0.16 Bis2a 04.0 energy and thermodynamics v1.2  (Page 37/21)

Introduction to energy

Energy is the ability to do work . Work is done when a force is applied to an object over a distance. When an object is in motion, there is energy associated with that object. In the example of an airplane in flight, there is a great deal of energy associated with the motion of the airplane. This is because moving objects are capable of enacting a change, or doing work. Think of a wrecking ball. Even a slow-moving wrecking ball can do a great deal of damage to other objects. However, a wrecking ball that is not in motion is incapable of performing work. Energy associated with objects in motion is called kinetic energy    . A speeding bullet, a walking person, the rapid movement of molecules in the air (which produces heat), and electromagnetic radiation like light all have kinetic energy. Any moving object has kinetic energy and thus can do work. Similarly, work has to be done on an object to change its kinetic energy. The kinetic energy of an object of mass $m$ and speed $v$ is given by the relation $E=1/2{\text{mv}}^2$ .

Sometimes energy can be stored and used at a later time. For example, a compressed spring and water held back by a dam both have the potential to do work ( [link] ). They are said to possess potential energy    . When the spring or water is released its potential energy is transformed into kinetic energy and other forms of energy such as heat. Remember that wrecking ball described above moving, even slowly has kinetic energy. What if it is motionless, lying on the ground; does it possess any form of energy? Now what if that same motionless wrecking ball is lifted two stories above a car with a crane? If the suspended wrecking ball is unmoving, is there energy associated with it? The answer is yes. The suspended wrecking ball has energy associated with it that is fundamentally different from the kinetic energy of objects in motion. This form of energy results from the fact that there is the potential for the wrecking ball to do work. If it is released, indeed it would do work. Because this type of energy refers to the potential to do work, it is called potential energy    . Objects transfer their energy between kinetic and potential in the following way: As the wrecking ball hangs motionless, it has 0 kinetic and 100 percent potential energy. Once it is released, its kinetic energy begins to increase because it builds speed due to gravity. At the same time, as it nears the ground, it loses potential energy. Somewhere mid-fall it has 50 percent kinetic and 50 percent potential energy. Just before it hits the ground, the ball has nearly lost its potential energy and has near-maximal kinetic energy. The energy associated to the gravitational force near the surface of the earth is potential energy. Other forms of energy are really combinations of kinetic and potential energy. For example, chemical energy, is the electrical potential energy stored in atoms. Heat energy is a combination of the potential and kinetic energy of the particles in a substance.