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Chapter 4 Homework
9.
A box rests on a frozen pond, which serves as a frictionless horizontal surface. If a
fisherman applies a horizontal force with magnitude 48.0 N to the box and produces an
acceleration of magnitude 3.00 m/s
2
, what is the mass of the box?
=
Fx max
.
=
.
48 0 N m3 00ms2
=
m 16 kg
10. A dockworker applies a constant horizontal force of 80.0 N to a block of ice on a smooth
horizontal floor. The frictional force is negligible. The block starts from rest and moves 11.0 m
in 5.00 s.
(a) What is the mass of the block of ice?

=
+
x x0 v0xt 12 axt2
.
=
.
+
.
11 0 m 0ms5 00 s 12 ax5 00 s2
= .
ax 0 880ms2
=
Fx max
.
=
.
80 0 N m0 880ms2
=
.
m 90 9 kg
(b) If the worker stops pushing at the end of 5.00 s, how far does the block move in the
next
5.00 s?
=
+
vx
v0x axt
=
+
.
.
0ms 0 880ms25 00 s
= .
/
4 40 m s

=
+
x x0 v0xt 12 axt2
= .
/
.
+
.
4 40 m s5 00 s 12 0ms25 00 s2
=
.
22 0 m
12. A crate with mass 32.5 kg initially at rest on a warehouse floor is acted on by a net horizontal
force of 140 N.
(a) What acceleration is produced?
=
Fx max
=
.
140 N 32 5 kgax
= .
ax 4 31ms2
(b) How far does the crate travel in 10.0 s?
Page  1
2.1.11

=
+
x x0 v0xt 12 axt2
=
/
.
+
.
.
0 m s10 0 s 12 4 31 ms210 0 s2
=
215 m
(c) What is its speed at the end of 10.0 s?
=
+
vx
v0x axt
=
+
.
.
0ms 4 31ms210 0 s
=
.
/
43 1 m s
16. An electron (mass = 9.11 x 10
31
kg) leaves one end of a TV picture tube with zero initial speed
and travels in a straight line to the accelerating grid, which is 1.80 cm away. It reaches the grid
with a speed of 3.00 x 10
6
m/s. If the accelerating force is constant, compute
(a) the acceleration
=
+

vx 2
v0x
2 2axx x0
.
×
=
+
.
3 00
106ms2 0ms2 2ax0 0180 m
= .
×
ax 2 50
1014ms2
(b) the time to reach the grid
=
+
vx
v0x axt
.
×
=
+
.
×
3 00
106ms 0ms 2 50
1014ms2 t
= .
×

t 1 20
10 8 s
(c) the net force, in newtons
=
Fx max
= .
×

.
×
9 11
10 31 kg2 50
1014ms2
= .
×

2 28
10 16 N
17. Superman throws a 2400N boulder at an adversary. What horizontal force must Superman
apply to the boulder to give it a horizontal acceleration of 12.0
m/s
2
?
=
w mg
=
.
2400 N m9 80ms2
=
m 245 kg
=
Fx max
=
.
245 kg12 0ms2
=
2940 N
18. A bowling ball weighs 71.2 N (16.0 Ib). The bowler applies a horizontal force of 160 N (36.0
lb) to the ball. What is the magnitude of the horizontal acceleration of the ball?
=
w mg
Page  2
2.1.11
.
=
.
71 2 N m9 80ms2
= .
m 7 27 kg
=
Fx max
= .
160 N
7 27 kgax
=
.
/
ax 22 0 m s2
19. At the surface of Jupiter's moon Io, the acceleration due to gravity is
g
= 1.81 m/s
2
. A
watermelon weighs 44.0 N at the surface of the earth.
(a) What is the watermelon's mass on the earth's surface?
=
w mg
.
=
.
44 0 N m9 80ms2
= .
m 4 49 kg
(b) What are its mass and weight on the surface of Io?
= .
m 4 49 kg
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