Ch 14

Waves and Energy Transfer

Ch 14 Bkwk

• 5 – 7, 11 – 15, 17 – 18, 24 – 26, 32 – 41

Starting Question

• How could we figure out the speed of a wave through the slinky?

Ways to transport Energy

• Particles– Throw a ball, it is a particle that moved and

carried energy with it

• What about a wave through the slinky? Does it matter physically move to a new spot?

Wave Properties

• Waves carry energy, but not matter– Throwing a ball carries energy and matter

– Waves carry the energy of vibration, but no matter

• Like sound waves

• Wave –– A rhythmic disturbance that carries energy

through matter or space

Mechanical Waves• Require a medium (something to travel

through)– Water, air, ropes, and springs

• Wave Pulse –– A single bump or disturbance that travels through

a medium

• Continuous Wave –– Multiple disturbances

Transverse Waves

• A wave that vibrates perpendicular to the direction of wave motion

Longitudinal Waves

• A wave that vibrates in the same direction or parallel to the direction of motion– Sound waves

– Fluids such as liquids and gases

Surface Waves

• A.k.a. – water waves

• Have characteristics of both transverse and longitudinal waves– Move parallel and perpendicular to the motion

Measuring a Wave

• Speed– Can be found just like any other moving object

• Distance traveled over time it took to travel

• Must use same part of wave to measure displacement

Measuring a Wave

Measuring a Wave

• Wavelength (λ)– Shortest distance between two points where a

wave repeats itself

– Trough – low points

– Crest – high points

Measuring a Wave

• Period and frequency can only be measured for continuous waves

• Period (T) –– Time it takes to complete one wavelength

Measuring a Wave

• Frequency (f) –– Number of complete oscillations in each second

• Measured in Hertz (Hz)

1fT

Measuring a Wave

• Speed of a wave

v f

Example

• A sound wave has a frequency of 262 Hz and a wavelength measured at 1.29m.– What is the speed of the wave?

– How long will it take the wave to travel the length of a football field, 91.4 m?

– What is the period of the wave?

Practice Problems

• P335: 1 – 7

Activity for Properties of a Wave

• Stretch out the coiled spring 2 or 3 meters• Send a longitudinal pulse down the spring

– What time did it take the pulse to reach the end?

• Increase the amplitude by pulling back more coils– What time did it take the pulse to reach the end?

• Stretch the spring to a different length and send a pulse– What time did it take the pulse to reach the end?

• What can you conclude?

14.2

Wave Behavior

Waves at Boundaries

• Speed depends only on properties of the medium

• Incident wave –– Wave that strikes a boundary

Waves at Boundaries

• Reflected wave –– Wave that is reflected back or returns

• Continuing wave is erect (upwards)

• Reflected wave can be– Erect – if beginning material is “thicker”

– Inverted – if beginning material is “thinner”

Superposition of Waves

• Waves combining

Wave Interference

Constructive Interference

Destructive Interference

Nodes and Antinodes

• Node – location where displacement is zero

• Antinode –location where displacement is the greatest

Waves in Two Dimensions

• Law of reflection –– Angle of incidence is equal to the angle of

reflection

• Refraction –– The change in the direction of waves at the

boundary between two media

– Or the wave that goes through to the new boundary

Waves in Two Dimensions

• Diffraction –– The spreading of waves around a barrier