Page 259 - Fisika Terapan for Engineers and Scientists

Problems 459

When an unknown mass is placed on the inner pan, the 59. Consider the differential windlass illustrated in Fig. 14.30.
microbalance can measure changes in mass as small as Calculate what clockwise torque must be applied to the
100 nanograms (1.0 10 7 g) and can measure masses up to handle to lift a load of mass m. What tangential force must be
2.0 milligrams. What would you expect that the resolution exerted on the handle? What is the mechanical advantage of
and maximum load for the outer pan might be? this windlass?
56. A man of 73 kg stands on one foot, resting all of his weight 60. Design a block and tackle with a mechanical advantage of 4,
on the ball of the foot. As described in Section 14.3, the bones and another with a mechanical advantage of 5. If you connect
of the foot play the role of a lever. The short end of the lever these two arrangements in tandem, what mechanical advan-
(to the heel) measures 5.0 cm and the long end (to the ball of tage do you get?
the foot) 14 cm. Calculate the force exerted by the Achilles *61. Figure 14.60 shows a compound bolt cutter. If the dimensions
tendon and the force at the ankle. are as indicated in this figure, what is the mechanical advantage?
57. A rope hoist consists of four pulleys assembled in two pairs
F'
with rigid straps, with a rope wrapped around as shown in
3.0 cm
Fig. 14.58. A load of 300 kg hangs from the lower pair of pul-
leys. What tension must you apply to the rope to hold the 5.0 cm
load steady? Treat the pulleys and the rope as massless, and
ignore any friction in the pulleys. 3.0 cm

16.0 cm

F 10 cm
FIGURE 14.60 Compound bolt cutter.

*62. The drum of a winch is rigidly attached to a concentric large
gear, which is driven by a small gear attached to a crank. The
dimensions of the drum, the gears, and the crank are given in
Fig. 14.61. What is the mechanical advantage of this geared
FIGURE 14.58 winch?
Rope hoist.
5.0 cm
58. A parbuckle is a simple device used by laborers for raising or 15 cm
lowering a barrel or some other cylindrical object along a
ramp. It consists of a loop of rope wrapped around the barrel 25 cm
(see Fig. 14.59). One end of the rope is tied to the top of the 10 cm
ramp, and the laborer pulls on the other end. Suppose that the
laborer exerts a pull of 500 N on the rope, parallel to the
ramp. What is the force that the rope exerts on the barrel?
What is the mechanical advantage of the parbuckle?

FIGURE 14.61 Geared winch.

*63. The screw of a vise has a pitch of 4.0 mm; that is, it advances
4.0 mm when given one full turn. The handle of the vise is
25 cm long, measured from the screw to the end of the
handle. What is the mechanical advantage when you push
perpendicularly on the end of the handle?
FIGURE 14.59 Parbuckle used to move a barrel up a ramp.