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MECHANICS. 303
Solution
:
The distance from c, to the center of the ball is two feet,
and making three revolutions per second, the velocity will be
2 X 3 X 3.1416 X 2 = 37.7 feet per second.
cf=
4 X m X m = 88.2 pounds.
32.2 X 2
In metric measure,
W X v2
cf=.
9.81 X r
cf= Centrifugal force in kilograms.
r = Radius in meters.
v = Velocity in meters per second.
W=- Weight of moving body in kilograms.
Example.
Suppose that the weight a, in Fig. 11, is five kilograms,
swinging around the center, c, one revolution per second ; the
distance from a to c isl^ meters. What is the stress on the
string due to centrifugal force ?
Solution
:
The velocity will be 1.5 X 3.1416 X2 = 9.4248 meters per
second.
5 X 9.4248 X 9.4248
= kilograms.
J
9.81 x \y
2
Friction.
The resistance which a body meets with from the surface
on which it moves is called friction. It is called sliding friction
when one body slides on another; for instance, a sleigh is pulled
along on ice—the friction between the runners of the sleigh and
the ice is sliding friction. It is said to be rolling friction when
one body is rolling on another so that new surfaces continually
are coming into contact; for instance, when a wagon is pulled
along a road, the friction between the wheels and the road is roll-
ing friction, but the friction between the wheels and their axles
is sliding friction. Sliding friction varies greatly between
different materials, as everybody knows from daily observation.
For instance, a sleigh with iron runners can be pulled with less
effort on ice than on sand, even if the road is ever so smooth.
This is because the friction between iron and ice is a great
deal less than the friction between iron and sand.
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