Unit 6: Pendulums, Waves, and Ocean Waves

Question 1

What is the period of a pendulum that takes one second to make a complete back and forth vibration?

Answer 1

T = 1 s.

Question 2

Suppose that a pendulum has a period of 1.5 seconds. How long does it take to make five complete back and forth vibrations?

Answer 2

T = 1.5 x 5 = 7.5 s.

Question 3

Is a pendulum with a 1.5 s period longer or shorter in length than a pendulum with a 1s period?

Answer 3

Longer.

Question 4

What is a wave? What is a vibration?

Answer 4

• Wave: a periodic wiggle in both space and time.

- Vibration: a periodic wiggle in time.

Question 5

Does a vibration or a wave spread out through space?

Answer 5

Both.

Question 6

Distinguish between the period and the frequency of a vibration or a wave. How do they relate to each other mathematically? [Note: T is for period, which is different than t for time.]

Answer 6

• Period (T): the time in which a vibration is completed.
• Frequency (f): for a vibrating body or medium, the number of vibrations per unit time. For a wave, the number of crests that pass a particular point per unit time.
• T = 1/f.
• f = 1/T.

Question 7

Distinguish among these different parts of a wave: equilibrium line, amplitude, crest, trough, and wavelength.

Answer 7

• Equilibrium line: the mid-line, or “flat-line.
• Amplitude: for a wave or vibration, the maximum displacement on either side of the equilibrium (midpoint) position.
• Crest: high points of a wave.
• Trough: low points of a wave.
• Wavelength: the distance between successive crests, troughs, or identical parts of a wave.

Question 8

Does the medium in which a wave travels move along with the wave itself? Defend your answer.

Answer 8

No, the only thing that moves along with the wave is energy.

Question 9

How does the speed of a wave relate to its wavelength and frequency?

Answer 9

• v = fλ
• v = velocity (speed), measured in m/s.
• f = frequency, measured in Hz.
• λ = lambda (wavelength), measured in m.

Question 10

As the frequency of sound is increased does the wavelength increase of decrease? Give an example.

Answer 10

• Decreases.

- Example: in beats, as the frequency increases, the wavelength decreases.

Question 11

Distinguish between a transverse wave and a longitudinal wave. Give examples of each.

Answer 11

• Transverse wave: a wave in which the medium vibrates perpendicularly (at right angles) to the direction in which the wave travels. Examples: light waves and waves on stringed instruments.
• Longitudinal wave: a wave in which the medium vibrates parallel to (along) the direction in which the wave travels. Example: sound waves.

Question 12

Distinguish between constructive interference and destructive interference.

Answer 12

• Constructive interference: when crest of one wave overlaps crest of another, their individual effects add together to produce wave of increased amplitude.
• Destructive interference: when crest of one wave overlaps trough of another, their individual effects are reduced (high part of one wave simply fills in low part of another).

Question 13

Is interference a property of only some types of waves or all types of waves?

Answer 13

All types of waves.

Question 14

What causes a standing wave?

Answer 14

Interference.

Question 15

Be able to do calculations involving and relating to frequency and period.

Answer 15

• T = 1/f.

- f = 1/T.

Question 16

What is a hertz?

Answer 16

The SI unit of frequency. One hertz (symbol Hz) equals one vibration per second.

Question 17

What are some possible units for frequency? Period?

Answer 17

• Frequency: Hertz (Hz).

- Period: seconds (s).

Question 18

What factor(s) influence the period of a pendulum. Which factors do not?

Answer 18

• Influential: length.

- Non-influential: mass.

Question 19

Be able to solve for velocity, frequency and wavelength.

Answer 19

• v = fλ.
• f = v/λ.
• λ = v/f.

Question 20

What does and does not affect the speed of a wave?

Answer 20

• Does affect: medium.

- Does not affect: pitch.

Question 21

What happens when a wave reflects off a fixed end? A non-fixed end?

Answer 21

• Fixed end: the reflected wave travels on the opposite side.
• Non-fixed end: the reflected wave travels on the same side.

Question 22

Describe what happens when two waves are in the same place at the same time.

Answer 22

Constructive or destructive interference.

Question 23

What is a standing wave?

Answer 23

A stationary interference pattern formed in a medium when two sets of identical waves pass through the medium in opposite directions.

Question 24

What are nodes and antinodes? (Include a discussion of constructive and destructive interference.)

Answer 24

• Nodes: regions of minimal or zero displacement, with minimal or zero energy.
• Antinodes: regions of maximum displacement and maximum energy.

Question 25

Be able to draw standing waves for ½, 1, 2 and 3 wavelengths. Determine how many nodes and antinodes for each one.

Answer 25

• ½ wavelength: nodes- 2; antinodes- 1; ^.
• 1 wavelength: nodes- 3; antinodes- 2; ^v.
• 2 wavelengths: nodes- 5; antinodes- 4; ^v^v.
• 3 wavelengths: nodes- 7; antinodes- 6; ^v^v^v.

Question 26

Make sure you review the questions from the ripple tank lab. Since we just did this, I don’t want you to panic but please know about the four properties, how we observed them in the ripple tank, and the overall procedure and major concepts of it.

Answer 26

See Lab: Ripple Tank.

Question 27

What causes ocean waves to break?

Answer 27

The wave starts cresting, and, because the water can’t support it, it breaks.

Question 28

What causes a Tsunami? (Earthquake, volcano, etc.)

Answer 28

An earthquake, a volcano, a landslide/avalanche, etc.

Question 29

How does the wavelength of a Tsunami compare to the wavelength of regular ocean waves (they are much longer)?

Answer 29

The wavelengths of tsunamis are much longer than the wavelengths of normal ocean waves.

Question 30

How does constructive and destructive interference apply to ocean waves? (See your KA.)

Answer 30

• Constructive interference is when waves from different directions superimpose such that the crests add together, and the waves become bigger.
• Destructive interference is when waves intersect such that the crests and troughs even each other out, and the waves become smaller.