5
Chap.5
HW2
Questions
5.
Freebody diagrams are shown for four
situations in which an object is pulled by
several forces across a frictionless floor, as
seen from overhead.
In which situations
does the object's acceleration have
(a) an x component and
Solution
(2), (3), (4)
For (1) the x components of the
forces cancel.
(b) a
y
component?
Solution
(1), (3), (4)
For (2) the y components of the
forces cancel.
(c) In each situation, give the direction of
a
by naming either a quadrant or a direction
along an axis.
(This can be done with a few
mental calculations.)
(1) along +y,
(2) along +x
(3) in 4
th
quadrant,
(4) in 3
rd
quadrant
8.
Figure 527 shows a train of four
blocks being pulled across a frictionless
floor by force
F
v
.
What total mass is
accelerated to the right by (a) force
F
v
,
(b) cord 3, and (c) cord 1?
(d) Rank the blocks according
to their accelerations, greatest first. (e) Rank the cords according to their tension, greatest first.
a) All mass is accelerated = 20 kg
b) All except the first (2 kg) mass, 18 kg
c) Only the last mass, 10 kg.
d) All blocks have the same acceleration
e) 3, 2, 1
Problems
25.
Tarzan, who weighs 820 N, swings from a cliff at the end of a
20.0 m vine that hangs from a high tree limb and initially makes an
angle of 22.0° with the vertical. Assume that an x axis extends
horizontally away from the cliff edge and a
y
axis extends upward.
Immediately after Tarzan steps off the cliff, the tension in the vine
is 760 N.
Just then, what are (a) the force on him from the vine in unitvector
notation
Rope
Tarzan
θ
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We need to rewrite the vector
T
= 760 N, 68
0
above the
x
axis
in unit vector form
()()
()
j
ˆ
N
705
i
ˆ
N
285
j
ˆ
68
cos
i
ˆ
68
sin
j
ˆ
sin
i
ˆ
cos
0
0
+
=
+
=
φ
+
φ
=
T
T
T
T
T
v
The net force on him (b) in unitvector notation,
Tarzan is now in midair and there are two forces on him,
the weight force and the tension in the vine.
Thus,
( ) ( ) ( )
j
ˆ
N
115
i
ˆ
N
285
j
ˆ
N
820

j
ˆ
N
705
i
ˆ
N
285
−
=
+
=
+
=
g
net
F
T
F
v
v
v
The net force on him (c) a magnitude and (d) an angle relative to the positive
direction of the
x
axis?
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 Spring '08
 oshea
 Acceleration, Force, Friction, r Newton

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