Problems
357
Section 11.4 Conservation of Angular Momentum
28. A cylinder with moment of inertia I
1
rotates about a verti-
cal, frictionless axle with angular speed )
i
. A second cylin-
der, this one having moment of inertia I
2
and initially not
rotating, drops onto the first cylinder (Fig. P11.28). Be-
cause of friction between the surfaces, the two eventually
reach the same angular speed )
f
. (a) Calculate )
f
.
(b) Show that the kinetic energy of the system decreases in
this interaction, and calculate the ratio of the final to the
initial rotational energy.
stant. The student pulls the weights inward horizontally to
a position 0.300 m from the rotation axis. (a) Find the new
angular speed of the student. (b) Find the kinetic energy
of the rotating system before and after he pulls the weights
inward.
31.
A uniform rod of mass 100 g and length 50.0 cm rotates in
a horizontal plane about a fixed, vertical, frictionless pin
through its center. Two small beads, each of mass
30.0 g, are mounted on the rod so that they are able to
slide without friction along its length. Initially the beads
are held by catches at positions 10.0 cm on each side of
center, at which time the system rotates at an angular speed
of 20.0 rad/s. Suddenly, the catches are released and the
small beads slide outward along the rod. (a) Find the angu-
lar speed of the system at the instant the beads reach the
ends of the rod. (b) What if the beads fly off the ends?
What is the angular speed of the rod after this occurs?
32.
An umbrella consists of a circle of cloth, a thin rod with
the handle at one end and the center of the cloth at the
other end, and several straight uniform ribs hinged to the
top end of the rod and holding the cloth taut. With the
ribs perpendicular to the rod, the umbrella is set rotating
about the rod with an angular speed of 1.25 rad/s. The
cloth is so light and the rod is so thin that they make negli-
gible contributions to the moment of inertia, in compari-
son to the ribs. The spinning umbrella is balanced on its
handle and keeps rotating without friction. Suddenly its
latch breaks and the umbrella partly folds up, until each
rib makes an angle of 22.5° with the rod. What is the final
angular speed of the umbrella?
A 60.0-kg woman stands at the rim of a horizontal
turntable having a moment of inertia of 500 kg · m
2
and a
radius of 2.00 m. The turntable is initially at rest and is
free to rotate about a frictionless, vertical axle through its
center. The woman then starts walking around the rim
clockwise (as viewed from above the system) at a constant
speed of 1.50 m/s relative to the Earth. (a) In what direc-
tion and with what angular speed does the turntable
rotate? (b) How much work does the woman do to set her-
self and the turntable into motion?
34.
A puck of mass 80.0 g and radius 4.00 cm slides along an
air table at a speed of 1.50 m/s as shown in Figure
P11.34a. It makes a glancing collision with a second puck
of radius 6.00 cm and mass 120 g (initially at rest) such
that their rims just touch. Because their rims are coated
with instant-acting glue, the pucks stick together and spin
after the collision (Fig. P11.34b). (a) What is the angular
momentum of the system relative to the center of mass?
(b) What is the angular speed about the center of mass?
33.
Figure P11.28
Figure P11.30
I
2
ω
i
ω
f
I
1
Before
After
ω
ω
(a)
(b)
ω
i
ω
ω
f
ω
29. A playground merry-go-round of radius R # 2.00 m has a
moment of inertia I # 250 kg · m
2
and is rotating at
10.0 rev/min about a frictionless vertical axle. Facing the
axle, a 25.0-kg child hops onto the merry-go-round and
manages to sit down on the edge. What is the new angular
speed of the merry-go-round?
30.
A student sits on a freely rotating stool holding two
weights, each of mass 3.00 kg (Figure P11.30). When his
arms are extended horizontally, the weights are 1.00 m
from the axis of rotation and he rotates with an angular
speed of 0.750 rad/s. The moment of inertia of the stu-
dent plus stool is 3.00 kg · m
2
and is assumed to be con-
Figure P11.34
(b)
(a)
1.50 m/s