Problems
241
where E is the seismic wave energy in joules. According to
this model, what is the magnitude of the demonstration
quake? (It did not register above background noise over-
seas or on the seismograph of the Wolverton Seismic Vault,
Hampshire.)
A bead slides without friction around a loop-the-loop
(Fig. P8.5). The bead is released from a height h " 3.50R.
(a) What is its speed at point !? (b) How large is the nor-
mal force on it if its mass is 5.00 g?
5.
M "
log E # 4.8
1.5
6. Dave Johnson, the bronze medalist at the 1992 Olympic
decathlon in Barcelona, leaves the ground at the high
jump with vertical velocity component 6.00 m/s. How far
does his center of mass move up as he makes the jump?
7.
A glider of mass 0.150 kg moves on a horizontal friction-
less air track. It is permanently attached to one end of a
massless horizontal spring, which has a force constant of
10.0 N/m both for extension and for compression. The
other end of the spring is fixed. The glider is moved to
compress the spring by 0.180 m and then released from
rest. Calculate the speed of the glider (a) at the point
where it has moved 0.180 m from its starting point, so that
the spring is momentarily exerting no force and (b) at the
point where it has moved 0.250 m from its starting point.
8.
A loaded ore car has a mass of 950 kg and rolls on rails
with negligible friction. It starts from rest and is pulled up
a mine shaft by a cable connected to a winch. The shaft is
inclined at 30.0° above the horizontal. The car accelerates
uniformly to a speed of 2.20 m/s in 12.0 s and then con-
tinues at constant speed. (a) What power must the winch
motor provide when the car is moving at constant speed?
(b) What maximum power must the winch motor provide?
(c) What total energy transfers out of the motor by work
by the time the car moves off the end of the track, which
is of length 1 250 m?
9.
A simple pendulum, which we will consider in detail in
Chapter 15, consists of an object suspended by a string.
The object is assumed to be a particle. The string, with its
top end fixed, has negligible mass and does not stretch. In
the absence of air friction, the system oscillates by swing-
ing back and forth in a vertical plane. If the string is
2.00 m long and makes an initial angle of 30.0° with the
vertical, calculate the speed of the particle (a) at the low-
est point in its trajectory and (b) when the angle is 15.0°.
10.
An object of mass m starts from rest and slides a distance d
down a frictionless incline of angle &. While sliding, it con-
tacts an unstressed spring of negligible mass as shown in
Figure P8.10. The object slides an additional distance x as
it is brought momentarily to rest by compression of the
spring (of force constant k). Find the initial separation d
between object and spring.
A block of mass 0.250 kg is placed on top of a light vertical
spring of force constant 5 000 N/m and pushed downward
so that the spring is compressed by 0.100 m. After the
block is released from rest, it travels upward and then
leaves the spring. To what maximum height above the
point of release does it rise?
12.
A circus trapeze consists of a bar suspended by two parallel
ropes, each of length !, allowing performers to swing in a
vertical circular arc (Figure P8.12). Suppose a performer
with mass m holds the bar and steps off an elevated plat-
form, starting from rest with the ropes at an angle &
i
with
respect to the vertical. Suppose the size of the performer’s
body is small compared to the length !, that she does not
pump the trapeze to swing higher, and that air resistance is
negligible. (a) Show that when the ropes make an angle &
with the vertical, the performer must exert a force
in order to hang on. (b) Determine the angle &
i
for which
mg(3
cos
& #
2
cos &
i
)
11.
h
R
!
Figure P8.5
Figure P8.12
Figure P8.10
m
d
k
θ
!
θ