Problems
145
(b) Determine the tension T and the magnitude of the ac-
celeration of the system.
46.
A block of mass 3.00 kg is pushed up against a wall by a
force P that makes a 50.0° angle with the horizontal as
shown in Figure P5.46. The coefficient of static friction be-
tween the block and the wall is 0.250. Determine the possi-
ble values for the magnitude of P that allow the block to
remain stationary.
value of F will move the block up the plane with constant
velocity?
50.
Review problem. One side of the roof of a building slopes
up at 37.0°. A student throws a Frisbee onto the roof. It
strikes with a speed of 15.0 m/s and does not bounce, but
slides straight up the incline. The coefficient of kinetic
friction between the plastic and the roof is 0.400. The Fris-
bee slides 10.0 m up the roof to its peak, where it goes into
free fall, following a parabolic trajectory with negligible air
resistance. Determine the maximum height the Frisbee
reaches above the point where it struck the roof.
Additional Problems
An inventive child named Pat wants to reach an apple in a
tree without climbing the tree. Sitting in a chair connected
to a rope that passes over a frictionless pulley (Fig. P5.51),
Pat pulls on the loose end of the rope with such a force
that the spring scale reads 250 N. Pat’s true weight is 320
N, and the chair weighs 160 N. (a) Draw free-body dia-
grams for Pat and the chair considered as separate systems,
and another diagram for Pat and the chair considered as
one system. (b) Show that the acceleration of the system is
upward and find its magnitude. (c) Find the force Pat ex-
erts on the chair.
51.
F
m
2
T
m
1
Figure P5.45
P
50.0
°
Figure P5.46
Figure P5.48
47.
You and your friend go sledding. Out of curiosity, you
measure the constant angle ! that the snow-covered
slope makes with the horizontal. Next, you use the fol-
lowing method to determine the coefficient of friction
-
k
between the snow and the sled. You give the sled a
quick push up so that it will slide up the slope away from
you. You wait for it to slide back down, timing the mo-
tion. It turns out that the sled takes twice as long to slide
down as it does to reach the top point in the round trip.
In terms of !, what is the coefficient of friction?
48.
The board sandwiched between two other boards in
Figure P5.48 weighs 95.5 N. If the coefficient of friction
between the boards is 0.663, what must be the magnitude
of the compression forces (assume horizontal) acting on
both sides of the center board to keep it from slipping?
49.
A block weighing 75.0 N rests on a plane inclined at 25.0°
to the horizontal. A force F is applied to the object at 40.0°
to the horizontal, pushing it upward on the plane. The co-
efficients of static and kinetic friction between the block
and the plane are, respectively, 0.363 and 0.156. (a) What
is the minimum value of F that will prevent the block from
slipping down the plane? (b) What is the minimum value
of F that will start the block moving up the plane? (c) What
52.
A time-dependent force, F " (8.00ˆi # 4.00tˆj) N, where t is
in seconds, is exerted on a 2.00-kg object initially at rest.
(a) At what time will the object be moving with a speed of
15.0 m/s? (b) How far is the object from its initial position
when its speed is 15.0 m/s? (c) Through what total dis-
placement has the object traveled at this time?
53.
To prevent a box from sliding down an inclined plane,
student A pushes on the box in the direction parallel to
the incline, just hard enough to hold the box stationary.
In an identical situation student B pushes on the box
horizontally. Regard as known the mass m of the box, the
coefficient of static friction -
s
between box and incline,
and the inclination angle !. (a) Determine the force A
Figure P5.51