CHAPTER 12
PROBLEM 12.1
Astronauts who landed on the moon during the Apollo 15, 16 and 17 missions brought back a large collection
of rocks to the earth. Knowing the rocks weighed 139 lb when they were on the moon, determine (a) the
weight of the rocks on the earth, (b) the mass of the rocks in slugs. The acceleration due to gravity on the
moon is 5.30 ft/s2.
PROBLEM 12.2
The value of g at any latitude
may be obtained from the formula
22
32.09(1 0.0053 sin ) ft/sg
which takes into account the effect of the rotation of the earth, as well as the fact that the earth is not truly
spherical. Knowing that the weight of a silver bar has been officially designated as 5 lb, determine to four
significant figures (a) the mass is slugs, (b) the weight in pounds at the latitudes of 0°, 45°, and 60°.
PROBLEM 12.3
A 400-kg satellite has been placed in a circular orbit 1500 km above the surface of the earth. The acceleration
of gravity at this elevation is 6.43 m/s2. Determine the linear momentum of the satellite, knowing that its
orbital speed is 3
25.6 10 km/h.
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PROBLEM 12.4
A spring scale A and a lever scale B having equal lever arms
are fastened to the roof of an elevator, and identical packages
are attached to the scales as shown. Knowing that when the
elevator moves downward with an acceleration of
2
1 m/s
the
spring scale indicates a load of 60 N, determine (a) the weight
of the packages, (b) the load indicated by the spring scale and
the mass needed to balance the lever scale when the elevator
moves upward with an acceleration of 1 m/s
2
.
PROBLEM 12.4 (Continued)
PROBLEM 12.5
In anticipation of a long 7° upgrade, a bus driver accelerates at a constant rate of
2
3 ft/s
while still on a level
section of the highway. Knowing that the speed of the bus is
60 mi/h
as it begins to climb the grade and that
the driver does not change the setting of his throttle or shift gears, determine the distance traveled by the bus
up the grade when its speed has decreased to
50 mi/h.
PROBLEM 12.6
A 0.2-lb model rocket is launched vertically from rest at time t
0 with a constant thrust
of 2 lb for one second and no thrust for t
1 s. Neglecting air resistance and the decrease
in mass of the rocket, determine (a) the maximum height h reached by the rocket, (b) the
time required to reach this maximum height.
PROBLEM 12.7
A tugboat pulls a small barge through a harbor. The
propeller thrust minus the drag produces a net thrust
that varies linearly with speed. Knowing that the
combined weight of the tug and barge is 3600 kN,
determine (a) the time required to increase the
speed from an initial value v
1
= 1.0 m/s to a final
value v
2
= 2.5 m/s, (b) the distance traveled during
this time interval.