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292 MECHANICS.
Example 0.
Under the influence of a constant force a body moves five
feet the first second. How far will it move in eight seconds,
friction not considered?
Solution
:
Distance = S2
X5 = £20 feet.
Centers.
Center of gravity is the point in a body about which all its
parts can be balanced. If a body is supported at its center of
gravity the whole body will remain at rest under the action of
gravity.
Center of gyration is a point in a rotating body at which
the whole mass could be concentrated (theoretically) without
altering the resistance, due to the inertia of the body, to angular
acceleration or retardation.
Center of oscillation is a point at which, if the whole matter
of a suspended body was collected, the time of oscillation
would be the same as it is in the actual form of the body.
Center of percussion is a point in a body moving about a
fixed axis at which it may strike an obstacle without communi-
cating the shock to the axis.
Moments.
The measures of tendency to
produce motion about a fixed point
or axis, is called moment. The pro-
duct of the length of a lever and the
force acting on the end of it. tending
to swing it around its center, is called
the moment, of force or the statical
moment, and may be expressed in
either foot-pounds or inch-pounds. In Fig. 3, the arm is 18
inches long and the force is 40 pounds; the moment is 18 X 40
= 720 inch-pounds, or iy
2 X 40"= 00 foot-pounds.
Levers.
When a lever is balanced, the distance a, fig. a
multiplied by the weight w, is always equal to ’^a-y—&-
the distance b, multiplied by the force F. 1 g c
In a bent lever (as Fig. 5) it is not the length [w\ ’
of the lever but the distance from the fulcrum
at right angles to the line in which the force is
acting, that must be multiplied. Thus:
a x w — b X F.
In Fig. 6, the force is acting at a right angle
to the lever, and. therefore, the distance a is equal
to the length of the long end of the lever.
1,
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