Multiple Choice
Identify
the letter of the choice that best completes the statement or answers
the question.
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1. |
The
free-body diagram of a wagon being pulled along a horizontal surface
is best represented by
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2. |
A
24-kg traffic light is suspended from the midpoint of a cable suspended
between two poles. The angle between the cable and the pole is 80° at both poles. The net force acting
on the traffic light has a value of
a. |
zero |
d. |
2.4
´
102 N |
b. |
47
N |
e. |
4.6
´
102 N |
c. |
82
N |
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3. |
An
object has two forces acting on it: 8.4 N [S] and 7.5 N [E]. The magnitude
of the net force is
a. |
1.3
´
102 N |
d. |
4.0
N |
b. |
16
N |
e. |
0.9
N |
c. |
11
N |
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4. |
An
object is pushed horizontally at a constant velocity. What can correctly
be said about the forces acting on the object?
a. |
The force(s) acting forward is/are greater
than the force(s) acting backward. |
b. |
The sum of all forces has a value directed
forward. |
c. |
The sum of all forces is zero. |
d. |
The
forces acting on the object can be said to be “unbalanced.” |
e. |
Newton’s
second law best summarizes the effect of the forces acting on
the object. |
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5. |
Which
of the following is NOT an example of “inertia”?
a. |
A
person’s head jerks back as the car he is riding in accelerates
forward. |
b. |
A
person’s head jerks forward as the car he is riding in suddenly
stops. |
c. |
A
person is pressed up against the car door as the car turns a corner. |
d. |
A
person is largely unaware of a car’s motion when his eyes
are closed. |
e. |
All
of the above are examples of inertia. |
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6. |
Which
of the following units is equivalent to a newton (N)?
a. |
kg·m/s |
d. |
kg·m/s2 |
b. |
g·cm/s |
e. |
kg·cm/s2 |
c. |
kg·s2/m |
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7. |
Forces of 2.4 N and 1.8 N act on an object at right angles to one
another. What is the magnitude of a third force acting on the same object
so that it remains stationary?
a. |
9.0
N |
d. |
2.7
N |
b. |
4.2
N |
e. |
0.6
N |
c. |
3.0
N |
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8. |
Two
masses, A and B, hang on opposite ends of a rope suspended over a pulley.
The mass of A is greater than the mass of B. If 
represents the force of tension exerted by the rope on mass A and  represents the force of tension exerted by the rope on mass B, then
which of the following statements concerning the forces of tension is
true?
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9. |
Two
masses hang on opposite ends of a rope suspended over a pulley. The
pulley is restrained from rotating and the two forces: 
(the force of tension exerted by the rope on mass A) and 
(the force of tension exerted by the rope on mass B) are found to be
equal in magnitude. If the pulley becomes free to rotate and the system
begins moving, the relationship between those forces becomes
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10. |
Three
masses are suspended vertically as shown in the diagram below. The system
is accelerating upward. What is the relationship among the forces of
tension?
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11. |
Streamlining is a technique that designers use to
a. |
increase
turbulent flow and decrease laminar flow |
b. |
increase turbulent flow and increase laminar
flow |
c. |
decrease
turbulent flow and increase laminar flow |
d. |
decrease turbulent flow and decrease laminar
flow |
e. |
stop
turbulent flow and stop laminar flow |
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12. |
Which
of the following would be considered an “inertial” frame of
reference?
a. |
a
moving escalator |
b. |
a car moving through a turn at a constant
speed |
c. |
an object in “free fall” |
d. |
a
car pulling away as a traffic light turns green |
e. |
all of the above |
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13. |
For
an object travelling with “uniform circular motion,” its acceleration
is
a. |
zero
because the speed is constant |
b. |
directed tangent to the circle |
c. |
directed
toward the centre of the circle |
d. |
changing in magnitude depending on its position
in the circle |
e. |
directed outward from the centre of the
circle |
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14. |
A
fighter jet flies at 328 km/h in an arc of radius 235 m. How many “g’s”
of centripetal acceleration does the pilot experience? (1g =
9.8 m/s2)
a. |
47 |
d. |
3.6 |
b. |
35 |
e. |
1.5 |
c. |
3.8 |
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15. |
A
wheel of diameter 85 cm spins at a rate such that a point on the rim
of the wheel has an acceleration of 45 m/s2. How many rotations
does the wheel make in 1.0 min?
a. |
1.7
´
102 |
d. |
6.9 |
b. |
98 |
e. |
0.93 |
c. |
69 |
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16. |
Imagine you are a passenger upside-down at the top of a vertical
looping roller coaster. The centripetal force acting on you at this
position
a. |
is
perhaps the least of anywhere in the loop |
b. |
is supplied at least partly by gravity |
c. |
is supplied partly by the seat of the roller
coaster |
d. |
is directed vertically downward |
e. |
all
of the above |
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17. |
For
objects travelling with uniform circular motion, the centrifugal force
they experience
a. |
is
radially outward |
b. |
is apparently present in the noninertial
frame of reference |
c. |
increases with speed |
d. |
doesn’t
actually exist |
e. |
all of the above |
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18. |
Planet X has a radius 4 times that of Earth and the acceleration
due to gravity at the surface of planet X is 4.9 m/s2. The
mass of Planet X compared to Earth’s mass is
a. |
16
times |
d. |
2
times |
b. |
8 times |
e. |
the same |
c. |
4
times |
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19. |
The
orbital speed of a satellite at an altitude equivalent to Earth’s
radius (rE = 6.38 ´
106 m) is (mE = 5.98 ´ 1024 kg, G = 6.67 ´
10–11 N·m2/kg2)
a. |
9.8
´
103 m/s |
d. |
4.9
´
103 m/s |
b. |
7.9
´
103 m/s |
e. |
2.5
´
103 m/s |
c. |
5.6
´
103 m/s |
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20. |
Astronauts on board an orbiting space station appear to be “floating”
because
a. |
they
are in the vacuum of space |
b. |
they are outside Earth’s gravitational
influence |
c. |
the force of gravity acting on them has
been reduced to an insignificant level |
d. |
they have become truly “weightless” |
e. |
they are in free fall along with the space
station itself |
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Completion
Complete each sentence
or statement.
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21. |
The
free-body diagram pictured below represents the forces acting on a toboggan
sliding down a hill. The force that has been omitted is the ____________________
force.
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22. |
The
force responsible for moving a propeller-driven airplane is the force
of the ____________________ acting on the ____________________.
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23. |
For
two given surfaces in contact with one another, the coefficient of kinetic
friction is generally ____________________ than the corresponding coefficient
of static friction.
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24. |
In
noninertial frames of reference, ____________________ forces are often
invented to try to account for observations.
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25. |
A
car travelling around a corner at a constant speed is considered to
be a(n) ____________________ frame of reference.
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26. |
For
an object travelling with uniform circular motion, the direction of
its acceleration at any point is ______________________________.
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27. |
For
an object travelling with uniform circular motion, the direction of
its instantaneous velocity at any point is _________________________.
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28. |
For
a planet orbiting the Sun, the centripetal force is supplied by ____________________.
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29. |
A
satellite in geosynchronous orbit has an orbital period of ____________________.
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30. |
The
speed of a satellite in orbit is determined by the ____________________
of the orbit.
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Matching
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Match the situations described below with the Newton’s
law that best accounts for the motion.
a. |
Astronauts feel pushed back into their seats
during launch. |
b. |
The force an astronaut exerts on his seat
is equal in strength and opposite in direction to the force the
seat exerts on the astronaut. |
c. |
The
force exerted by the rocket engine overcomes the forces of gravity
and air resistance, resulting in an upward acceleration of the
rocket. |
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31. |
Newton’s first law
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32. |
Newton’s second law
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33. |
Newton’s third law
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The diagram below shows an object travelling with uniform circular
motion as viewed from above. Match each of the following quantities
with its proper direction.
a. |
north |
c. |
east |
b. |
south |
d. |
west |
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34. |
instantaneous velocity
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35. |
centripetal acceleration
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36. |
centripetal force
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37. |
centrifugal force
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Match each of the following situations with the type of force
responsible for the object’s centripetal acceleration.
a. |
gravity |
d. |
static friction |
b. |
tension |
e. |
kinetic
friction |
c. |
normal |
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38. |
a
planet orbiting the Sun
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39. |
a
car driving around an icy banked curve
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40. |
a
car driving around a flat curve
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Short Answer
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41. |
For
an object sliding on a ramp, what is the most appropriate way of resolving
the forces? Why is this method superior to others?
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42. |
The
graph below shows the relationship between the force applied to an object
initially at rest and its acceleration. Why does the graph not pass
through the origin?
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43. |
An
object sits at rest on a ramp inclined at an angle q. As the angle of inclination increases,
the object suddenly starts moving. From a discussion of the force involved,
explain why.
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44. |
How
does an object’s speed typically affect its aerodynamics? How can
these aerodynamics be optimized?
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45. |
For
objects travelling with uniform circular motion, what are the directions
of the instantaneous velocity and acceleration at any point on the circle?
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46. |
Why
are curves in roadways often banked?
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47. |
Why
do roller coasters use clothoid loops instead of circular loops?
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48. |
Sketch two graphs. The first graph should illustrate the relationship
between the force of gravity Earth exerts on objects and the mass of
the objects. The second graph should show the variation of the force
of gravity a pair of objects exert on one another and the separation
distance between the objects’ centres of mass.
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49. |
Describe how the law of universal gravitation is closely associated
with Newton’s third law of motion.
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50. |
Given
the following quantities, show that the gravitational field strength
of Earth at its surface is 9.8 N/kg.
mE = 5.98 ´ 1024 kg,
rE = 6.38 ´
106 m, G = 6.67 ´ 10–11 N·m2/kg2
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Problem
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51. |
Crates of mass 50.0 kg must be hoisted onto a platform 8.0 m above
the ground. A person exerts 600.0 N of force on a rope that goes up
and over a pulley suspended from the ceiling. The other end of the rope
is attached to the 50.0-kg mass.
(a)
How long will it take the person to lift a crate from the ground to
the platform while exerting maximum force?
(b) If a 45.0-kg person grabs the free end of the
rope when the crate is next to the platform in its elevated position,
what will the acceleration of the crate be as it falls?
(c) What is the tension in the rope in this case?
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52. |
A
5.0-kg box accelerates across the floor at 1.2 m/s2 when
pulled by a force of 12 N acting horizontally. The same box is then
pulled by the same force acting at an elevation of 30.0°.
(a) Draw the free-body
diagrams for both situations.
(b) Determine the coefficient of sliding friction.
(c) Calculate the acceleration of the block if
the force is directed 30.0° above the horizontal.
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53. |
Three
books rest horizontally on a table as pictured below. What forces does
the middle book exert on each of the other two? Be specific in expressing
your answers.
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54. |
A
2.0 ´
102-g mass is tied on the end of a 1.6 m long string and
whirled around in a circle that describes a vertical plane.
(a) What is the minimum frequency of rotation
required to keep the mass moving in a circle?
(b) Calculate the maximum
tension in the string at this frequency.
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55. |
A
0.50-g insect rests on a compact disc at a distance of 4.0 cm from the
centre. The disc’s rate of rotation varies from 3.5 Hz to 8.0 Hz
in order to maintain a constant data sampling rate.
(a) What are the insect’s minimum and maximum
centripetal accelerations during its rotation around the disc?
(b)
What is the minimum value of the coefficient of static friction that
would prevent the insect from slipping off the disc at the slowest rotation
rate?
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56. |
A
pilot of mass 75 kg takes her plane into a dive, pulling out of it along
a circular arc as she nears the ground. If the plane is flying at 1.5
´ 102 km/h along the arc, what
is its radius such that the pilot feels four times heavier than normal?
Provide an appropriate free-body diagram.
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57. |
A
flea stands on the end of a 1.0 cm long sweep second hand of a clock
that rests horizontally on a table. What is the minimum coefficient
of static friction which would allow the flea to stay there without
slipping? Include an appropriate free-body diagram.
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58. |
What
force does Earth exert on a 80.0-kg astronaut at an altitude equivalent
to 2.5 times Earth’s radius?
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59. |
A
planet has a mass of 2.5 times that of Earth and a radius 1.2 times
Earth’s radius. How much would a 60.0-kg person weigh at the planet’s
surface?
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60. |
A
satellite has an orbital speed of 4.2 ´ 103 m/s. What is its altitude
above Earth’s surface?
(ME = 5.98 ´ 1024 kg, rE =
6.38 ´
106 m)
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Essay
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61. |
Consider a ride in a high-speed elevator from the ground floor
upward and from the top floor down. Describe the sensations one feels
during all parts of the trip and explain them in terms of the physics
involved.
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62. |
Using
the principles of uniform circular motion, explain how centrifuges are
able to separate materials that have a variety of densities.
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