Читать книгу Flight Theory and Aerodynamics - Joseph R. Badick - Страница 34

Energy is the ability to do work. In physics, work has a meaning different from the popular definition. You can push against a solid wall until you are exhausted but, unless the wall moves, you are not doing any work. Work requires that a force must move an object (displacement) in the direction of the force. Another way of saying this is that only the component of the force in the direction of movement does any work:

There are many kinds of energy: solar, chemical, heat, nuclear, and others. The type of energy that is of interest to us in aviation is mechanical energy.

There are two kinds of mechanical energy: The first is called potential energy of position, or more simply potential energy, PE. No movement is involved in calculating PE. A good example of this kind of energy is water stored behind a dam. If released, the water would be able to do work, such as running a generator. As a fighter aircraft zooms to a zenith point, it builds PE; once it starts to accelerate downward, it converts PE to KE. PE equals the weight, W, of an object multiplied by the height, h, of the object above some base plane:

(1.10)

The second kind of mechanical energy is called kinetic energy, KE. As the name implies, kinetic energy requires movement of an object. It is a function of the mass, m, of the object and its velocity, V:

(1.11)

The total mechanical energy, TE, of an object is the sum of its PE and KE:

(1.12)

The law of conservation of energy states that the total energy (of a closed system) remains constant. Both potential and kinetic energy can change in value, but the total energy must remain the same. For example, when a ball is thrown upward, if the height of the thrower is the reference plane, its energy is all kinetic when it leaves the thrower’s hand. As it rises, PE is continually increasing, but KE is always decreasing by the same amount, so the sum remains constant. At the top of its travel, PE is at its maximum (the same amount as the KE it had when it left the thrower’s hand) and KE is zero. Energy cannot be created or destroyed, but can change in form.