Problems
245
Section 8.6 Energy Diagrams and Equilibrium
of a System
44. A right circular cone can be balanced on a horizontal sur-
face in three different ways. Sketch these three equilib-
rium configurations, and identify them as positions of sta-
ble, unstable, or neutral equilibrium.
45. For the potential energy curve shown in Figure P8.45,
(a) determine whether the force F
x
is positive, negative, or
zero at the five points indicated. (b) Indicate points of sta-
ble, unstable, and neutral equilibrium. (c) Sketch the
curve for F
x
versus x from x " 0 to x " 9.5 m.
have force constant k and each is initially unstressed. (a) If
the particle is pulled a distance x along a direction perpen-
dicular to the initial configuration of the springs, as in
Figure P8.47, show that the potential energy of the system is
(Hint: See Problem 58 in Chapter 7.) (b) Make a plot of
U(x) versus x and identify all equilibrium points. Assume
that L " 1.20 m and k " 40.0 N/m. (c) If the particle is
pulled 0.500 m to the right and then released, what is its
speed when it reaches the equilibrium point x " 0?
U(x) " kx
2
%
2kL
(
L #
√
x
2
%
L
2
)
Additional Problems
48.
A block slides down a curved frictionless track and then up
an inclined plane as in Figure P8.48. The coefficient of ki-
netic friction between block and incline is )
k
. Use energy
methods to show that the maximum height reached by the
block is
y
max
"
h
1 % )
k
cot &
49. Make an order-of-magnitude estimate of your power out-
put as you climb stairs. In your solution, state the physical
quantities you take as data and the values you measure or
estimate for them. Do you consider your peak power or
your sustainable power?
50.
Review problem. The mass of a car is 1 500 kg. The shape
of the body is such that its aerodynamic drag coefficient is
D " 0.330 and the frontal area is 2.50 m
2
. Assuming that
the drag force is proportional to v
2
and neglecting other
sources of friction, calculate the power required to main-
tain a speed of 100 km/h as the car climbs a long hill slop-
ing at 3.20°.
4
U ( J)
!
"
$
%
6
2
0
–2
–4
2
8
6
4
x(m)
#
Figure P8.45
46. A particle moves along a line where the potential energy of
its system depends on its position r as graphed in
Figure P8.46. In the limit as r increases without bound, U(r)
approaches %1 J. (a) Identify each equilibrium position for
this particle. Indicate whether each is a point of stable, un-
stable, or neutral equilibrium. (b) The particle will be
bound if the total energy of the system is in what range?
Now suppose that the system has energy #3 J. Determine
(c) the range of positions where the particle can be found,
(d) its maximum kinetic energy, (e) the location where it
has maximum kinetic energy, and (f) the binding energy of
the system—that is, the additional energy that it would have
to be given in order for the particle to move out to r : 1 .
0
r(mm)
+2
U( J)
+4
+6
+2
–2
–4
–6
2
4
6
Figure P8.46
Figure P8.48
47.
A particle of mass 1.18 kg is attached between two identical
springs on a horizontal frictionless tabletop. The springs
Top View
L
L
x
m
k
k
x
Figure P8.47
y
max
θ
h