1. Chapter 13: Energy Flow and Power
13.1 Harmonic Motion
13.2 Why Things Oscillate
13.3 Resonance and Energy 1

2. Objectives
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. 2

3. Chapter 13 Vocabulary Terms
amplitude
damping
frequency
harmonic motion
hertz (Hz)
natural frequency
oscillator
period
periodic force
periodic motion
phase
phase difference
piezoelectric effect
resonance
stable equilibrium
unstable equilibrium

4. Harmonic motion Investigation Key Question:
How do we describe the back and forth motion of a pendulum? 4

5. Cycles, systems, and oscillators
A cycle is a unit of motion that repeats.

6. Harmonic motion is common
sound
communications
clocks
nature

7. Describing harmonic motion
The period of an oscillator is the time to complete one cycle.

8. Describing harmonic motion
Frequency is closely related to period.
The frequency of an oscillator is the number of cycles it makes per second.
At a frequency of 100 Hz, an oscillating rubber band completes 100 cycles per second.

9. Describing harmonic motion
The unit of one cycle per second is called a hertz (Hz).
When you tune into a station at on the FM dial, you are setting the oscillator in your radio to a frequency of megahertz (MHz).

10. Amplitude Amplitude describes the size of a cycle.
The value of the amplitude is the maximum amount the system moves away from equilibrium.

11. 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.

12. Harmonic Motion Graphs
Graphs of linear motion do not show cycles.

13. Harmonic motion graphs
Graphs of harmonic motion repeat every period, just as the motion repeats every cycle.
Harmonic motion is sometimes called periodic motion.

14. Circles & 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.

15. Circles & 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.

16. Harmonic Motion 1 Nature of Harmonic Motion 2 Why Things Oscillate
3 Resonance and Energy 16

17. 2 Why Things Oscillate Investigation Key Question:
What kinds of systems oscillate? 17

18. 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.

19. 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.

20. 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).

21. 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.

22. 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.

24. Harmonic Motion 1 Nature of Harmonic Motion 2 Why Things Oscillate
3 Resonance and Energy 24

25. Resonance and Energy Investigation Key Question:
What is resonance and why is it important?
1. The restoring force pulls the pendulum toward the center (equilibrium).
2. The pendulum overshoots the center because of its inertia.
3. The restoring force pulls back toward the center, slowing and reversing the pendulum’s direction.
4. The pendulum overshoots the center again, because of inertia.
5. The cycle repeats, creating harmonic motion. 25

26. 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.

27. Resonance
A good way to understand resonance is to think about three distinct parts of any interaction between a system and a force.

28. 13.2 Resonance
Resonance occurs when the frequency of a periodic force matches the natural frequency of a system in harmonic motion.

29. 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.

30. Quartz Crystals
The precise heartbeat of nearly all modern electronics is a tiny quartz crystal oscillating at its natural frequency.
In 1880, Pierre Curie and his brother Jacques discovered that crystals could be made to oscillate by applying electricity to them.
This is known as the piezoelectric effect.