0.20 Phy1180: energy -- potential energy  (Page 3/7)

Potential energy and the rubber band

Similarly, stretching and then releasing the rubber band as described above first stored, and then released elastic potential energy in the rubber band.

Work is required to increase the potential energy of an object

In both cases, it was necessary for you to expend energy in order to increase the amount of energy stored in the rock and the rubber band.

Gravitational potential energy

Gravitational potential energy is the energy stored in an object as the result of the gravitational attraction between the earth and the object. Inpractical terms, the energy is stored as the result of the objects height above the surface of the earth.

Every object that has a position above the surface of the earth has stored gravitational potential energy. Some common examples are

• Books on a bookshelf
• An air conditioning unit in a window on the third story of an apartment building
• You, when you climb a tree and sit on a limb or walk up the stairs to your classroom.

Because all objects are attracted to a point in the center of the earth, all objects that don't rest on the surface of the earth have potential energy.

All objects have stored gravitational potential energy

Even those objects that do rest on the surface of the earth have potential energy, because they have a strong tendency to make their way to the center of theearth. In most cases, however, that desire cannot be fulfilled so we usually consider the surface of the earth to be the reference point for gravitationalpotential energy.

The deep

In the final analysis, however, only those objects that rest at the bottom of the deepest point in the ocean cannot be forced to give up stored gravitationalpotential energy. In theory, every other object could be transported to and dropped to that point on the ocean bed, thereby giving up stored potentialenergy in the process.

The height and the mass are critical

The magnitude of the potential energy possessed by the rock balanced on the tree limb in the earlier example depends on two things: the height of the rockabove the ground and the mass of the rock.

Gravitational potential energy is proportional to mass

The gravitational potential energy of an object is proportional to the mass of the object. Objects with more mass are capable of having more potential energy than objectswith less mass. Therefore, if the rubber band discussed in the earlier example were to be placed on the same limb as the rock, the rock would have moregravitational potential energy than the rubber band due to its greater mass.

Gravitational potential energy is proportional to height

Gravitational potential energy is also proportional to the distance of the object from the center of the earth. Thus, a 10 kg rock falling from 50 metersonto your head would do more damage than the same 10 kg rock falling onto your head from 2 meters. In both cases, the potential energy of the rock wouldbe converted to kinetic energy, (which is a topic for a future module), and that kinetic energy would be converted to pain when the rock strikes your head.