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MECHANICS. 287
Kinetic Energy.
The kinetic energy stored in any moving body is always
expressed in foot-pounds, by the product of the force in pounds
acting to overcome the inertia of the body, and the distance in
feet through which the force was acting in starting the body>
and is always equal to the weight of the body multiplied by the
square of the velocity and this product divided by twice the
acceleration of gravity. Thus
:
K = Kinetic energy in foot-pounds.
W= Weight of the body in pounds.
v = Velocity of the body in feet per second.
2^=G4.4.
In a free fall the height, /i, corresponding to a given
velocity, is found by the formula, -^— ; therefore, A^= W X h.
Thus, multiplying the weight of a moving body by the height
which in a free fall corresponds to its velocity, the product will
be the kinetic energy stored in the body.
W X v1
The formula K — —^
transposes to K = }4 ni v2
.
Hence the simple rule
:
Multiply half the mass of a moving body by the square of
its velocity in feet per second, and the product is the kinetic
energy in foot-pounds stored in the body.
The kinetic energy stored in any moving body always
represents a corresponding amount of mechanical work which
is required in order to again bring the body to rest.
Example.
A body weighing 1(510 pounds is moving at a constant
velocity of IS feet per second. How many foot-pounds of
kinetic energy is stored in the body ?
Solution
:
„ WX v2 1 610 X 18 X 18
K = —g = g^ = 8,100 foot-pounds.
If this moving body was brought to rest and all its stored
energy could be utilized to do work it could lift 8,100 pounds
one foot, or it could lift 81 pounds 100 feet, or any other combi-
nation of distance and resistance which, when multiplied by
one another, will give 8,100 foot-pounds.
It is very important always to keep in mind a clear dis-
tinction between -work 2nd. power, as power is the rate of doing
work, and time must, therefore, always be considered in the
question of power. For instance, when 33,000 foot-pounds of
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