Tutorial Answers 85-100
 

Your answer to Q85.  Sorry, your answer is not correct. Think how might you control the values of any of these three quantities?

Help: Fundamentals of Sound, Secs. 1-G, 1-H.

Or would you like a HINT?

You should try to work out the answer on your own, but if you insist on reading it, the correct answer is here

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Hint for question 85.  Think how you might change the value of frequency, wavelength, or wave velocity with a given  medium. If you must change the medium to change the size of X, then that eliminates being able to create a wave of arbitrary X.
 
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Your answer to Q85.  Congratulations, your answer is correct!

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Correct answer to Q85: The wave velocity depends on the properties of the medium, especially elasticity and inertia. The more elastic the medium is, and the less the mass density, the faster the wave travels. So, if those are "given" properties of the medium, we cannot change them or the wave velocity. However, one can change the frequency of the wave by changing how you make it. If I am making a wave on a rubber rope, I can shake my hand up and down faster to raise the frequency. If I increase the frequency, the wavelength becomes shorter. Thus I can control wavelength as well as frequency. The answer is d).

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Your answer to Q90.  Sorry, your answer to question 90 is not correct. Think what it is that determines wave velocity.

Help: Fundamentals of Sound, Secs. 1-G, 1-H.

Or would you like a HINT?

You should really try to work out the answer on your own, but if you insist on reading it, the correct answer is here.

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Hint for question 90.  Wave velocity is determined strictly by the properties of the medium.

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Your answer to Q90.  Congratulations, your answer is correct!

If you like, you can compare your answer to the "official" correct answer.

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Correct answer to Q90.  Wave velocity is determined by elasticity and inertia. If I stretch the string tighter or have a less dense string, the wave will move faster. Shaking it up and down at a higher frequency does not change the wave velocity. The correct answer is d).

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Answer to Q95: The answer is 16 feet.

Help: Fundamentals of Sound, Secs. 1-D, 1-G.

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To see how to get the correct answer click HERE

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Hint for Q95:  Wavelength is the distance between two adjacent crests of a wave, i.e., the peak-to-peak distance.

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Correct answer to Q95: The distance from one peak to the next is 16 feet. You can also take the distance from any one point to the next precisely similar point—for example, from a point where the displacement is  zero, such as at x = 0, to the next similar zero (at x = 16ft) where it has completed one cycle in space.

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Answer to Q96:    The correct answer is 8 seconds.

Help: Fundamentals of Sound, Secs. 1-D, 1-G.

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To see how to reach the correct answer click HERE.

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Hint for Q96.  The period is the time for one complete cycle.

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Correct answer to Question 96: The period is the time for one complete cycle. From the time graph we see that the point at x = 16 ft starts at height 3 ft at t = 0, goes down to zero at t = 2 s, continues down to height –3 ft at t = 4 s, and comes back up to height 3 ft at t = 8 s. The time for this complete cycle is the period and is 8 seconds.

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Answer to Q97:  The correct answer is 3 feet.

Help: Fundamentals of Sound, Secs. 1-D, 1-G.

If you got that wrong, would you like a HINT.

To see how to reach the correct answer click HERE.

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Hint for Q97:  Amplitude is the distance from the zero position (at-rest or equilibrium position) of any point to the point where is at a maximum distance away from equilirium.

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Correct answer to Q97: The amplitude is 3 feet. This is the distance from the at-rest position to the position of a peak of a wave. You can use either graph to find this.

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Answer to Q98:  The correct answer is 2 ft/s.

Help: Fundamentals of Sound, Secs. 1-D, 1-G.

If you got that wrong, would you like a HINT?

To see how to reach the correct answer click HERE.

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Hint for Q98:  Wave velocity is the distance traveled divided by the time to travel that distance.  Any wave travels one wavelength in one period.

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Correct answer to Q98:  The wave travels one wavelength in one period. We know that the wavelength is 16 feet and the period is 8 seconds. Thus the wave velocity (distance over time) is 16 ft/8 s= 2 ft/s. Another way to do this is to use the formula velocity = frequency times wavelength. The frequency is the inverse of the period =1/8 Hz. Thus v =  (1/8)16 = 2 ft/s.
 

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Your answer to Q99:  Sorry, your answer is not correct.  Use the top graph in your analysis.

Help: Fundamentals of Sound, Secs. 1-D, 1-G.

Would you like a HINT?

You should really try to work out the answer on your own, but if you insist on reading it the correct answer is here.

Return to Question 99.


Your answer to Q99:  Congratulations, your answer is correct!

If you like you can compare your answer to the "official" correct answer.

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Hint for Q99: Use the graph "At 2 s" to locate what the height is when the "distance in feet" is 18 feet; this is near the middle of the graph.

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Correct answer to Q99: The displacement is about 2 feet. Note that at distance (x) = 16 feet, the displacement is 0 ft; at x = 20 ft it is at maximum displacement of 3 feet. In between at 18 feet you should see it up at 2 feet. Answer is d).

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Your answer to Q100:  Sorry, your answer is not correct.

Help: Fundamentals of Sound, Secs. 1-D, 1-G.

Or, would you like a HINT?

You should really try to work out the answer on your own, but if you insist on reading it the correct answer is here.

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Hint for Q100:  This problem uses both graphs. You find how any point moves in time from the second graph and use that information to see how a particular point on the first graph will move as time goes on.Try dividing any cycle into four segments.
 

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Your answer to Q100:  Congratulations, your answer is correct!

If you like, you can compare your answer to the "official" correct answer.

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Correct answer to Q100:  Each period is 8 seconds. To make things easy divide a cycle of the motion of the medium point at x = 12 into four parts, 1) up from a displacement of  -3 ft to zero; 2) from zero up to maximum height of 3 ft; 3) back down to zero; and 4) back down to -3 ft. Each segment takes one-fourth of a period or 2 seconds. Thus from 2 seconds to 6 seconds involves just two segments. The point moves from -3 to zero and then to +3 feet. Thus the answer is e).

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