Problems
413
5. Three uniform spheres of mass 2.00 kg, 4.00 kg, and
6.00 kg are placed at the corners of a right triangle as in
Figure P13.5. Calculate the resultant gravitational force on
the 4.00-kg object, assuming the spheres are isolated from
the rest of the Universe.
6. During a solar eclipse, the Moon, Earth, and Sun all lie on
the same line, with the Moon between the Earth and the
Sun. (a) What force is exerted by the Sun on the Moon?
(b) What force is exerted by the Earth on the Moon?
(c) What force is exerted by the Sun on the Earth?
Section 13.2 Measuring the Gravitational Constant
In introductory physics laboratories, a typical
Cavendish balance for measuring the gravitational con-
stant G uses lead spheres with masses of 1.50 kg and 15.0 g
whose centers are separated by about 4.50 cm. Calculate
the gravitational force between these spheres, treating
each as a particle located at the center of the sphere.
8.
A student proposes to measure the gravitational constant
G by suspending two spherical objects from the ceiling of a
tall cathedral and measuring the deflection of the cables
from the vertical. Draw a free-body diagram of one of the
objects. If two 100.0-kg objects are suspended at the lower
ends of cables 45.00 m long and the cables are attached to
the ceiling 1.000 m apart, what is the separation of the
objects?
Section 13.3 Free-Fall Acceleration and
the Gravitational Force
When a falling meteoroid is at a distance above the
Earth’s surface of 3.00 times the Earth’s radius, what is its
acceleration due to the Earth’s gravitation?
The free-fall acceleration on the surface of the Moon is
about one sixth of that on the surface of the Earth. If the
radius of the Moon is about 0.250R
E
, find the ratio of their
average densities, )
Moon
/)
Earth
.
11.
On the way to the Moon the Apollo astronauts reached a
point where the Moon’s gravitational pull became stronger
than the Earth’s. (a) Determine the distance of this point
from the center of the Earth. (b) What is the acceleration
due to the Earth’s gravitation at this point?
10.
9.
7.
Section 13.4 Kepler’s Laws and the Motion
of Planets
12. The center-to-center distance between Earth and Moon is
384 400 km. The Moon completes an orbit in 27.3 days.
(a) Determine the Moon’s orbital speed. (b) If gravity
were switched off, the Moon would move along a straight
line tangent to its orbit, as described by Newton’s first law.
In its actual orbit in 1.00 s, how far does the Moon fall be-
low the tangent line and toward the Earth?
Plaskett’s binary system consists of two stars that revolve in
a circular orbit about a center of mass midway between
them. This means that the masses of the two stars are
equal (Fig. P13.13). Assume the orbital speed of each star
is 220 km/s and the orbital period of each is 14.4 days.
Find the mass M of each star. (For comparison, the mass of
our Sun is 1.99 # 10
30
kg.)
13.
14.
A particle of mass m moves along a straight line with con-
stant speed in the x direction, a distance b from the x axis
(Fig. P13.14). Show that Kepler’s second law is satisfied by
showing that the two shaded triangles in the figure have
the same area when t
4
"
t
3
!
t
2
"
t
1
.
Io, a moon of Jupiter, has an orbital period of 1.77 days
and an orbital radius of 4.22 # 10
5
km. From these data,
determine the mass of Jupiter.
16.
The Explorer VIII satellite, placed into orbit November 3,
1960, to investigate the ionosphere, had the following or-
bit parameters: perigee, 459 km; apogee, 2 289 km (both
distances above the Earth’s surface); period, 112.7 min.
Find the ratio v
p
/v
a
of the speed at perigee to that at
apogee.
15.
y
2.00 kg
F
24
(0, 3.00) m
x
O
6.00 kg
(–
4.00, 0) m
F
64
4.00 kg
Figure P13.5
220 km/s
M
220 km/s
M
CM
Figure P13.13
x
t
1
t
2
t
3
t
4
y
b
O
v
0
m
Figure P13.14