Physics Solutions Manual

Chapter 4 continued
d. When does the direction of the instruments’
velocity first become downward?
The velocity becomes negative after
it passes through zero. Thus, use
v f v i gt, where v f 0, or
t
v i
g
(
98
m/
s)
( 9.80
m/
s2)
1.010 1 s after release
90. When a horizontal force of 4.5 N acts on a
block on a resistance-free surface, it produces
an acceleration of 2.5 m/s 2 . Suppose
a second 4.0-kg block is dropped onto the
first. What is the magnitude of the acceleration
of the combination if the same force
continues to act? Assume that the second
block does not slide on the first block.
F m first block a initial
m first block
F m both blocks a final
(m first block m second block )a final
so, a final
F
F
F
ainitial
F
m first block m second block
m
ain second block
itial
4.5 N
4.5
N
2. 5 m/s2
4.0 kg
0.78 m/s 2
91. Two blocks, masses 4.3 kg and 5.4 kg, are
pushed across a frictionless surface by a
horizontal force of 22.5 N, as shown in
Figure 4-23.
■ Figure 4-23
a. What is the acceleration of the blocks?
Identify the two blocks together as
the system, and right as positive.
F net ma, and m m 1 m 2
a
F
m1 m 2
22.5 N
4.3 kg 5.4 kg
2.3 m/s 2 to the right
b. What is the force of the 4.3-kg block on
the 5.4-kg block?
Identify the 5.4-kg block as the
system and right as positive.
F net F 4.3-kg block on 5.4-kg block
ma
(5.4 kg)(2.3 m/s 2 )
12 N to the right
c. What is the force of the 5.4-kg block on
the 4.3-kg block?
According to Newton’s third law, this
should be equal and opposite to the
force found in part b, so the force is
12 N to the left.
80 Solutions Manual Physics: Principles and Problems
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