physics form 5: waves

Wave Motion

In general, an oscillation or vibrating motion in which a point or body moves back and forth along a line about a fixed central point produces

waves.

How does waves transfer energy?How does waves transfer energy?

SourceOr

Disturbance Medium

Energy Transfer

•Waves are vibration that transmit energy away from an energy Waves are vibration that transmit energy away from an energy sourcesource•The energy source is often in the form of a vibration or oscillation.The energy source is often in the form of a vibration or oscillation.

The water in an ocean wave, for example, moves mainly up and down – as it passes, you bob up and down with it rather than being

carried onto the shore.

An oscillating or vibrating system acts as the source of waves which transfer energy from one point to another without transferring mass

Propagation / Travelling of Waves

1. When a wave travels through a medium, the particles of the medium vibrate about their equilibrium positions.

2. The particles of the medium do not travel in the direction of the waves.

Medium : water moleculers

Wave motion

Water waves

Disturbance

Medium

Energy Transfer

Propagation / Travelling of Waves

WavefrontWavefront

•A wavefront is a line or plane on which the vibrations of every points on it are in a phase and are at the same distance from the source of the waves AD,BE, CF the lines that join the point along the troughs of the waves.•Points in a wave are in a phase if they vibrate in the same direction with the same displacement.

wavefront

Direction of travel

A round dipper

The wavefronts of tranverse wave and longitudinal wave are perpendincular to the direction of propogation of the wave

Circular wavefrontCircular wavefront

A bar dipper

Plane wavefrontPlane wavefront

Two types of waves

Transverse waves Longitudinal waves

Transverse waves

A transverse waves is a wave in which the vibration of particles in the medium is perpendicular (at right angle) to the direction of propagation of the wave. Example : water wave, light wave and radio wave.

This animation is taken from Absorb Physics for GCSE - © Crocodile Clips Ltd.  For more information, visit:  www.crocodile-clips.com

Longitudinal waves

A longitudinal waves is a waves in which the vibration of particles in the medium is parallel (along) to the direction of propagation of the wave

Transverse waves Longitudinal waves

Transverse waveTransverse wave Longitudinal waveLongitudinal wave

Longitudinal waveLongitudinal wave

Wavelength

Amplitude

Frequency

Wave speed

Period

Displacement time graph

Describing wavesDescribing waves

Describing wavesDescribing waves

Wave speedWave speed

velocity of waves, velocity = frequency x wavelength

v = f

The frequency of the vibrator of a ripple tanks is 8 Hz. The wave generated has a wavelength of 0.02m.The speed of the wave is….

f = 8 Hz, = 0.02 m , V = ?

v = f

V = 8 x 0.02 = 0.16 ms-1

Sort out the jumble!

Velocity

Frequency

The speed of the wave

The number of waves per second

in m/s

in Hz

Amplitude

Wavelength in metres

Velocity

Frequency

The speed of the wave

The number of waves per second

in m/s

in Hz

Amplitude

Wavelength in metres

Answers

Wavelength

in metres

Amplitude

Frequency The number of waves per second in Hz

Velocity The speed of the wave in m/s

Reflection Refraction

Diffraction

Waves spreading out after passing through a narrow gap

Waves changing direction because of changing speed

Waves bouncing off a surface

Group the correct names, descriptions and pictures together

Reflection

Refraction

Diffraction Waves spreading out after passing through a narrow gap

Waves changing direction because of changing speed

Waves bouncing off a surface

Answers

Group the correct names, descriptions and pictures together

Factors that effect period of oscillation, TFactors that effect period of oscillation, T

Factor Period of oscillation, T

Pendulum Spring Jigsaw blade

increase

Gravitationalfield strength, gincrease

Stiffness, k increase

Mass, mincrease

Solving Problem Involving WavesSolving Problem Involving Waves

Damping and ResonanceDamping and Resonance

In an oscillating system such as oscillation of a spring, the oscillation does not continue with the same amplitude indefinitely except when the system is oscillating in the vacuum.

The amplitude of oscillation will gradually decrease and become zero when the oscillation stops. The decrease in the amplitude of an oscillating system is called damping.

An oscillating system experiences damping when its energy is drained out as heat energy.

a) External damping : loss of energy to overcome frictional forces or air resistance b) Internal damping : loss of energy due to the extension and compression of the molecules in the system.

To enable an oscillating system to go on continuously, an external force must be applied to the system. Such a motion is called a forced motion. The frequency of the system which oscillates freely without the action an external force is called natural frequency.

Resonance occurs when a system is made to oscillate at Resonance occurs when a system is made to oscillate at a frequency equivalent to its natural frequency by an a frequency equivalent to its natural frequency by an external force. The resonating system oscillates at its external force. The resonating system oscillates at its maximum amplitudemaximum amplitude

When pendulum X oscillates, all the other pendulums are forced to oscillate. It is found that pendulum D oscillates with the largest amplitude, that is, pendulum D resonates.

The frequency of a simple pendulum depends on the length of the pendulum. Note that pendulum A and pendulum C are at the same length.( same frequency ).

Lecture 23, Resonance Motion

Consider the following set of pendulum all attached to the same string

D

A

B

CIf I start bob D swinging which of the others will have the largest swing amplitude ?

(A) (B) (C)

Dramatic example of resonance• In 1940, turbulent winds set up a torsional

vibration in the Tacoma Narrow Bridge

Dramatic example of resonance

• when it reached the natural frequency

Dramatic example of resonance

• it collapsed !

Some effects of resonance in daily life:Some effects of resonance in daily life:

Trumpet Soprano

The loudness of music produced by musical instruments such as flute and trumpet is the result of resonance in the air. A soprano sings with a high note, a thin piece of glass may break. A bridge can collapse when the amplitude of its vibrationincreases as a result of resonance