Problems
173
der at a high rotation rate forces the solidifying metal
strongly to the outside. Any bubbles are displaced toward
the axis, so unwanted voids will not be present in the cast-
ing. Sometimes it is desirable to form a composite casting,
such as for a bearing. Here a strong steel outer surface is
poured, followed by an inner lining of special low-friction
metal. In some applications a very strong metal is given a
coating of corrosion-resistant metal. Centrifugal casting re-
sults in strong bonding between the layers.
Suppose that a copper sleeve of inner radius 2.10 cm
and outer radius 2.20 cm is to be cast. To eliminate bubbles
and give high structural integrity, the centripetal accelera-
tion of each bit of metal should be 100g. What rate of rota-
tion is required? State the answer in revolutions per minute.
Section 6.2 Nonuniform Circular Motion
A 40.0-kg child swings in a swing supported by two chains,
each 3.00 m long. If the tension in each chain at the lowest
point is 350 N, find (a) the child’s speed at the lowest
point and (b) the force exerted by the seat on the child at
the lowest point. (Neglect the mass of the seat.)
14.
A child of mass m swings in a swing supported by two
chains, each of length R. If the tension in each chain at
the lowest point is T, find (a) the child’s speed at the low-
est point and (b) the force exerted by the seat on the child
at the lowest point. (Neglect the mass of the seat.)
13.
Tarzan (m ! 85.0 kg) tries to cross a river by swinging
from a vine. The vine is 10.0 m long, and his speed at the
bottom of the swing (as he just clears the water) will be
8.00 m/s. Tarzan doesn’t know that the vine has a breaking
strength of 1 000 N. Does he make it safely across the river?
16.
A hawk flies in a horizontal arc of radius 12.0 m at a con-
stant speed of 4.00 m/s. (a) Find its centripetal acceleration.
(b) It continues to fly along the same horizontal arc but in-
creases its speed at the rate of 1.20 m/s
2
. Find the accelera-
tion (magnitude and direction) under these conditions.
A pail of water is rotated in a vertical circle of radius
1.00 m. What is the minimum speed of the pail at the top
of the circle if no water is to spill out?
18.
A 0.400-kg object is swung in a vertical circular path on a
string 0.500 m long. If its speed is 4.00 m/s at the top of
the circle, what is the tension in the string there?
19.
A roller coaster car (Fig. P6.19) has a mass of 500 kg when
fully loaded with passengers. (a) If the vehicle has a speed
of 20.0 m/s at point !, what is the force exerted by the
track on the car at this point? (b) What is the maximum
speed the vehicle can have at " and still remain on the
track?
17.
15.
20.
A roller coaster at the Six Flags Great America amusement
park in Gurnee, IL, incorporates some clever design tech-
nology and some basic physics. Each vertical loop, instead of
being circular, is shaped like a teardrop (Fig. P6.20). The
cars ride on the inside of the loop at the top, and the speeds
are high enough to ensure that the cars remain on the
track. The biggest loop is 40.0 m high, with a maximum
speed of 31.0 m/s (nearly 70 mi/h) at the bottom. Suppose
10 m
15 m
!
"
Figure P6.19
Axis of rotation
Molten metal
Preheated steel sheath
Figure P6.12
3.00 m
2.00 m
2.00 m
Figure P6.11