CPO Science
Foundations of Physics
Unit 5, Chapter 13
Unit 5: Waves and Sound
Chapter 13 Harmonic Motion
 13.1 Harmonic Motion
 13.2 Why Things Oscillate
 13.3 Resonance and Energy
Chapter 13 Objectives
1.
2.
3.
4.
5.
6.
7.
8.
9.
Identify characteristics of harmonic motion, such as cycles,
frequency, and amplitude.
Determine period, frequency, and amplitude from a graph of
harmonic motion.
Use the concept of phase to compare the motion of two
oscillators.
Describe the characteristics of a system that lead to harmonic
motion.
Describe the meaning of natural frequency.
Identify ways to change the natural frequency of a system.
Explain harmonic motion in terms of potential and kinetic energy.
Describe the meaning of periodic force.
Explain the concept of resonance and give examples of
resonance.
Chapter 13 Vocabulary Terms










harmonic motion
cycle
period
frequency
amplitude
hertz (Hz)
damping
periodic motion
periodic force
resonance










phase
phase difference
equilibrium
restoring force
stable equilibrium
unstable equilibrium
oscillator
natural frequency
steady state
piezoelectric effect
13.1 Harmonic motion
Key Question:
How do we describe the
back and forth motion
of a pendulum?
*Students read Section 13.1
AFTER Investigation 13.1
13.1 Cycles, systems, and oscillators
A cycle is a unit of motion that repeats.
13.1 Harmonic motion is common
sound
communications
clocks
nature
13.1 Amplitude
Amplitude describes the size of a cycle.
13.1 Amplitude
The energy of an oscillator is proportional to the
amplitude of the motion.
 Friction drains energy away from motion and slows
the pendulum down.
 Damping is the term used to describe this loss.
13.1 Linear Motion vs. Harmonic Motion
Graphs
13.1 Circles and the phase
of harmonic motion
 Circular motion is very similar to
harmonic motion.
 Rotation is a cycle, just like
harmonic motion.
 One key difference is that cycles
of circular motion always have a
length of 360 degrees.
13.1 Circles and the phase
of harmonic motion
 The word “phase” means where the oscillator is in the
cycle.
 The concept of phase is important when comparing one
oscillator with another.
13.2 Why Things Oscillate
Key Question:
What kinds of systems
oscillate?
*Students read Section 13.2
AFTER Investigation 13.2
13.2 Why Things Oscillate
 Systems that have harmonic
motion move back and forth
around a central or equilibrium
position.
 Equilibrium is maintained by
restoring forces.
 A restoring force is any force that
always acts to pull the system back
toward equilibrium.
13.2 Inertia
 Newton’s first law explains why harmonic motion
happens for moving objects.
 According to the first law, an object in motion stays in
motion unless acted upon by a force.
13.2 Stable and unstable systems
 Not all systems in equilibrium show harmonic motion
when disturbed.
 In unstable systems there are forces that act to pull the
system away from equilibrium when disturbed.
 Unstable systems do not usually result in harmonic
motion (don't have restoring forces).
13.2 The natural frequency
 The natural frequency is the
frequency at which systems
tend to oscillate when
disturbed.
 Everything that can oscillate
has a natural frequency, and
most systems have more
than one.
Adding a steel nut greatly increases the inertia of a stretched
rubber band, so the natural frequency decreases.
13.2 Changing the natural frequency
 The natural frequency is proportional to the acceleration
of a system.
 Newton’s second law can be applied to see the
relationship between acceleration and natural frequency.
13.3 Resonance and Energy
Key Question:
What is resonance and
why is it important?
*Students read Section 13.3
AFTER Investigation 13.3
13.3 Resonance and Energy
 Harmonic motion involves both potential energy and
kinetic energy.
 Oscillators like a pendulum, or a mass on a spring,
continually exchange energy back and forth between
potential and kinetic.
13.3 Resonance
 A good way to understand resonance is to think
about three distinct parts of any interaction
between a system and a force.
13.3 Energy, resonance and damping
 Steady state is a balance between damping from
friction and the strength of the applied force.
 Dribbling a basketball on a
floor is a good example of
resonance with steady state
balance between energy loss
from damping and energy
input from your hand.
Application: Quartz Crystals