Ch8 Nature of Wave
Physics for Tomorrow 3
Ch 8 Nature of Wave
8.1
8.2
8.3
8.4
Nature of wave
 Oscillation in wave motion
 Transmission of energy in wave
 Medium for wave motion
Transverse and longitudinal waves
 Transverse wave
 Longitudinal wave
Description of wave
Particle motion in transverse and longitudinal
 Particle motion in transverse wave
 Particle motion in longitudinal wave
waves
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Ch8 Nature of Wave
8.1 Nature of wave (p. 3)
1. Oscillation in wave motion (p. 3)
Activity 1 Wave motion in spring (p. 3)
(a) Pulse on a spring:
Generated by:
giving the spring a disturbance.
- shaking the spring sideways.
- pushing the spring along its length.
(b) Wave:
(i) Generated by:
repeating the disturbance periodically.
(ii) What is a wave?
Wave motion is a periodic motion.
(iii)Types of waves:
water waves, sound waves, light waves, radio
waves, microwaves
2. Water wave (p. 5) Fig. 8.1 (p. 5)
(a) Circular water wave:
(i) Generated by:
A stone hits the water surface.
(ii) Result:
Circular waves spread radially outwards.
(b) Plane water wave:
(i) Generated by:
A straight vibrator hits the water surface.
(ii) Result:
Plane waves are generated.
3. Wavefront (p. 5) Fig. 8.2 (p. 5)
(a) Wavefront:
A line joins a row of the peaks of pulses.
(b) Ray:
An arrow indicates the direction of propagation
of a wave.
(c) Ray is perpendicular to the wavefronts.
4. Transmission of energy in wave (p. 6)
Fig. 8.3, Fig. 8.4 (p. 6)
(a) A wave:
(i) is a phenomenon of energy transport.
(ii) transports energy from one place to another
without transfer of matter.
(b) When a wave propagates:
(i) the particles
- have no net translation.
- only oscillate about a fixed position
(e.g. the cork moves up and down).
(ii) Energy is transported along with the wave in
all directions.
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Ch8 Nature of Wave
5. Medium for wave motion (p. 7)
Activity 2 Speed of wave in spring (p. 7)
(a) Medium:
(i) A medium is a substance or a material
which carries the wave.
(ii) Example:
For a water wave, the medium is water.
(b) Mechanical wave:
(i) Waves that require media to propagate.
(ii) Example:
water wave, sound wave
(c) Electromagnetic wave:
(i) Waves do not require media to propagate.
(ii) They can travel through a vacuum.
(iii)Example:
visible light
6. Speed of wave (p. 8)
(a) The wave speed depends on the medium and its
properties.
(b) For example:
(i) Wave in spring:
- The longer the spring, the higher the wave
speed.
- Shaking the spring harder results in a “larger”
wave only but no change in the wave speed.
(ii)Water wave:
Changing the depth of water can change the
wave speed.
8.2 Transverse and longitudinal waves
(p. 8)
Activity 3 Transverse and longitudinal waves (p. 8)
7. Transverse wave (p. 9) Fig. 8.5 (p. 9)
(a) Generated by:
flicking a spring sideways repeatedly.
(b) Properties:
(i) Form successive crests (the peaks) and
troughs (the lowest points).
(ii) The particles of the medium vibrate
perpendicular to the direction of propagation of
the wave.
(c) Example:
water wave, electromagnetic wave
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Ch8 Nature of Wave
8. Longitudinal wave (p. 9) Fig. 8.6 (p. 9)
(a) Generated by:
pushing a spring back and forth repeatedly.
(b) Properties:
(i) Form successive rarefactions and
compressions.
(ii) The particles of the medium vibrate parallel
to the direction of propagation of the wave.
(c) Example:
sound wave
8.3 Description of wave (p. 10)
9. Equilibrium positions (p. 10) Fig. 8.7 (p. 10)
Definition:
A wave is formed as the particles of a medium
oscillate about their equilibrium positions
(undisturbed positions).
10. Amplitude (A) (p. 10) Fig. 8.7 (p. 10)
(a) Definition:
The maximum displacement of a particle from
its equilibrium position
(i) Transverse wave: maximum vertical
displacement
(ii) Longitudinal wave: maximum horizontal
displacement
(b) A measure of energy carried by the wave:
(i) The higher the wave energy, the larger the
amplitude.
(ii) Shake a spring harder, a wave with larger
amplitude is obtained.
(c) Unit:
metre (m)
11. Wavelength () (p. 11) Fig. 8.7 (p. 10)
(a) Definition:
The shortest separation between two particles,
which move in an identical way
(i) For transverse wave:
 is the distance between two adjacent wave
crests (or troughs).
(ii) For longitudinal wave:
 is the distance between two adjacent
compressions (or rarefactions).
(b) Unit:
metre (m)
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Ch8 Nature of Wave
12. Period (T) (p. 11) Fig. 8.8 (p. 11)
(a) Definition:
The time required to generate one complete
pulse
(b) Unit:
second (s)
13. Frequency (f) (p. 11)
(a) Definition:
The number of complete waves generated in one
second
(b) f is the same as the frequency of the source.
(c) Mathematically:
f= 1
T
(d) Unit:
Hertz (Hz), 1 Hz = 1 s1
14. Wave speed (v) (p. 12) Fig. 8.10 (p. 12)
(a) Definition:
The distance travelled by a wave in 1 second
(b) Velocity = Frequency  Wavelength (v = f)
(c) For the same property of the medium:
Concept recall: Section 8.1, p. 8
Increase f of the wave
- brings no change in v.
- only  decreases.
(d) Unit:
m s 1
Example 1 (p. 12), Class Practice 1 (p. 13),
Class Practice 2 (p. 14)
8.4 Particle motion in transverse and
longitudinal waves (p. 14)
15. Particle motion in transverse wave (p. 14)
Fig. 8.11 (p. 15)
(a) The figure shows the waveforms of a transverse
wave at a quarter-period interval.
(b) The wave is travelling to the right.
16. Motion of particle P (p. 16)
Fig. 8.11 (p. 15)
(a) t = 0 to 1 T:
4
(i) At t = 0 s, P
- is at the crest of the wave.
- starts to move down.
(ii) At t = 1 T, P reaches its equilibrium
4
position.
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Ch8 Nature of Wave
(b) t = 1 T to 1 T:
4
2
1
(i) At t = T, P moves further downwards.
4
1
(ii) At t = T, P reaches the trough of the
2
wave.
(c) t = 1 T to 3 T:
4
2
1
(i) At t = T, P rises.
2
(ii) t = 3 T, P returns to its equilibrium
4
position.
(d) t = 3 T to T:
4
(i) At t = T, P returns to its initial position.
(ii) The wave has travelled through a distance of
.
17. Motion of particle R (p. 16) Fig. 8.11 (p. 15)
Particle R and particle P are in phase.
Reason:
(a) They have exactly the same oscillation.
(b) They have, at any time, the same displacement.
18. Motion of particle Q (p. 16) Fig. 8.12 (p. 16)
Particle Q particle P are 180º out of phase or in
antiphase.
Reason:
(a) They move in the opposite way.
(e.g., at t = 1 T,Q reaches the crest but P
2
reaches the trough)
(b) Their displacements, at any time, are both equal
in magnitude but opposite in direction.
19. Relation between phase and wavelength (p. 17)
Fig. 8.11 (p. 15)
(a) The distance between two adjacent particles that
are in phase (e.g. P and R) is .
(b) The distance between two adjacent particles that
are in antiphase (e.g. P and Q) is  .
2
20. Displacement-distance graph (p. 17)
Fig 8.13 (p. 17)
(a) Show the displacements of particles of a wave
from their equilibrium positions at a certain
time.
(b) The displacement-distance graph of a transverse
wave shows its waveform.
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Ch8 Nature of Wave
21. Displacement-time graph (p. 18) Fig 8.14 (p. 18)
(a) Obtained by plotting the displacement of a
certain particle with time.
(b) From d-t graph, we can found
(i) period
(ii) frequency
of the wave.
Class Practice 3 (p. 18)
22. Particle motion after a short time (p. 19)
Fig 8.15 (p. 19)
Predict the motion of particles:
(a) The waveform of a transverse wave at a certain
instant is given.
(b) Draw another waveform (a dotted curve) next to
the original one.
(c) Add vertical arrows from the initial curve to the
final curve.
(d) The directions of motion of particles are shown
by the arrows.
Example 2 (p. 19) , Class Practice 4 (p. 20)
23. Particle motion in longitudinal wave (p. 20)
Concept recall: Section 8.2, p. 9
(a) Longitudinal wave Fig. 8.16 (p. 20)
(i) Consist of compressions and rarefactions.
(ii)  = the distance between two successive
compressions (or rarefactions).
(b) Waveform of a longitudinal wave at a
quarter-period interval is shown. Fig. 8.17 (p. 21)
24. In phase (p. 21) Fig. 8.17 (p. 21)
For particles 0 and 12, they are in phase.
(a) t = 0 to 1 T:
4
At t = 0 s, they
- start from their equilibrium positions.
- move to the left.
(b) t = 1 T to 1 T:
4
2
1
(i) At t = T, they
4
- attain their maximum displacements on the
left.
- return to the right.
(ii) At t = 1 T, they reach their equilibrium
2
positions.
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Ch8 Nature of Wave
(c) t = 1 T to 3 T:
4
2
1
(i) At t = T, they continue to move to the
2
right.
(ii) At t = 3 T, they reach their maximum
4
displacements on the right.
(d) t = 3 T to T:
4
At t = T , they move back to their initial
positions.
25. In antiphase (p. 22) Fig. 8.17 (p. 21)
(a) At the centre of compression:
particles (e.g. particle 6) around it are crowded
together.
(b) At the centre of rarefaction:
particles (e.g. particles 0 and 12) around it move
apart.
(c) Particle at the centre of compression is in
antiphase with the particle at the centre of
rarefaction.
(d) The distance between two successive
compressions (or rarefactions) is  ( = 12 cm).
(e) The maximum displacement of particles from
their equilibrium positions is A (A = 1.2 cm).
Class Practice 5 (p. 22)
26. Displacement-distance graph (p. 23)
Fig. 8.17 (p. 21), Table 8.1, Fig. 8.18(p. 23)
The figure shows the displacement-distance graph
of particles of the longitudinal wave at t =
1 T.
2
27. Particle motion after a short time (p. 24)
Fig 8.19 (p. 24)
(a) Predict the motion of particles:
(i) The displacement –distance graph of a
longitudinal wave at a certain instant is given.
(ii) Draw another waveform (a dotted curve)
next to the original one.
(iii)Add vertical arrows from the initial curve to
the final curve.
(iv) The directions of motion of particles are
shown by the arrows.
(b) Take the direction of right as positive:
(i) A particle is moving towards the positive
direction in the graph:
it (e.g. particle 2) is in fact moving to the right.
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Ch8 Nature of Wave
(ii) A particle is moving towards the negative
direction in the graph:
it (e.g. particle 8) is in fact moving to the left.
28. Displacement-time graph (p. 24) Fig 8.20 (p. 25)
(a) The displacement-time graph can be obtained by
plotting the displacement of a certain particle
(e.g. particle 6) with time.
(b) The direction of motion of particle 6 can be
obtained by considering the slope of its
displacement-time graph (p. 25).
(c) Properties of the graph:
(i) Particle 6 vibrates at the highest speed when
the slopes of the graph are the steepest
(at t = 1 T and T).
2
(ii) Negative slope (at t = 1 T ) shows that
2
particle 6 is moving towards the negative
direction (to the left).
(iii)Positive slope (at t = T) shows that particle 6
is moving towards the positive direction (to the
right).
(iv) When displacement = 0 (at t = 1 T and T),
2
particle 6 may either at the centre of rarefaction
or compression.
Class Practice 6 (p. 25),
STS Corner 1 Earthquake (p. 26)
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