Problems
333
80.
A thin rod of mass 0.630 kg and length 1.24 m is at rest,
hanging vertically from a strong fixed hinge at its top
end. Suddenly a horizontal impulsive force (14.7ˆ
i) N is
applied to it. (a) Suppose the force acts at the bottom
end of the rod. Find the acceleration of its center of mass
and the horizontal force the hinge exerts. (b) Suppose
the force acts at the midpoint of the rod. Find the accel-
eration of this point and the horizontal hinge reaction.
(c) Where can the impulse be applied so that the hinge
will exert no horizontal force? This point is called the
center of percussion.
81.
A bowler releases a bowling ball with no spin, sending it
sliding straight down the alley toward the pins. The ball
continues to slide for a distance of what order of magni-
tude, before its motion becomes rolling without slipping?
State the quantities you take as data, the values you mea-
sure or estimate for them, and your reasoning.
82.
Following Thanksgiving dinner your uncle falls into a deep
sleep, sitting straight up facing the television set. A
naughty grandchild balances a small spherical grape at the
top of his bald head, which itself has the shape of a sphere.
After all the children have had time to giggle, the grape
starts from rest and rolls down without slipping. It will
leave contact with your uncle’s scalp when the radial line
joining it to the center of curvature makes what angle with
the vertical?
83.
(a) A thin rod of length h and mass M is held vertically
with its lower end resting on a frictionless horizontal sur-
face. The rod is then released to fall freely. Determine the
speed of its center of mass just before it hits the horizontal
surface. (b) What If? Now suppose the rod has a fixed
pivot at its lower end. Determine the speed of the rod’s
center of mass just before it hits the surface.
84.
A large, cylindrical roll of tissue paper of initial radius R
lies on a long, horizontal surface with the outside end of
the paper nailed to the surface. The roll is given a slight
shove (v
i
! 0) and commences to unroll. Assume the roll
has a uniform density and that mechanical energy is con-
served in the process. (a) Determine the speed of the cen-
ter of mass of the roll when its radius has diminished to r.
(b) Calculate a numerical value for this speed at
r " 1.00 mm, assuming R " 6.00 m. (c) What If? What
happens to the energy of the system when the paper is
completely unrolled?
A spool of wire of mass M and radius R is unwound under
a constant force
F (Fig. P10.85). Assuming the spool is a
85.
uniform solid cylinder that doesn’t slip, show that (a) the
acceleration of the center of mass is 4
F/3M and (b) the
force of friction is to the right and equal in magnitude to
F/3. (c) If the cylinder starts from rest and rolls without
slipping, what is the speed of its center of mass after it has
rolled through a distance d ?
86.
A plank with a mass M " 6.00 kg rides on top of two
identical solid cylindrical rollers that have R " 5.00 cm
and m " 2.00 kg (Fig. P10.86). The plank is pulled by a
constant horizontal force
F of magnitude 6.00 N applied
to the end of the plank and perpendicular to the axes of
the cylinders (which are parallel). The cylinders roll with-
out slipping on a flat surface. There is also no slipping
between the cylinders and the plank. (a) Find the accel-
eration of the plank and of the rollers. (b) What friction
forces are acting?
87.
A spool of wire rests on a horizontal surface as in Figure
P10.87. As the wire is pulled, the spool does not slip at
the contact point P. On separate trials, each one of
the forces
F
1
,
F
2
,
F
3
, and
F
4
is applied to the spool. For
each one of these forces, determine the direction the
spool will roll. Note that the line of action of
F
2
passes
through P.
88.
Refer to Problem 87 and Figure P10.87. The spool of wire
has an inner radius r and an outer radius R. The angle !
between the applied force and the horizontal can be var-
ied. Show that the critical angle for which the spool does
not roll is given by
If the wire is held at this angle and the force increased, the
spool will remain stationary until it slips along the floor.
cos !
c
"
r
R
F
M
R
Figure P10.85
M
m
R
m
R
F
Figure P10.86
c
F
1
F
2
F
3
F
4
P
θ
R
r
Figure P10.87 Problems 87 and 88.