Question

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9 ) A 170 kg crate hangs from the end of
affope of length L = 12.0 m. You push
人
horizontally on the crate with a varying
force É to move it distance d = 3.50 m to
L
the side (Fig. 7-20). (a) What is the mag-
nitude of † when the crate is in this final
position? During the crate's displace-lid 250 bisw
ment, what are (b) the total work done
32
on it, (c) the work done by the grav-
16 Hoold
itational force on the crate, and (d) the
kd
work done by the pull on the crate from
the rope? (e) Knowing that the crate is Figure 7-20 Problem 9.
motionless before and after its displace-
ment, use the answers to (b), (c), and (d) to find the work
force + does on the crate. (f) Why is the work of your force not
. (
equal to the product of the horizontal displacement and the answer
to (a)?
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your

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35 Engineering a highway curve. If a car goes through a curve too
fast, the car tends to slide out of the curve. For a banked curve with
friction, a frictional force acts on a fast car to oppose the tendency
to slide out of the curve; the force is directed down the bank (in the
direction water would drain). Consider a circular curve of radius
R= 200 m and bank angle 0, where the coefficient of static fric-
tion between tires and pavement is us. A car (without negative
.
lift) is driven around the curve as shown in Fig. 6-11. (a) Find an
expression for the car speed Vmax that puts the car on the verge
of sliding out. (b) On the same graph, plot Vmax versus angle o for
the
range 0° to 50°, first for us = 0.60 (dry pavement) and then for
Hs = 0.050 (wet or icy pavement). In kilometers per hour, evalu-
ate Vmax for a bank angle of 0 = 10° and for (c) us = 0.60 and
(d) us = 0.050. (Now you can see why accidents occur in highway
curves when icy conditions are not obvious to drivers, who tend to
drive at normal speeds.)
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