Potential energy

POWER

Mechanical power is defined as the rate at which work is done. If an amount of work ∆ W is done in a time interval ∆t, then the average power is defined to be

Average power (4.10)

The SI unit of power is J/s which is given the name watt (W) in honor of James Watt; thus, 1 W = 1 J/s. The instantaneous mechanical power is the limiting value of P_av as ∆t→0; that is

Instantaneous power (4.11)

The work done by a force F on a object that has an infinitesimal displacement d s is dW = F ·ds. Since the velocity of the object is, v = d s / dt the instantaneous mechanical power may be written as

P = F · v (4.12)

In order to raise an apple from the ground to some height, one must either lift it by hand or throw it with enough initial kinetic energy. When it reaches its new height, what happens to the lost kinetic energy or the work done by the hand? We know from our study of free-fall that when an apple is thrown up. it returns to its starting point with a speed equal to the initial value. The initial kinetic energy, or the work done by the hand, is somehow stored and is later fully recovered in the form of kinetic energy. The apple must have something at the new height that it does not have at ground level. By virtue of its position it has potential energy: Potential energy is energy associated with the relative positions of two or more interacting particles.

Дата добавления: 2014-01-05