0.4 Springs

A favorite device for storing potential energy is the spring . Everything from clocks to catapults make use of springs. One common use for springs is toreturn something to its original position. Another interesting application is the measurement (and creation of) force, such as springs in a scale. The third use is to store energy. All springs perform allthree functions all of the time, but specific devices are built to exploit certain functions of the spring.

There are many physical forms of springs, created for specific purposes. The most basic form is the bending bar, shown in Figure 1. The familiar coil spring is just a bar spring in a different form. A rubber band is also a spring that makes use of the elastic properties of polymer materials.

To understand how a spring works, one must zoom in to the microscopic level where molecules interact. Molecules are held together in rigidbodies because of electromagnetic forces. Some of these forces are repulsive, and some of them are attractive. Normally they balanceout so that the molecules are evenly spaced within an object; however, by bending a bar, some molecules are forced farther apartand others are shoved closer together, as in Figure 1. Where the molecules have been spread out, the attractive forces strive to return theoriginal spacing. Where molecules have been forced together, the repulsive forces work to return the object to the original shape. A coil spring works in more or less the sameway as a bar spring; when its shape is deformed, molecular forces act to return it to its original shape.

Rubber bands

A rubber band is slightly more versatile than a metal spring because of its flexibility, just as pulleys are more versatilethan their rigid cousin the lever. Using springs in a small robot might take a small amount of imagination, but rubber bands almost scream tobe used. There are often several small tasks that a robot performs only once during a game. It would not make sense todevote an entire motor to such a task. It's not worth carrying around the extra weight if the task could be accomplished justas well with rubber bands. Examples include shooting a ball, pulling up a gate, or closing a jaw.

Rubber bands also prove useful in the case of repetitive motions. Rather than turning a motor forward thenbackwards then forwards and so on, one could make use of a cam and a rubber band to allow the motor to always turn in onedirection. Look at the assembly in Figure for an example.