0.20 Phy1180: energy -- potential energy  (Page 2/7)

An accident waiting to happen

Perhaps the change in potential energy can best be illustrated by stating that the rock now has the potential to crack someone's skull open if they happento be standing under the limb when the rock falls towards the ground with an acceleration of 9.8 m/s^2.

When the rock was laying on the surface of the earth, it did not have that potential. You created that potentialby imparting potential energy into the rock when you listed it from the ground and balanced it on the limb.

Elastic potential energy

Consider a common rubber band. Because of its small mass, a rubber band can never be expected to acquire much in the way of gravitational potentialenergy. However, it can acquire a considerable amount of elastic potential energy.

While I don't recommend that you try the experiment with the rock described earlier, this is an experiment that you can try with no permanent damage to yourperson.

Thread your arm through the rubber band

Find a strong rubber band and thread your left arm through the rubber band up to the wrist. Make certain that the rubber band fits loosely on your arm. Atthat point, the rubber band has little or no potential energy.

Change the potential energy

Now grasp the rubber band between the thumb and forefinger of your right hand and stretch it to the point where it is almost ready to break.

At that point, you have expended your energy in stretching the rubber band, and the rubber band hasacquired elastic potential energy.

How much elastic potential energy?

How much elastic potential energy did the rubber band acquire? You can get a qualitative estimate of the amount of potential energy by releasing the rubberband and letting it strike your bare wrist.

When the rubber band contacts your skin, it will impart energy into the nerve ending in your skin as its potential energy goes back to zero. If it hurts alot, there was a lot of elastic potential energy stored in the stretched rubber band. If it doesn't hurt very much, there probably wasn't much potentialenergy stored in the rubber band, or you have very tough skin with few nerve endings.

Potential energy

There are several ways that an object can acquire and store energy, which we will refer to as potential energy . Two of the most common ways are:

• Raising the object to a new height above the surface of the earth (gravitational potential energy)
• Deforming the object within its elastic limit (elastic potential energy)

Increasing the potential energy of the rock

For example, elevating the rock by 2 meters above the ground as described earlier imparted energy into the rock. (Work was done on the rock in order toincrease its height above the ground.)

Storing the potential energy

Balancing the rock on the limb caused that energy to be (temporarily) stored in the rock as gravitational potentialenergy. The rock stores that energy for so long as it is held at that elevated position.

Releasing the potential energy

Tipping the rock off the limb so that it would fall back to the ground caused that potential energy to be released and caused the potential energy of the rockto go back to zero.