Electricity, Sound and Light

1
(No Transcript)
2
Electricity, Sound and Light
3
Chapter Nine Waves and Sound

• 9.1 Harmonic Motion
• 9.2 Waves
• 9.3 Sound

4
Investigation 9A
The Motion of a Pendulum

• How do we describe back and forth, or repeating
  motion?

5
9.1 Harmonic motion

1. Harmonic motion is motion that repeats over and
   over.
2. Linear motion gets us from one place to another.

6
9.1 Harmonic motion

• One full back-and-forth swing of a child on a
  swing is one cycle.
• A pendulum has a cycle like the one below.

7
9.1 Harmonic motion

• An oscillator is a physical system that has
  repeating cycles.
• Systems that oscillate always move back and forth
  around a center or equilibrium position.
• A restoring force is any force that always acts
  to pull a system back toward equilibrium.

8
9.1 Harmonic motion

• If a pendulum is pulled forward, gravity creates
  a restoring force that pulls it back, toward
  equilibrium.
• If the pendulum is moved backward, gravity pulls
  it forward, back to equilibrium again.

9
9.1 Harmonic motion

• According to the first law, an object in motion
  tends to stay in motion.
• Inertia causes the pendulum to overshoot its
  equilibrium position every time.

10
9.1 Harmonic motion

• Harmonic motion can be fast or slow, but speed
  constantly changes during its cycle.
• We use period and frequency to describe how
  quickly cycles repeat themselves.
• The time for one cycle to occur is called a
  period.
• The frequency is the number of complete cycles
  per second.
• Frequency and period are inversely related.
• One cycle per second is called a hertz,
  abbreviated (Hz).

11
(No Transcript)
12
9.1 Amplitude

• Amplitude describes the size of a cycle.
• The amplitude is the maximum distance the
  oscillator moves away from its equilibrium
  position.

13
9.1 Amplitude

• A pendulum with an amplitude of 20 degrees swings
  20 degrees away from the center in either
  direction.
• Friction slows a pendulum down, just as it slows
  all motion.
• We use the word damping to describe the gradual
  loss of amplitude.

14
9.1 Amplitude

• A good way to find the amplitude of a water wave
  is to measure the distance between the highest
  and lowest points on the wave.
• The amplitude is half this distance.

15
9.1 Graphs of harmonic motion

• A graph is a good way to show harmonic motion
  because you can quickly recognize cycles.
• Graphs of linear motion do not show cycles.

16
(No Transcript)
17
(No Transcript)
18
9.1 Natural frequency and resonance

• The natural frequency is the frequency (or
  period) at which a system naturally oscillates.
• Every system that oscillates has a natural
  frequency.

19
9.1 Natural frequency and resonance

• You can get a swing moving by pushing it at the
  right time every cycle.
• A force that is repeated over and over is called
  a periodic force.

20
9.1 Natural frequency and resonance

• Resonance happens when a periodic force has the
  same frequency as the natural frequency.
• When this happens, each push adds to the next one
  and the amplitude of the motion grows.
• The big amplitude of a swing is an example of
  resonance.