The Transformation of Energy
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-72
2025-11-03
46
he pendulum affords a ready means of showing that potential energy may be changed into kinetic, and vice versa. Let a ball A be suspended by a cord from a fixed-point P (Fig. 1). The ball when at rest will take the position A, where, since it is at rest at its lowest point, it has neither potential nor kinetic energy. In order to move it to В, work must be done on it equivalent to raising it through the vertical distance DB. At B it has potential energy only, and if it is allowed to swing, it will move down the arc, losing potential energy and gaining kinetic, until it reaches A, when its energy will all be kinetic and will be sufficient to carry it up the other branch of the arc to the point C, a distance CE above the horizontal line, practically equal to DB; and here its energy is again all potential. If a spiral spring, the spring for a screen door, for example, is used as the suspending cord, and a kilogram weight for the pendulum bob, a further transformation takes place, the to-and-fro vibration changing into a vertical vibration and then back again repeatedly.
Fig. 1
The kinetic energy of the pendulum is employed in raising it against the force of gravity and restoring its potential energy. The case of a rifle ball striking against a stone wall is somewhat different. The motion of the ball is stopped and its kinetic energy is transformed chiefly into mechanical. work and heat, for the ball itself is shattered, the wall is defaced, and if the velocity is very great, heat enough is produced to melt part of the ball. The potential energy stored in coal may be transformed into heat energy by combustion, this into kinetic energy, if applied to a boiler and steam engine, and this into electrical energy, if the engine is used to turn a dynamo.
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