0.20 Phy1180: energy -- potential energy  (Page 5/7)

A steel beam

A steel beam that supports a bridge can normally flex by a small amount when loaded by traffic on the bridge and return to its original shape once the loadis removed. However, there is a point beyond which it cannot recover if flexed too far.

A coil spring

A coil spring in an old-fashioned fisherman's scale can stretch up to a certain limit when loaded with a fish, and then return to its original lengthwhen the fish is removed. However, if it is stretched beyond its elastic limit,it won't return to its original length when the load is removed. (The enclosure on a fisherman's scale is usually designed to prevent the spring from beingstretched beyond its elastic limit.)

Many objects store elastic potential energy

Elastic potential energy can be stored in steel beams, rubber bands, golf balls, springs, automobile tires, etc. The amount of elastic potential energystored in such a device is related to the amount by which the object is deformed (usually stretched or compressed). The more the object is deformed, up to apoint, the more elastic potential energy is stored in the object.

Springs are a special case

Springs are a special case of an object that can store elastic potential energy either through stretching or compression. Some springs are probablymanufactured with one or the other in mind while other springs may be manufactured to serve both purposes.

For example, the spring in a fisherman's scale is probably manufactured with only stretching in mind. The process of weighing a fish on a fisherman's scaleis to hang the fish on a hook on the bottom end of the spring and measure how far the spring stretches.

On the other hand, I believe, but am not certain, that the coil springs in the suspension of a car serve their purpose by both stretching an compressing.

A force is required

A force is required to compress or stretch a spring. The more the spring is compressed or stretched (depending on its purpose), the more force will berequired to compress or stretch it further.

The spring constant

For certain springs, the amount of force required to compress or stretch the spring (up to a limit) is directly proportional to the amount of stretch orcompression. This can be expressed in equation form as

Fs = k * x

where

• Fs is the force applied to the spring
• k is the constant of proportionality
• x is the amount of stretch or compression of the spring

Hooke's law

The constant of proportionality in the above equation is known as the spring constant . Springs that behave this way are said to follow Hooke's law, named after the 17thcentury British physicist Robert Hooke.

The equilibrium state

If the spring is not compressed or stretched, it is in its equilibrium state, andthere is no potential energy stored in it. The equilibrium state is the state that the spring naturally assumes when no forces are acting on it. This statecould be called the zero-potential energy state.

(This is the state of the rubber band that hung loosely around your wrist in the experiment at the beginning of this module.)