Basics of Waves Review
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15. The number of cycles of a periodic wave occurring per unit time is defined as a wave's ____.
a. wavelength.

b. period.

c. amplitude.

d. frequency.

Answer: D
This is a basic definition which you should know and be able to apply.

16. A periodic and repeating disturbance in a lake creates waves which emanate outward from its source to produce circular wave patterns. If the frequency of the source is 2.00 Hz and the wave speed is 5.00m/s then the distance between adjacent wave crests is ___ meter.
a. 0.200

b. 0.400

c. 1.25

d. 2.50

e. 10.0

Answer: D
Let w=wavelength; then v = w*f. In this problem, it is given that v=5.00 m/s and f = 2.00 Hz. Substitution and algebra yields w = v / f = 2.50 m.

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17. What is the frequency of a wave that has a speed of 0.4 m/s and a wavelength of 0.020 meter?
a. 10 hertz.

b. 20 hertz.

c. 0.008 hertz.

d. 0.5 hertz.

Answer: B
Let w=wavelength; then v = w*f. In this problem, it is given that v=5 m/s and w = .02 m. Substitution and algebra yields f=v/w=20 Hz.

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18. Many wave properties are dependent upon other wave properties. Yet, one wave property is independent of all other wave properties. Which one of the following properties of a wave is independent of all the others?
a. wavelength

b. frequency

c. period

d. velocity


Answer: D
The speed of a wave is dependent upon the properties of the medium through which it moves, not upon the properties of the wave itself.

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19. A pendulum makes exactly 40 vibrations in 20.0 s. Its period is ____. (Be cautious of the units.)
a. 0.500 Hz.

b. 0.500 s.

c. 2.00 Hz.

d. 2.00 s.

e. 8.00 x 10^{2} Hz.

Answer: B
The period is the time for one complete cycle. If the pendulum takes 20.0 seconds for exactly 40 vibrational cycles, then it must take 0.500 second for one cycle.

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20. A period of 0.005 seconds would be equivalent to a frequency of ____ Hz.
a. 20

b. 50

c. 200

d. 500

e. 2000

Answer: C
The period and the frequency are related by a reciprocal relationship; that is, f = 1/T.

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21. TRUE or FALSE:
The number of waves generated per second by a source is called the frequency of the source.
Answer: A
This is a basic definition which you should know and be able to apply.

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22. TRUE or FALSE:
The SI unit for frequency is hertz.
Answer: A
Know this like the back of your hand.

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23. TRUE or FALSE:
Doubling the frequency of a wave source (without altering the medium) doubles the speed of the waves.
Answer: B
Don't be fooled. Wave speed may equal frequency*wavelength. Yet doubling the frequency only halves the wavelength; wave speed remains the same. To change the wave speed, the medium would have to be changed.

24. If the frequency of a wave is doubled and if the speed remains constant, its wavelength is ____.
a. quartered.

b. halved.

c. unchanged.

d. doubled.

Answer: B
Wave speed equals frequency*wavelength. So doubling the frequency must halve the wavelength in order for wave speed to remain the same.

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25. Two different ropes with different mass densities are attached to each other. A pulse is introduced into one end of the rop and approaches the boundary as shown at the right. At the boundary, a portion of the energy is transmitted into the new medium and a portion is reflected. Which one of the diagrams below depicts the possible location and orientation of the pulse shortly after the incident pulse reaches the boundary?
Answer: C
The speeds in the two media can be deduced by the distance of the pulses from the boundary. In A and E, the speed is shown as fastest on the right, which makes the transmitted medium the less dense. Rule out A and E since a reflected pulse should not invert when moving from more dense to less dens. Rule out B for just the opposite reasons; the wave is moving from less to more dense and should invert upon reflection. Rule out D because the transmitted pulse never inverts. That leaves C as the answer.

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26. When a pulse reaches a boundary between two different media, it will be____.
a. reflected, only.

b. transmitted, only.

c. partly reflected and partly transmitted.

d. neither reflected nor transmitted.

Answer: C
This is basic information about the boundary behavior of waves.

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