this point it has potential or stored energy. But once it is started down the stairs and gravity affects it, the potential energy is converted to the energy of motion or kinetic energy and the Slinky gracefully tumbles coil by coil down the stairs. The physical properties of the slinky determine how quickly it moves under the influence of gravity. Although its movement may look simple, from a scientific point of view the motion is quite complex. As the slinky moves down the steps, energy is transferred along its length in a longitudinal or compressional wave, which resembles a sound wave that travels through a substance by transferring a pulse of energy to the next molecule. How quickly the wave moves depends on the spring constant and the mass of the metal. Other factors, such as the length of the slinky, the diameter of the coils and the height of the step must be considered to completely understand why a slinky moves as it does. James originally developed the Slinky for the Navy as an anti-vibration device for ship instruments. When the Slinky failed to work for the Navy, it became one of the most successful toys of all time!

Activity

You can overcome an object's inertia and watch physical forces act on it as it moves. You will also be able to create energy transfers. Materials:

• Empty spool of thread
• Small rubber band about the same length as the spool
• Metal washer
• Tack or pin
• Match stick
• Paper
• Pencil

1. Slip the rubber band through the hole of the empty spool. 2. Attach the end of the rubber band to the end of the spool with the tack or pin. 3. Pull the loose end of the rubber band through the metal washer. 4. Slip the match stick through the open end of the rubber band. Wind the rubber band around the match stick several times until there is no slack left in the rubber band. 5. Let go of the match stick and watch what happens!

Resources

• Ardley, Neil. Muscles to Machine. New York: Glouchester Press, 1990.
• Hewitt, Paul. Conceptual Physics, 2nd Edition. New York: Addison-Wesley, 1991.
• Milgrom, Harry. First Experiments with Gravity. New York: E.P. Dutton & Co., 1966.
• Panati, Charles. Extraordinary Origins of Everyday Things. New York: Harper & Row, 1987.

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