The Force of the ‘Slinky’

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Is it a toy or is it an industrial equipment stabiliser?

Most of you will recognise this as a toy that is lots of fun to play with. When set in motion, this coiled spring can ‘walk’ down a whole flight of stairs, but it was not originally intended to be a toy at all.

 

In 1943  naval engineer Richard James was attempting to develop a way of keeping fragile equipment steady on ships using springs. One day James knocked one of his new springs from a shelf and to his surprise watched it do that famous Slinky walk down instead of just hitting the ground. He took the creation home to show his wife, Betty, who saw the potential for a new toy. After looking in a dictionary,they came across the name Slinky, a Swedish term meaning “sleek and sinuous.” In Christmas of 1945 the toy was demonstrated in front of Gimbels Department Store in Philadelphia and was an instant hit. The industrial machine James had could coil 80 feet of wire into two inches (5.08cms), and he began producing hundreds of Slinkys.

Other uses

Like many other great inventions, the Slinky has found other uses, including as mobile radio antennas by soldiers in Vietnam and NASA has used them in zero-gravity physics experiments in the Space Shuttle. Slinkys and similar springs can be used to create a ‘laser gun’ like sound effect. This is done by holding up a slinky in the air and striking one end, resulting in a metallic tone which sharply lowers in pitch. This is due to the properties of the metal; higher frequencies travel faster than the lower ones, so the high-pitched sound is heard first, then gets progressively lower.

 

Physical properties

The rules that govern the mechanics of a Slinky are due to Hooke’s law and the effects of gravitation.

 

Flight of stairs

When set in motion on a stepped platform such as a stairway, the slinky transfers energy along its length in a longitudinal wave. The whole spring descends end over end in a periodical motion, as if it were somersaulting down one step at a time.

 

This explains what happens when a Slinky is released at the top of a flight of stairs but what happens if it is held in mid-air and dropped?

 

Take a look at the video of Slinkys in slow motion.

Pause the video at 0.56 secs – take a class vote. What do you think will happen?

Pause the video at 3.22 secs – take a class vote. What do you think will happen?

 

Whether it’s dropped from eye level, or from the roof of a building, the bottom of a fully extended slinky will appear to hover in midair when released. What lets the Slinky’s bottom hover like that?

 

It all boils down to the summation of forces. Remember, a Slinky is just a big, loose spring. When the spring is fully extended like you see in the video, it has reached a point of equilibrium wherein the downward force of gravity is equal and opposite to the upward tension of the coils above the bottommost end of the Slinky.

When released, the top of the Slinky has two forces acting on it: the force of gravity, and the force of the spring, which is also pulling it downward. The bottom of the slinky has two forces acting on it, as well; the difference is that while gravity is still pulling downward, the force of the spring’s tension is instead pulling the bottommost coils upward. The two forces cancel each other out, hence the hovering effect.

 

Digging  Deeper

 

Investigate how the angle of an inclined plane affects how a Slinky can walk.

Toys of the Past:

The Slinky features as one of the most popular Christmas gifts from the last 100 years. What other toys made the most favourite list? Check out this poster!

 

Human Slinky:

Check out this amazing human slinky performance.