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Q05. Using Newtons Laws
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1. A 13-N weight and a 12-N weight are connected by a
massless string over a massless, frictionless pulley.
The 13-N weight has a downward acceleration with
magnitude equal to that of a freely falling body times:
1. 1
2. 1/12
3. 1/13
4. 1/25
5. 13/25
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T
T
a
a
13 N
12 N
13 13m a T N
12 12m a T N
12 13 1m m a N
12 13
1Na
m m
1
12 13 25
N gN N
g g
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2. A 40-N crate rests on a rough horizontal floor. A 12-N
horizontal force is then applied to it. If the coefficients of
friction are s = 0.5 and k = 0.4, the magnitude of the
frictional force on the crate is:
1. 8 N
2. 12 N
3. 16 N
4. 20 N
5. 40 N
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40 N
12 N
s = 0.5 k = 0.4
f
max 0.5 40 20 12s sf n N N N
crate is stationary
f = 12 N.
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3. A bureau rests on a rough horizontal surface (s = 0.50,
k = 0.40). A constant horizontal force, just sufficient
to start the bureau in motion, is then applied. The
acceleration of the bureau is:
1. 0
2. 0.98 m/s2
3. 3.9 m/s2
4. 4.9 m/s2
5. 8.9 m/s2
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m g
F
s = 0.50 k = 0.40
fk
s sF f mg
s ka g
2 20.50 0.40 9.8 / 0.98 /m s m s
k s km a F f mg
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4. A car is traveling at 15 m/s on a horizontal road. The
brakes are applied and the car skids to a stop in 4.0 s.
The coefficient of kinetic friction between the tires and
road is:
1. 0.38
2. 0.69
3. 0.76
4. 0.92
5. 1.11
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k kF f mg m a
ka g
0v v at
0k
v
g t
2
15 /0.38
10 / 4.0
m s
m s s
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5. A crate resting on a rough horizontal floor is to be moved
horizontally. The coefficient of static friction is s . To
start the crate moving with the weakest possible applied force,
in what direction should the force be applied?
1. Horizontal
2. sin1s below the horizontal
3. tan1s above the horizontal
4. sin1s above the horizontal
5. cot1s above the horizontal
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m g
F
fs
cos sins sF f mg F
cos sins
s
mgF
2
sin cos0
cos sins s
s
mgd F
d
tan s
1tan s
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6. A professor holds an eraser against a vertical chalkboard by pushing
horizontally on it. He pushes with a force that is much greater than is
required to hold the eraser. The force of friction exerted by the board on
the eraser increases if he:
1. pushes with slightly greater force
2. pushes with slightly less force
3. stops pushing
4. pushes so his force is slightly downward but has the same magnitude
5. pushes so his force is slightly upward but has the same magnitude
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m g
F
fs
s sf mg F
1. pushes with slightly greater force fs unchanged
2. pushes with slightly less force fs unchanged
3. stops pushing fs = 0
4. pushes so his force is slightly downward but has the same magnitude
fs = m g + Fy
5. pushes so his force is slightly upward but has the same magnitude
fs = m g Fy
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7. A 5.0-kg crate is resting on a horizontal plank. The coefficient of
static friction is 0.70 and the coefficient of kinetic friction is 0.50.
After one end of the plank is raised so the plank makes an angle of
30° with the horizontal, the force of friction is:
1. 13 N
2. 18 N
3. 22 N
4. 25 N
5. 30 N
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= 30 cosn mg
s = 0.70 k = 0.50
5.0 kg
/ / sinmgf
max coss s sn gf m
2//
15.0 10 / 25
2sf f kg m s N
1
2mg
30.70 0.61
2mg mg
/ /f
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8. A block is suspended by a rope from the ceiling of a car. When
the car rounds a 40-m radius horizontal curve at 10 m/s, what
angle does the rope make with the vertical?
1. 0°
2. 14 °
3. 45 °
4. 76 °
5. 90 °
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m g
m v2 / R
T2
sinv
m TR
cosmg T
2
tanv
R g
2
2
10 /0.25
40 10 /
m s
m m s
14
1.
0°
2.
14 °
3.
45 °
4.
76 °
5.
90 °