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304 MECHANICS.
Coefficient of Friction.
300 lbs.
The ratio between the force required to overcome the re-
sistance due to friction and the weight of a body sliding along
a horizontal plane is called coefficient of friction.
For instance, in Fig. 12 a f»g. 12
piece of iron weighing 300 lbs. ^
rests on a horizontal plate b. A /^, ^ ^-
string fastened to a, goes over a v§~7
’
pulley, c. At the end of the
^~^
string is applied a weight, d. If L^
this weight is increased until r-^-\ 50Jb
the body a just starts to move - *
along on b, and the weight is found to be 50 pounds, the co-
efficient of friction will be
50
— — — 0.1667
300
— 6
When the weight of a moving body is multiplied by the
coefficient of friction, the product is the force required to keep
the body in motion. Of course, any pressure applied to the
moving body, perpendicular to its line of motion, may be sub-
stituted for its weight. For instance, the frictional resistance
of the slide in a slide-valve engine is not due to the weight of the
valve, but to the unbalanced steam pressure on the valve. In all
cases the rule is :
Multiply the coefficient of friction by the pressure perpen-
dicular to the line of motion, and the product is the force
required to overcome the frictional resistance.
Example.
The coefficient of friction is 0.1, and the weight of the
sliding body is 800 pounds. What force is required to slide it
along a horizontal surface ?
Solution
:
Force = S00 X 0.1 = 80 pounds.
Rolling Friction.
If the body, a, (see Fig. 12) was lifted up from the plane, b,
high enough so that two rollers could be placed between a and
b, it would be found that the body would move with much less
force than 50 pounds because, instead of sliding friction, as in
the first experiment, it would be rolling friction. Suppose it is
found that a commenced to move when the load, d, was four
pounds, then the coefficient of friction for this particular case
would be
4
- = -L — 0.0133
300 75
In these experiments the whole force at d is not used to
move the load a, as a small part of it is used to move the pulley
at c, but in order to make the principle plain, this loss has not
been considered.
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