Chapter 25

Vibrations and Waves

Waves transmit energy through space and

time.

A repeating, back-and-forth motion about an equilibrium position is a vibration.

The source of allwaves is somethingthat vibrates.

Some Definitions

A wave is a disturbance thatpropagates through space and time, usually with transference of energy. Propagate - to move Transference - a transfer

A wave moves energy from one place to another.

All waves have

• Crest - Highest point on the wave• Trough - lowest point on the wave

• Wavelength () lambda. – length from crest to crest– Measured in meters.

• Amplitude - height of wave– Measured in meters about the midpoint.

crest

trough

Frequency• The amount of cycles per second.• Measured in Hertz (Hz.)

• Ex. A dog wags its tail 5 times a second. – Tail wags with a frequency of 5 Hz.

• Math• Wavelength times frequency = wave speed.

Try This!

• Use the math formula from the previous slide.

• Sound travels at 340 m/sec.

• A good low bass note of c# has a frequency of about 34Hz.

• What is the wavelength of the note?

Period

• The time it takes to move through one cycle is the period

• The period is the reciprocal of the frequency.• They are inversely

related.• We use the Greek

letter (tau) to

represent the period.

The Pendulum

The time of a back-and-forth swing of the pendulum is called the period.

The period is measured in seconds.

We use the Greek letter (tau)

to represent the period

More about the Pendulum

The period of the pendulum depends on

the length of a pendulum

and

the acceleration of gravity.

Simpleharmonic motion

• The back-and-forth vibratory motion (often called oscillatory motion) of a swinging pendulum is called simple harmonic motion.

• The pendulum is an example of simple harmonic motion.

Two Kinds of Waves

• Fundamentally, there are two types of waves.

• Before we get to the two kinds of waves, you need to know a word.

• Medium - any solid, liquid, or gas.

Transverse Waves

• Whenever the amplitude of the wave is at right angles to the direction in which a wave travels, the wave is a transverse wave.

• Transverse waves do not need a medium to propagate through.

Longitudinal Waves

• When the particles oscillate parallel to or along the direction of the wave rather than at right angles to it, the wave is a longitudinal wave.

• Sometimes called compression waves.

• Longitudinal waves need a medium to propagate through.

Difference between Transverse and Longitudinal

Waves

Standing Waves

• When waves hit solid boundaries, like the walls of a room, standing waves can appear.

• It appears that the waves is stationary, but it is really moving in a way that makes it look like that.

Nodes and Antinodes

Node

Anti-Node

Node

Anti-Node

Anti-Node

Node- Place that does not move

Antinode- Place of maximum displacementAntinodes are half way between nodes

Interference

• When two waves cross paths, they are said to interfere.

• This interference can be broken up into two categories.

Constructive or Destructive

• Constructive Interference- When two or more waves interfere and the resultant wave has a larger amplitude.

• Destructive Interference- When two or more waves interfere and the resultant wave has a smaller amplitude than the original waves.

Doppler Effect

• The shift in frequency due to the motion of the source of waves.

• As a wave source approaches, an observer encounters waves with a higher frequency. As the wave source moves away, an observer encounters waves with a lower frequency.

Bow Waves

• A bow wave occurs when a wave source moves faster than the waves it produces.

Shock Waves

Slinky Lab Wrap Up

1. Describe the amplitude of the first waves that you made. What are these waves called?

2. Describe the amplitude of the second kind of waves that you made. What are these waves called?

3. How are the amplitudes of the two waves different in relation to the way the waves propagate?

4. What happened to the waves as you made the slinky tighter?

Summary of Tick Tock LabAnswer the following questions

1. Define Period

2. Why does the mass of the pendulum fall towards the ground?

3. What are the two things that change the period of the pendulum?

4. What was the length of the pendulum with a period of 1 sec.?

Mini QuizGet out a piece of paper.

1. Define Transverse wave

2. Define Longitudinal Wave

3. Are sound waves different than light waves?