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Conceptual Physics: Form TR5.16A
Name ______________________________
MACHINES/WAVES
Date _________________ Period _____
Test Review # 5
Inclined Planes. An inclined plane is a ramp. It makes work easier because the incline
Wout = Fout × dout
helps to support the weight of the object as it is raised and because the work is done
Win = Fin × Din
over a greater distance when an object is pushed up an incline rather than lifted. As a
W
result, a smaller force can be used. That doesn't mean you get to do less work using an
f = Win – Wout
AMA = Fout/Fin
incline. In fact, you may have to do more, because of friction. The input distance of an
incline is its length, while the output distance is its height. The equations for calculating
IMA = din/dout
the work output (Wout), the work input (Win), the work overcoming friction (Wf), the
Eff = (Wout/Win) × 100
actual mechanical advantage (AMA), the ideal mechanical advantage (IMA), and the
Eff = (AMA/IMA) × 100
efficiency (Eff) of an incline are shown to the right.
Momentum. Momentum (p) is the product of mass (m) and velocity (v). The greater the
mass and velocity of an object is, the greater its momentum is. During
a collision, such as the one between a bowling ball and a bowling pin,
the total momentum of the objects is conserved (unless, of course,
there are outside forces such as friction.) There are two types of
collisions in which momentum can be conserved – one where objects
bounce off each other, and another where objects stick together. The mathematics for “sticking together” is less complex.
p = mv
Sample Problem
You are standing still on skates. Your mass is 40 kg. Some one tosses you a 5 kg back pack with a velocity of 3 m/s east. What is your velocity
after you catch the back pack?
Step 1: Find the total momentum by adding the momentums of each object
•
ptotal = pbackpack + pyours
•
ptotal = mbackpackvbackpack + myoursvyours
•
ptotal = (5 kg)(3m/s east) + (40 kg)(0 m/s) = 15 kg•m/sec east
Step 2: Use the total momentum and the total mass to solve for the velocity.
•
ptotal = mtotalvtotal
•
15kg•m/s east = (5 kg + 40 kg)vtotal
•
15kg•m/s east = (45 kg)vtotal
•
0.33 m/s east = vtotal
Recall that Newton’s second law is F = ma.
Since acceleration is the change in velocity
over time, any change in acceleration is also a
change in velocity, so, changes in velocity also
require a force. Therefore, to change an
object’s momentum requires an unbalanced
force, because the object’s momentum only
changes when it’s velocity changes. A few
algebraic substitutions show that force is the
change in momentum over time.
Definitions
• F = ma
∆v
t
• ∆p = m∆v
• a=
Substitutions
m∆v
t
∆p
F=
t
F=
Sample Problem
A bowling ball applies a force to a bowling pin of 175 N for 0.25 s. What is the change in the bowling ball’s momentum?
∆p = Ft = (–175 N)(0.25 s) = –43.75 kg·m/s
[Note: The force on the bowling ball is negative because it is a reaction force and opposite in
direction from the force applied by the bowling ball.]
Conceptual Physics: Form TR5.16A
MACHINES/WAVES
Test 5 Review
Page 2
Harmonic motion. Harmonic motion is motion that repeats in identical or nearly identical cycles. A cycle is a unit of motion
that repeats. A system in harmonic motion is an oscillator. It oscillates. Systems at rest are in equilibrium. A system at rest
at relatively high potential energy is in unstable equilibrium. When disturbed, forces act to pull it away from equilibrium.
A system at rest at relatively low potential energy is in stable equilibrium. When disturbed, forces act to pull it back toward
equilibrium. Because of inertia it may move past the equilibrium point and begin oscillating back and forth. As a result,
systems in stable equilibrium undergo harmonic motion when they are disturbed. Equilibrium is maintained by restoring
forces. A restoring force is any force that acts to pull a system back toward equilibrium. The time it takes to complete a
cycle is called the period. The number of cycles per second is the frequency. Frequency is the inverse of the period, and is
measured in Hertz (Hz). The amplitude is the size of the cycle, or how far the system moves from its resting state. Larger
amplitudes have higher energy. The gradual loss of amplitude by an oscillator due to friction is called damping. The natural
frequency is the frequency at which a system tends to oscillate when disturbed. The natural frequency results from the interaction
between the restoring force and inertia. A force that oscillates in strength and direction is a periodic force. When the frequency
of a periodic force matches the natural frequency, the amplitude of the motion increases. This is called resonance.
The Nature of Waves. A wave is a rhythmic disturbance that carries energy without carrying matter. For example, when a
rock is thrown in a pond, ripples move across the pond in concentric rings, but the water is not moving across the pond. There
are two main types of Waves, mechanical waves and electromagnetic waves. Mechanical waves use matter, or a medium, to
transfer energy. There are two types of mechanical waves, transverse and longitudinal or compressional waves. A transverse
wave causes the medium to move at right angles to the direction the wave is traveling.A water wave, for example, spreads
across a body of water but the water moves up and down. The high points are called crests, and the low points are called
troughs. In a compressional or longitudinal wave the medium moves back and forth in the same direction that the wave
passes. A sound wave, for example causes the particles in the medium to be alternately pushed together (compression) and
spread apart (rarefaction).The series of compressions and rarefactions moving through a medium is a longitudinal wave.
Electromagnetic waves are waves that can travel through space where no matter is present. Examples are radio waves,
infrared, visible light, ultraviolet, X rays, and gamma rays. Ninety two percent of the electromagnetic waves that reach earth
from the sun is infrared and visible light. Infrared makes you warm, while visible light enables you to see. Some of the radiant
energy from the sun is ultraviolet, which causes tanning and sunburn.
Wave Properties. Amplitude refers to the size of a wave. For transverse waves, amplitude is how high a wave rises above
or falls below the rest position, or half the distance between the top of the crest and the bottom of the trough. For longitudinal
waves, it is the degree to which the particles are squeezed together during a compression, and spread apart during a
rarefaction. As the amplitude of a wave increases, its energy also increases. Wavelength is the distance between the repeating
parts of the wave. For transverse waves, it is the distance from crest to crest, or from trough to trough. For longitudinal waves,
it is the distance from compression to compression, or from rarefaction to rarefaction. Electromagnetic waves vary in length
from kilometers for radio waves to less than the diameter of an atom for X-rays and gamma rays. Frequency is the number
of wave lengths that pass a given point per second. For transverse waves, it is the number of crests
or troughs passing a point per second. For longitudinal waves, it is the number of compressions or
rarefactions passing a point per second. Frequency is measured in wavelengths per second or hertz
(Hz). For waves of the same speed, the shorter the wavelength is, the higher the frequency is.
Wavelength or frequency determine the color of light, and the pitch of a sound. Waves travel at
different speeds. Light waves travel faster than sound waves in air. Mechanical waves travel fastest
through solids and slowest through gases but not at all in a vacuum. Electromagnetic waves, on the
other hand, travel fastest in a vacuum, but in a medium, they travel fastest through gases and
slowest through solids. Wave speed can be determined using the equation v = f λ, where v is speed
in meters per second [m/s], f is frequency in wavelengths per second or hertz [Hz], and λ is wave
length in meters [m].
Conceptual Physics: Form TR5.16A
Test 5 Review
MACHINES/WAVES
Page 3
Wave Behavior . Waves can bend around a barrier. This is called diffraction. The closer an object’s size is to the wavelength
of the wave, the more the wave bends, and the greater the diffraction is. Waves can also interact with each other. When the
waves meet and overlap, they combine to form a new wave. The ability of two waves to combine and form a new wave when
they overlap is called interference. There are two types, constructive interference and destructive interference. Constructive
interference results in a wave with greater amplitude. Destructive interference results in a wave with smaller amplitude.
Constructive interference occurs when the crest of one wave overlaps the crest of another wave to form a wave with higher
amplitude. Destructive interference occurs when the crest of one wave overlaps the trough of another wave to form a wave
with lower amplitude. When a sound wave interferes destructively with another sound wave its amplitude or volume is
reduced. Since a wave that interferes destructively with one wave does not have the same effect on waves with different
properties it is possible to reduce noise without reducing the volume of what you want to hear. Interference and diffraction
are properties of waves only, not particles. When a wave strikes an object or surface it can bounce off. A bouncing wave is
a reflection. Sound reflections are echos, while light reflections are mirror images. When light bounces off a smooth surface,
a sharp mirror like image forms. Uneven or rough surfaces scatter light making the image unclear. When a wave passes from
one substance to another, it may bend. Bending of a wave as it passes from one medium to another is called refraction.
Waves travel at different speeds in different mediums. Refraction occurs when a wave changes speed as it passes from one
medium to another. The direction the wave bends depends on whether it is slowing down or speeding up. It is best understood
relative to an imaginary line perpendicular to the surface of the medium called the normal. A wave speeding up as it enters
a medium bends away from the normal. A wave slowing down as it enters a medium bends toward the normal. When light
passes through a raindrop or a prism, it is refracted twice – once on the way in, and once on the way out. Red light (the longest
wave length) is refracted least, and violet light (the shortest wavelength) is refracted most. As a result the colors of light are
separated. Rainbows form from many separate water droplets acting as prisms.
Answer the questions below by circling the number of the correct response
1.
What is the mechanical advantage of a 6-m long ramp that extends
from a ground-level sidewalk to a 2-m high porch? (1) 1 (2) 2 (3) 3
(4) 4
2.
The mechanical advantage
of the ramp is 10. What is
the height? (1) 2 m (2) 5
m (3) 10 m (4) 20 m
3.
Two objects have the same mass. They will have the same
momentum if they also have the same (1) weight, (2) speed,
(3) velocity, (4) acceleration.
4.
What is the momentum of a 57 kg cheetah running north at 27 m/s?
(1) 1,539 kg•m/s (2) 84 kg•m/s (3) 30 kg•m/s (4) – 30 kg•m/s
5.
A sports car and a moving van are traveling at a speed of 30 km/h.
Which vehicle will be easier to stop and why? (1) the moving van,
because it has more mass (2) the moving van, because it has greater
acceleration (3) the sports car , because it has less mass (4) the
sports car , because it has lower acceleration
6.
A hunter uses a blowgun to hunt for supper. A force of 2.0 newtons is
applied to a 0.05-kilogram dart for 0.75 seconds. The speed of the
dart as it leaves the blowgun is about: (1) 0.13 m/s. (2) 1.5 m/s.
(3) 30 m/s. (4) 300 m/s.
7.
The time required to stop a 200-kilogram wagon moving at 5 m/s with
a 40-newton force is: (1) 1 second. (2) 5 seconds. (3) 80 seconds.
(4) 25 seconds.
Use the figure below to answer questions 8 and 9.
8.
What happens to the momentum of the bowling ball when it hits the
pins? (1) It increases. (2) It decreases. (3) It remains the same.
(4) There isn’t enough information to tell
9.
What happens to the speed of the ball? (1) It increases. (2) It
decreases. (3) It remains the same. (4) There isn’t enough
information to tell
10. What is the momentum of a 0.145 kg baseball pitched at 40.2 m/s?
(1) 5.83kg·m/s (2) 0.0036 kg·m/s (3) 277.2 kg·m/s (4) 40.35 kg·m/s
11. Jonathan throws a 0.2 kg snowball at 7 m/s at Rachel. Rachel throws
a 0.15 kg snowball at 12 m/s at Jonathan. The snowballs collide in
mid air and stick together. What is the final momentum of the new
snowball? (1) 1.4 kg·m/s toward Rachel (2) 0.4 kg·m/s toward
Jonathan (3) 1.8 kg·m/s toward Jonathan (4) 3.2 kg·m/s toward
Rachel
Conceptual Physics: Form TR5.16A
Test 5 Review
MACHINES/WAVES
Page 4
12. What force is required to stop a 7.3 kg bowling ball in 5.0 s if it is
thrown at a speed of 8.5 m/s? (1) 12.4 N (2) 4.3 N (3) 5.8 N
(4) 3.2 N
13. A pitcher exerts a force of 832.8 N on a 0.145 kg baseball for 0.007 s
just as the ball is released. How fast is the pitch? (1) 17,251 m/s
(2) 40.2 m/s (3) 1.2 × 10–6 m/s (4) 5,479 m/s
14. How long does a cue stick pushed with a force of 1.5 N need to be in
contact with a 0.165 kg cue ball at rest in order for it to take off with a
speed 13.4 m/s? (1) 0.018 s (2) 121.8 s (3) 7.9 s (4) 1.47 s
15. A ping pong ball that is in contact with the ping pong paddle for a
mere 0.00156 s is smashed with a force of 54 N. It takes off at
31.25 m/s. What is its mass? (1) 0.0009 kg (2) 1.1 × 106 kg
(3) 0.0027 kg (4) 0.0036 kg
26. An object in stable equilibrium could be (1) at rest at the top of a hill,
(2) at rest in a valley, (3) rolling off a hill, (4) oscillating in a valley.
27. A swinging pendulum has an amplitude of 5 centimeters. Which
statement about the pendulum must be TRUE? (1) The pendulum
moves 5 centimeters every second. (2) The pendulum moves a
maximum of 5 centimeters away from its equilibrium position. (3) The
pendulum completes one cycle every 5 centimeters of motion.
(4) The pendulum has a string length of 5 centimeters.
28. The type of wave that has rarefactions is a –?–. (1) mechanical wave
(2) longitudinal wave (3) transverse wave (4) electromagnetic wave
29. The distance between two adjacent crests of a transverse wave is the
–?–. (1) wavelength (2) rarefaction (3) diffraction (4) amplitude
Answer questions 16-18 by referring to
the diagram to the right which shows a
long spring at one instant in time that is
stretched between two friends and
shaken back and forth to produce
harmonic motion.
30. The more energy a wave carries, the greater its –?–
(1) wavelength (2) rarefaction (3) diffraction (4) amplitude
16. Which of the following points
represents the equilibrium position? (1) B (2) D (3) G (4) H
17. Which interval represents the amplitude of the motion?
(2) CD (3) BD (4) AI
25. An object in unstable equilibrium could be (1) at rest at the top of a
hill, (2) at rest in a valley, (3) rolling off a hill, (4) oscillating in a valley.
(1) AC
18. Which interval represents the length of one cycle? (1) AB (2) AC
(3) AD (4) AE
is.
31. A(n) –?– can travel through space without a medium.
(1) electromagnetic wave (2) mechanical wave (3) transverse wave
(4)reflection
32. What is the material through which mechanical waves travel?
(1) charged particles (2) space (3) a vacuum (4) a medium
33. What is carried from particle to particle in a water wave? (1) speed
(2) amplitude (3) energy (4) matter
34. What are the lowest points on a transverse wave called? (1) crests
(2) troughs (3) compressions (4) rarefactions
19. Your favorite radio station is 106.7. To what are the numbers 106.7
referring? (1) frequency (2) amplitude (3) period (4) the length of
one cycle
20. The natural frequency of a vibrating string can be changed by
changing the (1) way the string is plucked, (2) the tension in the
string, (3) the periodic force applied to the string, (4) the amplitude of
the vibration.
21. A system in harmonic motion is called a(n) (1) oscillator, (2) vibrator,
(3) cycler, (4) pendulum.
22. A force that returns a system to equilibrium is called a (1) damping
force, (2) periodic force, (3) restoring force, (4) resonant force.
35. What determines the pitch of a sound wave?
(2) frequency (3) speed (4) refraction
(1) amplitude
36. What is the distance between adjacent wave compressions? (1) one
wavelength (2) 1 km (3) 1 m/s (4) 1 Hz
37. What type of wave is the
sound wave pictured to the right?
(1) transverse
(2) electromagnetic
(3) longitudinal
23. A force that reduces the amplitude of a system in harmonic motion is
called a (1) damping force, (2) periodic force, (3) restoring force,
(4) resonant force.
24. What two factors cause a system to oscillate? (1) the damping force
and the periodic force, (2) the restoring force and inertia, (3) the
periodic force and resonance, (4) the damping force and friction.
(4) refracted
38. What color light has the shortest wave-length and the highest
frequency? (1) red (2) green (3) orange (4) blue
Conceptual Physics: Form TR5.16A
Test 5 Review
MACHINES/WAVES
Page 5
39. What do waves carry as they move? (1) matter (2) energy
(3) matter and energy (4) particles and energy
47. The diagram below shows the position versus time for a harmonic
oscillator. Which of the following pairs of points are in phase?
Use the figure below to answer questions 40 and 41.
(1) A and B (2) A and E (3) A and F (4) A and G
Use the figure below to answer questions 48 and 50.
40. What property of the wave is shown at F?
(2) wavelength (3) crest (4) trough
(1) amplitude
41. What property of the wave is shown at J?
(2) wavelength (3) crest (4) trough
(1) amplitude
42. What kind of wave does NOT need a medium through which to travel?
(1) mechanical (2) sound (3) light (4) refracted
43. What happens as a sound wave’s energy decreases? (1) Wave
frequency decreases. (2) Wavelength decreases. (3) Amplitude
decreases. (4) Wave speed decreases.
44. What unit is used to measure frequency?
(2) meters/second (3) decibels (4) hertz
(1) meters
45. What properties of a light wave determines its color? (1) wavelength
(2) amplitude (3) speed (4) interference
46. Which diagram below shows a system that is NOT stable?
(1) Mass on a spring (2) Pendulum (3) Ball in a valley (4) Ball on a
hill
48. How does the wavelength of wave A compare to the wave length of
wave B? (1) it is half as big, (2) It is twice as big. (3) It is 3 times as
big. (4) It is 1½ times as big.
49. What is the frequency of wave B? (1) 1 Hz (2) 2 Hz (3) 3 Hz (4) 1.5
Hz
50. If both waves are traveling through the same medium, how do their
speeds compare? (1) Wave A is faster. (2) Wave B is faster.
(3) Waves A and B move at the same speed. (4) There is no way to
tell.
51. An earthquake in the middle of the Indian Ocean produces a tsunami
that hits an island. Is the water that hits the island the same water that
was above the place where the earthquake occurred? (1) Yes,
because the earthquake pushed it there. (2) Yes, because the wave
carried it there. (3) No, because a wave only transfers energy, not the
medium. (4) There is no way to tell.
Conceptual Physics: Form TR5.16A
Test 5 Review
MACHINES/WAVES
Page 6
52. Referring to the diagram below, which pair of spheres have the lowest
kinetic energy and the highest kinetic energy respectively?
62. What is the name for a change in the direction of a wave when it
passes from one medium into another?
(1) refraction
(2) interference (3) reflection (4) diffraction
63. In a science fiction movie, a spaceship explodes. The people in a
nearby spaceship see the explosion, but can’t hear it because
(1) light is an electromagnetic wave and sound is a mechanical wave,
(2) light is a mechanical wave and sound is an electromagnetic wave,
(3) light is diffracted and sound is not, (4) sound is diffracted and light
is not.
64. A tone that is lower in pitch is lower in what characteristic?
(1) frequency (2) wavelength (3) loudness (4) resonance
(1) 7 and 4 (2) 6 and 1 (3) 3 and 1 (4) 4 and 2
53. The speed of a wave is equal to: (1) the amplitude divided by the
frequency. (2) the amplitude multiplied by the frequency. (3) the
wavelength divided by the frequency. (4) the wavelength multiplied
by the frequency.
Use the figure below to answer questions 65 and 66.
54. In dry air at 20.EC, sound travels at 343.3 m/s. Middle C has a
frequency of 261.6 Hz. What is its wavelength? (1) 604.9 m
(2) 1.31 m (3) 81.7 m (4) 89,807 m
55. The speed of light is 3.00 × 108 m/s. Green light has a wavelength of
6.30 × 10–7 m. What is its frequency? (1) 4.76 × 1014 Hz (2) 9.30 ×
101 Hz (3) 3.30 × 1015 Hz (4) 189 Hz
56. What determines the pitch of a sound wave?
(2) frequency (3) speed (4) refraction
(1) amplitude
57. The change in direction of a wave going from one medium to another
is (1) refraction (2) reflection (3) diffraction (4) wavelength
58. What occurs when a wave strikes an object or surface and bounces
off? (1) diffraction (2) refraction (3) a transverse wave (4) reflection
Use the figure below to answer questions 59 and 60.
59. What behavior of light waves lets you
see a sharp, clear image of yourself?
(1) refraction
(2) diffraction
(3) reflection (4) interference
60. Why can't you see a clear image of
yourself if the water's surface is rough?
(1) The light bounces off the surface in
only one direction. (2) The light scatters
in many different directions. (3) There is
no light shining on the water's surface.
(4) The light changes speed when it strikes the water.
61. What happens when light travels from air into glass? (1) It speeds
up. (2) It slows down. (3) It travels at 300,000 km/s. (4) It travels at
the speed of sound.
65. Why does the light ray bend toward the normal when is passes from
air into water? (1) Water is wet. (2) Light slows down when it enters
water. (3) Light speeds up when it enters water. (4) Water is blue.
66. Compared to its actual position, to a fish in the water, a bird in the air
appears to be (1) closer to the normal, (2) further from the normal,
(3) at the normal, (4) in its actual position.
67. When two waves overlap and interfere constructively, what does the
resulting wave have? (1) a greater amplitude (2) less energy (3) a
change in frequency (4) a lower amplitude
68. You are standing outside a classroom with an open door. You hear
people talking but cannot see them because (1) the sound is
diffracted but the light is not, (2) the light is diffracted but the sound
is not, (3) light is an electromagnetic wave and sound is a mechanical
wave, (4) light is a mechanical wave and sound is an electromagnetic
wave.
69. How does the size of an obstacle affect the diffraction of a wave?
(1) The smaller the wavelength is compared to the size of the
obstacle, the more the wave is diffracted. (2) The larger the
wavelength is compared to the size of the obstacle, the more the
wave is diffracted. (3) The more similar the wavelength is to the size
of the obstacle, the more the wave is diffracted. (4) The is no
relationship between the wavelength and the size of the obstacle with
respect to diffraction.
Conceptual Physics: Form TR5.16A
Test 5 Review
MACHINES/WAVES
Page 7
70. If a wave speed stays the same, which of the following decreases as
the frequency increases? (1) pitch (2) wavelength (3) loudness
(4) resonance
71. What is an echo? (1) diffracted sound (2) resonating sound
(3) reflected sound (4) an overtone
79. Constructive interference occurs when two waves: (1) add up to
create a wave that is smaller than either single wave. (2) add up to
create a wave that is larger than either single wave. (3) are moving
at the same speed. (4) have equal and opposite amplitudes.
80. Which of the diagrams below best illustrates the movement of a wave
through a small opening?
72. Sound waves are which type of wave? (1) slow (2) transverse
(3) longitudinal (4) electromagnetic
73. How can the pitch of the sound made by a guitar string be lowered?
(1) by shortening the part of the string that vibrates (2) by tightening
the string (3) by replacing the string with a thicker string (4) by
plucking the string harder
Use the figure below to answer questions 74 and 75.
(1) A (2) B (3) C (4) D
Answers
52.
53.
54.
55.
56.
57.
58.
59.
60.
61.
62.
63.
64.
65.
66.
67.
68.
1
4
2
1
2
1
2
3
2
2
1
2
1
2
1
1
1
69.
70.
71.
72.
73.
74.
75.
76.
77.
78.
79.
80.
81.
3
2
3
3
3
2
3
2
4
3
2
1
2
78. When a sound wave passes
through an opening, what
does the amount of
diffraction depend on? (1) the size of the opening only (2) the
wavelength only (3) both the size of the opening and the wavelength
(4) neither the size of the opening nor the wavelength
2
1
3
4
2
3
2
3
3
4
1
4
4
2
2
3
3
77. Which wave phenomenon is
pictured in the photograph
to the right showing the
waves flowing around the
rocks?
(1) refraction
(2 infarction (3) reflection
(4) diffraction
35.
36.
37.
38.
39.
40.
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
76. What happens to the particles of matter when a longitudinal wave
moves through the matter? (1) The particles do not move. (2) The
particles move back and forth along the wave direction. (3) The
particles move back and forth and are carried along with the wave.
(4) The particles move at right angles to the direction the wave
travels.
4
1
2
1
3
1
2
1
2
2
2
1
4
1
4
3
2
rarefaction
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
75. What part of the wave is shown at H?
(1)
(2) compression (3) wavelength (4) amplitude
3
1
3
1
3
3
4
2
2
1
2
1
2
4
3
3
2
rarefaction
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
74. What part of the wave is shown at F?
(1)
(2) compression (3) wavelength (4) amplitude
81. Which of the following will INCREASE the natural frequency of a
vibrating string? (1) Making the string longer, and keeping the
tension the same (2) Making the string shorter, and keeping the
tension the same (3) Keeping the length the same, and decreasing
the tension (4) Keeping the length the same, and keeping the tension
the same