CHAPTER
5
Applications of Newton’s Laws
1* ·
Various objects lie on the floor of a truck moving along a horizontal road. If the truck accelerates, what force
acts on the objects to cause them to accelerate?
Force of friction between the objects and the floor of the truck.
2
·
Any object resting on the floor of a truck will slide if the truck’s acceleration is too great. How does the
critical acceleration at which a light object slips compare with that at which a much heavier object slips?
They are the same.
3
·
True or false:
(
a
) The force of static friction always equals
m
s
F
n
. (
b
) The force of friction always opposes the
motion of an object. (
c
) The force of friction always opposes sliding. (
d
) The force of kinetic friction always
equals
m
k
F
n
.
(
a
) False (
b
) True
(
c
) True (
d
) True
4
·
A block of mass
m
rests on a plane inclined at an angle
q
with the horizontal. It follows that the coefficient of
static friction between the block and the plane is
(
a
)
m
s
≥
1
.
(
b
)
m
s
= tan
q
. (
c
)
m
s
≤
tan
q
. (
d
)
m
s
≥
tan
q
.
(
d
)
5* ·
A block of mass
m
is at rest on a plane inclined at angle of 30
o
with the horizontal, as in Figure 538. Which
of the following statements about the force of static friction is true?
(
a
)
f
s
>
mg
(
b
)
f
s
>
mg
cos 30
o
(
c
)
f
s
=
mg
cos 30
o
(
d
)
f
s
=
mg
sin 30
o
(
e
) None of these statements is true.
(
d
)
f
s
must equal in magnitude the component of the weight along the plane.
6
·
A block of mass
m
slides at constant speed down a plane inclined at an angle
q
with the horizontal. It follows
that
(
a
)
m
k
=
mg
sin
q
. (
b
)
m
k
= tan
q
. (
c
)
m
k
= 1  cos
q
. (
d
)
m
k
= cos
q
 sin
q
.
(
a
) Acceleration = 0, therefore
f
k
=
mg
sin
q
. With
F
n
=
mg
cos
q
, it follows that
m
k
= tan
q
7
·
A block of wood is pulled by a horizontal string across a horizontal surface at constant velocity with a force
of 20 N. The coefficient of kinetic friction between the surfaces is 0.3. The force of friction is (
a
) impossible to
determine without knowing the mass of the block. (
b
) impossible to determine without knowing the speed of the
block.
(
c
) 0.3 N.
(
d
) 6 N.
(
e
) 20 N.
(
e
)
The net force is zero.
8
·
A 20N block rests on a horizontal surface. The coefficients of static and kinetic friction between the surface
and the block are
m
s
= 0.8 and
m
k
= 0.6. A horizontal string is attached to the block and a constant tension
T
is
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Applications of Newton’s Laws
maintained in the string. What is the force of friction acting on the block if (
a
)
T
= 15 N, or (
b
)
T
= 20 N.
(
a
) If
m
s
mg >
15,
then
f
=
f
s
= 15 N
(
b
)
T
>
f
s,max
;
f
=
f
k
=
m
k
mg
0.8
×
(20 N) = 16 N;
f
=
f
s
= 15 N
f
=
f
k
= 0.6
×
(20 N) = 12 N
9* ·
A block of mass
m
is pulled at constant velocity across a horizontal surface by a string as in Figure 539. The
magnitude of the frictional force is
(
a
)
m
k
mg
. (
b
)
T
cos
q
. (
c
)
m
k
(
T

mg
). (
d
)
m
k
T
sin
q
. (
e
)
m
k
(
mg + T
sin
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 Spring '08
 staff
 Force, Friction, freebody diagram

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