Chapter 4
Newton’s Laws
CHAPTER
4
Newton’s Laws
Note: For all problems we shall take the upward direction as positive unless otherwise stated.
1* ··
How can you tell if a particular reference frame is an inertial reference frame?
If Newton’s first law is obeyed, the reference frame is an inertial reference frame.
5* ·
If an object is acted upon by a single known force, can you tell in which direction the object will move
using no other information?
No; only its acceleration is known (assuming one knows the mass).
9* ·
A particle of mass
m
is traveling at an initial speed
v
0
= 25.0 m/s. It is brought to rest in a distance of 62.5
m when a net force of 15.0 N acts on it. What is
m
?
(
a
) 37.5 kg
(
b
) 3.00 kg
(
c
) 1.50 kg
(
d
) 6.00 kg (
e
) 3.75
kg
a
=
v
2
/2
s
=
F
/
m
;
m
=
v
2
/2
Fs
m
= 3.00 kg;
(
b
)
13*
·
A force
F
=
6 N
i
 3 N
j
acts on an object of mass 1.5 kg. Find the acceleration
a
. What is the magnitude
a
?
a
=
F
/
m
= (4
i
 2
j
) m/s
2
;
a
= (16 + 4)
1/2
m/s
2
= 4.46 m/s
2
.
17* ·
A 4kg object is subjected to two forces,
F
1
= 2 N
i
 3 N
j
and
F
2
= 4 n
i
 11 N
j
. The object is at rest at
the origin at time
t
= 0. (
a
) What is the object’s acceleration? (
b
) What is its velocity at time
t
= 3 s? (
c
) Where
is the object at time
t
= 3 s?
(
a
)
Find the net force,
F
=
F
1
+
F
2
a
=
F
/
m
(
b
)
v
=
a
t
(
c
)
r
=
v
av
t
;
v
av
=
1/2
v
(3 s)
F
= (2
i
+ 4
i
) N  (3
j
+ 11
j
) N = (6
i
 14
j
) N
a
= (1.5
i 
3.5
j
) m/s
2
v
= (4.5
i
 10.5
j
) m/s
r
= (6.75
i
 15.75
j
) m
21*
·
On the moon, the acceleration due to gravity is only about 1/6 of that on earth. An astronaut whose weight
on earth is 600 N travels to the lunar surface. His mass as measured on the moon will be (
a
) 600 kg.
(
b
) 100
kg.
(
c
) 61.2 kg. (
d
) 9.81 kg. (
e
) 360 kg.
(
c
) The mass is (600 N)/(9.81 m/s
2
) = 61.2 kg, and remains the same on the moon.
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Chapter 4
Newton’s Laws
25*
··
Caught without a map again, Hayley lands her spacecraft on an unknown planet. Visibility is poor, but she
finds someone on a local communications channel and asks for directions to Earth. “You are already on Earth,”
is the reply, “Wait there and I’ll be right over.” Hayley is suspicious, however, so she drops a lead ball of mass
76.5 g from the top of her ship, 18 m above the surface of the planet. It takes 2.5 s to reach the ground. (
a
) If
Hayley’s mass is 68.5 kg, what is her weight on this planet? (
b
) Is she on Earth?
(
a
) Use
s
=
1/2
at
2
to find accel. of gravity,
g
'
g
' = (2
×
18/2.5
2
) m/s
2
= 5.76 m/s
2
w
=
mg
'
w
= (68.5
×
5.76) N = 395 N
(
b
)
Evidently, she is not on Earth.
29* ·
A baseball player hits a ball with a bat. If the force with which the bat hits the ball is considered the action
force, what is the reaction force? (
a
) The force the bat exerts on the batter’s hands. (
b
) The force on the ball
exerted by the glove of the person who catches it. (
c
) The force the ball exerts on the bat. (
d
) The force the
pitcher exerts on the ball while throwing it. (
e
) Friction, as the ball rolls to a stop.
(
c
)
33*
·
A vertical spring of force constant 600 N/m has one end attached to the ceiling and the other to a 12kg
block resting on a horizontal surface so that the spring exerts an upward force on the block. The spring is
stretched by 10 cm. (
a
) What force does the spring exert on the block? (
b
) What is the force that the surface
exerts on the block?
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 Spring '09
 Eduardo
 Physics, Force, Inertia, kg, Jay

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