Transcript Chapter 11 Vibrations and waves

Chapter 11 Vibrations
and waves
Goals: Harmonic Motion, Pendulums,
wave properties, period, frequency,
amplitude, wave speed and interactions
of waves
How does Energy move?
Section 11.1
 Forms
of Energy – mechanical,
thermal, chemical, electromagnetic,
electrical and nuclear
 Moves by conduction, convection and
radiation
 In the form of waves
 Wave = repeated disturbance in
matter or electromagnetic fields
Simple Harmonic Motion – repeated
motion with a displacing force and a
restoring force
 Back and forth motion
 ex: springs and pendulums
 Pendulums demonstrate simple harmonic
motion where the displacing force is the
force applied and the restoring force being
gravity
 at low angles as amplitude or mass is
changed the period remains the same
 Remember: there is a balance between
acceleration due to gravity and inertia

How does a Pendulum represent
Harmonic Motion?
Max displacement to equilibrium to max
displacement to equilibrium to max
displacement
 one cycle or one wave
 Amplitude is the max displacement
measured by the angle of the string
 Period is the length of time for one cycle
T
 Frequency is the number of cycles per
second
Hz
kHz
MHz


string
T = 2πΓL/g
L = length of
f = cycles/sec
f = 1/T

practice problems B pg 378
 Pendulum Lab – varied length of the
string and mass of the bob
 the Period is not affected by mass or
amplitude (not much)
 the Period is affected by the length
of the string

What are the different types of
waves? Section 11.3
Remember waves transfer Energy E = A²
 Two types of waves include: Mechanical
and Electromagnetic
 Mechanical waves transfer energy through
matter - the medium disturbing matter
not moving the medium
ex: water
waves, sound, seismic and tsunami
 Electromagnetic- disturbances in electric
and magnetic fields
empty space

Mechanical waves can be either transverse
or longitudinal, but electromagnetic waves
are transverse
 Transverse waves disturb matter at right
angles to the transfer of energy (water)
 have a crest and a trough = 1λ
 sine wave and can be shown on wave
form
 Longitudinal waves disturb matter a series
of compressions (crests) and rarefactions
(troughs)
(sound)
 wave velocity
v=ΔX/Δt v=λ/T and
f=1/T
 so
v=f·λ
practice problems
D

How do waves interact?
Section 11.4
Mechanical waves displace matter and can
occupy the same space and time
 overlap or superimpose
 produce interference patterns
 Resultant wave is the sun of the waves in
the same space and time
 Constructive interference – waves add to
produce higher amplitudes
 Destructive interference – waves with
opposite signs cancel partially or
completely

Reflection – when a wave returns from a
different medium
 Demo:
hold slinky fixed at one end open
at the other and wave will return upright
 with both ends fixed the wave will return
inverted
 Standing wave – appear stationary and
are alternating constructive and
destructive interferences at certin
frequencies
 Nodes (des)
Antinodes (con)
 Fundamental – one antinode, lowest
natural frequency or first harmonic

 Lab
on harmonics

λ = 2L/ harmonic

ex: third harmonic
2/3 m
λ = 2L/3
or
 Waves
are not only a lot of fun to
study
 they also bring music to our ears, so
 Study for tomorrows Exam
 that is some sound advice!!!