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PROBLEM 8.85
A scooter is to be designed to roll down a 2 percent slope at a constant speed. Assuming that the
coefficient of kinetic friction between the 25-mm-diameter axles and the bearings is 0.10, determine the
required diameter of the wheels. Neglect the rolling resistance between the wheels and the ground.
PROBLEM 8.86
The link arrangement shown is frequently used in highway bridge
construction to allow for expansion due to changes in temperature. At each of
the 60-mm-diameter pins A and B the coefficient of static friction is 0.20.
Knowing that the vertical component of the force exerted by BC on the link is
200 kN, determine (a) the horizontal force that should be exerted on beam BC
to just move the link, (b) the angle that the resulting force exerted by beam
BC on the link will form with the vertical.
PROBLEM 8.87
A lever AB of negligible weight is loosely fitted onto a 2.5-in.-diameter
fixed shaft. Knowing that the coefficient of static friction between the
fixed shaft and the lever is 0.15, determine the force
P
required to start
the lever rotating counterclockwise.
PROBLEM 8.88
A lever AB of negligible weight is loosely fitted onto a 2.5-in.-diameter fixed
shaft. Knowing that the coefficient of static friction between the fixed shaft
and the lever is 0.15, determine the force
P
required to start the lever rotating
counterclockwise.
PROBLEM 8.89
A lever AB of negligible weight is loosely fitted onto a 2.5-in.-diameter
fixed shaft. Knowing that the coefficient of static friction between the
fixed shaft and the lever is 0.15, determine the force
P
required to start
the lever rotating clockwise.
PROBLEM 8.90
A lever AB of negligible weight is loosely fitted onto a 2.5-in.-diameter fixed
shaft. Knowing that the coefficient of static friction between the fixed shaft
and the lever is 0.15, determine the force
P
required to start the lever rotating
clockwise.
PROBLEM 8.91
A loaded railroad car has a mass of 30 Mg and is supported by eight 800-mm-diameter wheels with 125-
mm-diameter axles. Knowing that the coefficients of friction are
0.020
s
and
0.015,
k
determine the
horizontal force required (a) to start the car moving, (b) to keep the car moving at a constant speed.
Neglect rolling resistance between the wheels and the track.
PROBLEM 8.92
Knowing that a couple of magnitude 30 N m
is required to start the
vertical shaft rotating, determine the coefficient of static friction between
the annular surfaces of contact.
PROBLEM 8.93
A 50-lb electric floor polisher is operated on a surface
for which the coefficient of kinetic friction is 0.25.
Assuming that the normal force per unit area between
the disk and the floor is uniformly distributed, determine
the magnitude Q of the horizontal forces required to
prevent motion of the machine.
PROBLEM 8.94*
The frictional resistance of a thrust bearing decreases as the shaft and bearing surfaces wear out. It is
generally assumed that the wear is directly proportional to the distance traveled by any given point of the
shaft and thus to the distance r from the point to the axis of the shaft. Assuming, then, that the normal
force per unit area is inversely proportional to r, show that the magnitude M of the couple required to
overcome the frictional resistance of a worn-out end bearing (with contact over the full circular area) is
equal to 75 percent of the value given by Eq. (8.9) for a new bearing.
