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WAVES
a) describe what is meant by wave motion as illustrated by vibrations in ropes and springs and by waves in a ripple tank
b) show understanding that waves transfer energy without transferring matter
c) compare transverse and longitudinal waves and give suitable examples of each
LEARNING OBJECTIVES
How does a Kallang Wave look like?
The Kallang wave moves around the stadium, but the people do not.
How do people move in a Kallang Wave?
KALLANG WAVE
A wave is a disturbance that transfers energy from one place to another.
It is made up of periodic motions – motion repeated at regular intervals.
We can also call it periodic oscillations. (recall pendulum)
INTRODUCING WAVES
1. Kinetic energy from the moving hand is transferred to the rope.
2. This forms a rope wave (a wave that travels within the rope).
3. The rope wave moves from the hand to the wall ( left to right).
4. As the wave moves through the rope, from left to right, the rope particles (P and Q) move up and down , about their rest positions.
5. Eventually, the kinetic energy is transferred from the hand to the wall.
INTRODUCING WAVESWAVES IN A ROPE
Kinetic energy from the dipper is transferred to the water.
This forms a water wave (i.e. a
water ripple).
The water wave moves outwards from the dipper.
The water particles move up and down, about their rest positions.
In other words, the kinetic energy gets transferred from the dipper to the edges of the ripple tank.
INTRODUCING WAVESWAVES IN A RIPPLE TANK
The source of a wave is a vibration or an oscillation.
Waves transfer energy from one point to another.
Waves transfer energy without transferring the medium (i.e. rope or water).In another words, the particles do not move forward with the wave.
INTRODUCING WAVESSUMMARY
Observe how I move the spring. Describe the motion of the spring.
If we move the spring in an up-down motion:
The spring coils move perpendicular to the direction of the wave.
INTRODUCING WAVESWAVES IN A SPRING
(top view)up
down
Wave direction
If we move the spring in a push-and-pull motion:
The spring coils move parallel to the direction of the wave.
INTRODUCING WAVESWAVES IN A SPRING
Wave direction
From the Slinky Spring, we can see that there are 2 types of waves.Transverse wavesLongitudinal waves
INTRODUCING WAVESTYPES OF WAVE MOTION
How do transverse waves differ from longitudinal waves?
The coils move up and down, while the wave moves from left to right.
The movement of the coils is perpendicular to the wave motion.
E.g. Electromagnetic waves i.e. light, infrared-red, ultraviolet
INTRODUCING WAVES TRANSVERSE WAVES
Transverse waves are waves that travel perpendicular to the direction of the medium’s particle vibration.
The coils move left and right, while the wave moves from left to right.
The movement of the coils is parallel to the wave motion.
Examples: Sound waves
INTRODUCING WAVESLONGITUDINAL WAVES
Longitudinal waves are waves that travel parallel to the direction of the medium’s particle vibration.
In longitudinal waves, the part where the particles of matter are close together is called the compression.
The part where the particles are spread apart is called the rarefaction.
Longitudinal waves are formed by a series of compression and rarefactions.
INTRODUCING WAVESLONGITUDINAL WAVES
Describe and explain the motion of the Styrofoam balls (if any) when the rod is dipped quickly into the water and then removed.
INTRODUCING WAVES
rod
water
Styrofoam balls
Is the wave formed transverse or longitudinal?
INTRODUCING WAVES
rod
water
Styrofoam balls
What is a wave?
2 kinds of waves Transverse – particles move perpendicular to wave motion Longitudinal – particles move parallel to wave motion
SUMMARY
a) define speed, frequency, wavelength, period and amplitude
b) state what is meant by the term wavefrontc) Recall and apply the relationship velocity =
frequency × wavelength to new situations or to solve related problems
LEARNING OBJECTIVES
When describing a wave, there are a lot of scientific terms.
Therefore we want to understand what does each term means.
PROPERTIES OF WAVE MOTION
When water waves are produced, the peaks of the waves forms a circle and move outwards from the sources of disturbance.
DefinitionWavefront is the line that
joins all the peaks of a wave or all identical points on a wave.
PROPERTIES OF WAVE MOTIONDESCRIBING WAVES
Plane waves can be produced by disturbing the water surface with a vibrating wooden bar.
The wavefronts of plane wave are straight lines that moves away from the source of disturbance.
PROPERTIES OF WAVE MOTIONDESCRIBING WAVES
Crests – the highest point of a transverse waveTroughs – the lowest point of a transverse
wave
Which of the points labelled on the wave are crests and which are troughs??
PROPERTIES OF WAVE MOTIONDESCRIBING WAVES
P
Q
R
S
T
U
V
PROPERTIES OF WAVE MOTIONDESCRIBING WAVES
The amplitude A of a wave is the maximum possible displacement of a point from its rest position.
P
Q
R
S
T
U
V
amplitude (height of crest)
amplitude (depth of trough)
PROPERTIES OF WAVE MOTIONDESCRIBING WAVES
Points along a wave are in phase if they have the same• direction;• speed;• displacement from their rest positions.
P
Q
R
S
T
U
VQuestionWhich points on the wave are in phase?
1. P, S, V2. Q, T3. R, U
PROPERTIES OF WAVE MOTIONDESCRIBING WAVES
The wavelength l of a wave is the shortest distance between any two points in phase.
P
Q
R
S
T
U
Vwavelength
wavelength
wavelength
PROPERTIES OF WAVE MOTIONDESCRIBING WAVES
The frequency, f, is the number of complete waves produced per second.
SI Unit of frequency is the hertz (Hz).
SI Unit of period is the seconds (s).
The period, T, is the time taken to produce one complete wave.
PROPERTIES OF WAVE MOTIONDESCRIBING WAVES
Looking at the ripple tank.
Frequency Period
5 waves produced in 1 second.
𝐅𝐫𝐞𝐪𝐮𝐞𝐧𝐜𝐲 , 𝒇=𝟓𝑯𝒛 𝐏𝐞𝐫𝐢𝐨𝐝 ,𝐓=𝟏𝒇
=𝟏𝟓
=𝟎 .𝟐𝐬
PROPERTIES OF WAVE MOTIONDESCRIBING WAVES
In one period, what is the distance moved by a wave crest? The wave moves by 1 wavelength.
Therefore is the formula for speed of waves.
Wave speed v is the distance travelled by a wave per second.
PROPERTIES OF WAVE MOTIONDISPLACEMENT-DISTANCE GRAPH
A picture capture of the wave motion at a specific time, for e.g. at t = 0 s.
From the displacement-distance graph, we can find the amplitude, A and wavelength, l of the wave.
P
Q
R
S
T
U
V
PROPERTIES OF WAVE MOTIONDISPLACEMENT-TIME GRAPH
Follow 1 particle on the wave, say ribbon Q.
We then used the information to plot the displacement of ribbon Q over a period of time.
P
Q
R
S
T
U
V
PROPERTIES OF WAVE MOTIONDISPLACEMENT-TIME GRAPH
From this graph, we can find the amplitude, A and period, T of the wave.
Wave Terms: Wavefront Amplitude Wavelength Frequency Period Wave speed
Wave graphs Displacement-distance graph Displacement-time graph
SUMMARY
A wave in a string is travelling to the right at 2 m s –1. The diagram below shows its displacement–distance graph at t = 0 s.
a) Sketch the graph to show how the wave will appear at t = 3 s.
b) Draw and label the position of P and Q at t = 3 s.
PRACTICE QUESTION 1
Displacement/m
Distance/m
P
82 4 6
Q
Solution
PRACTICE QUESTION 1
x/m
d/mP
t = 3 s
x/m
d/m
P
82 4 6
Q
Q
t = 0 s
A wave in a string is travelling to the right at 2 m s –1. The diagram below shows its displacement–distance graph at t = 0 s.
Plot a graph to show how the displacements of particles P and Q vary with time.
PRACTICE QUESTION 2
Displacement/m
Distance/m
P
82 4 6Q
Solution
PRACTICE QUESTION 2
Displacement/mP
Time/sQ421 3
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