TUTORIAL

Answers A

 

 

 

Your Answer to Q5. Sorry, your answer is not correct. The ribbon must move; the question is which direction and how. You may be confusing the wave motion with the medium motion.

Help: Fundamentals of Sound, Secs. 1-A, 1-C.

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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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Your Answer to Q5. Sorry, your answer is not correct. The wave on a rope is NOT a sound wave.

Help: Fundamentals of Sound, Secs. 1-A, 1-C.

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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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Your Answer to Q5. Congratulations, your answer is correct.

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Hint for Question 5: The question is meant to point out the distinction between medium velocity and propagation or wave velocity. Ask yourself which kind of wave is on this rope, longitudinal or transverse? What is it that is moving down the rope to the right with the propagation velocity? What is it that is moving up and down with the medium velocity?

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Correct Answer to Question 5: The ribbon is attached to the medium, the rubber rope itself. When a wave propagates down the rope, the medium moves up and down while wave energy moves to the right. The rope itself does not move to the right, just the bulge of displacement of the rope caused by the impulsive wave. First the bulge is at the left and later it is farther to the right. The ribbon stays at a fixed position x, which does move up and down. Answer a) is correct.

The following animation shows this is detail. If the animation is not moving click on the Reload or Image button.

 

The ribbon is represented by a black dot. As the pulse moves to the right and passes the dot, the dot lifts up and back down with the medium.

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Your Answer to Q10. Sorry, your answer is not correct. Make sure you understand the distinctions among impulsive, sinusoidal, oscillatory, and traveling waves.

Help: Fundamentals of Sound, Sec. 1-A.

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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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Your Answer to Q10. Congratulations, your answer is correct.

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Hint for Question 10:

The question is meant to point out the distinctions among the terms "traveling", "impulsive", "sinusoidal" , and "oscillatory". These terms are not all mutually exclusive. A wave may be traveling and impulsive. Not all oscillatory waves are sinusiodal but may have other shapes as well. What does the term "traveling" mean?

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Correct Answer to Question 10:

Wave 1 is impulsive; the arrow indicates that it is traveling to the right; that is, we have an impulsive traveling wave. Wave 2 is a sinusoidal traveling wave. Wave 3 is a "sawtooth" wave; it is not sinusoidal, but it is oscillatory, that is it repeats its pattern regularly. and it is traveling. Thus all three waves are traveling waves of different sorts. Answer f) is correct.

 

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Your Answer to Q12. Sorry, your answer is not correct. Make sure you understand precisely what is involved in a sound wave. One of the answers, while accurately describing one aspect of a sound wave, is wrong because it is not precise enough.

Help: Fundamentals of Sound, Sec. 1-F.

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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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Your Answer to Q12. Congratulations, your answer is correct.

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Hint for Question 12: When one has a sound wave there are alternating compressions and rarefactions. In a rush of wind the molecules have varying position and a velocity, and energy is being carried along, but that is not a sound wave.

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Correct Answer to Question 12: A sound wave involves the elasticity and inertia of air. In a sinusoidal sound wave there are alternating compressions and rarefactions. Where the air is denser at a compression, the pressure is higher, and where the air is less dense at a rarefaction, the pressure is lower. If the air pressure is just higher or lower as when a storm is approaching, but without variation, we don't have a sound wave. A wind involves a motion (velocity) of the air and is not a sound wave, although it can often generate sound waves. Energy can flow through the air, say, by heat convection (feel the air flow above a radiator), but this is not a sound wave. However, when a sound wave exists, energy is indeed transferred by it from one place to another. The correct answer is c).

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Your Answer to Q15. Sorry, your answer is not correct. Make sure you understand the distinction between transverse and longitudinal waves. Have you included all possible transverse waves among the choices in your answer?

Help: Fundamentals of Sound, Sec. 1-A.

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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Your Answer to Q15. Congratulations, your answer is correct.

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Hint for Question 15: Three of the waves described are transverse and only one is longitudinal. A transverse wave has a medium motion that is perpendicular to the direction of travel of the wave.

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Correct Answer to Question 15: The pulse wave on the string and the water wave each have the medium moving perpendicularly to the wave direction. For example, recall that the ribbon in Question 5 moves up and down as the pulse wave goes to the right. That is a transverse wave with the ribbon showing the medium motion. In a wave made by fans in a stadium, the fans hands go up and down while the "wave" moves horizontally around the stadium; this too is analogous to a transverse wave (it is not really a true wave since it does not depend on a connected elastic medium for the "wave" to propagate). A sound wave is longitudinal with the air molecules moving back in forth along the direction of motion of the sound wave. The correct answer is g).

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Your Answer to Q17. Sorry, your answer is not correct. Make sure you understand the distinction between impulsive and oscillatory waves.

Help: Fundamentals of Sound, Sec. 1-B.

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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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Your Answer to Q17. Congratulations, your answer is correct.

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Hint for Question 17: An impulsive wave is just a sudden blip or two, while an oscillatory wave repeats the same pattern over and over again. Note however, that not all oscillatory waves are sinusoidal. A sinusoidal wave has a very special shape.

 

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Correct Answer to Question 17: When you drop a stone into a lake there is a ripple that is short-lived; it does not repeat over and over again. Thus it is impulsive. The heartbeat signal on a hospital monitor is regular (one hopes); it keeps repeating, but it is not sinusoidal; it has a more complex shape. A sinusoidal sound wave (also called a pure tone) such as made by a tuning fork has the smooth rippling shape shown, for example, in Figure 1-3b in Fundamentals of Sound. The correct answer is d).

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Your Answer to Q20. Sorry, your answer is not correct. Make sure you have read about reflection of waves on strings at a hard wall.

Help: Fundamentals of Sound, Sec. 1-J.

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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Your Answer to Q20. Congratulations, your answer is correct.

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Hint for Question 20: A wave hitting a wall where the rope is tied tight to the wall is going to stretch the rope especially much. Think what kind of a "rebound" that might cause for the wave.

 

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Correct Answer to Question 20: When a wave hits a wall where the rope is tied down it stretches the rope more than normally. That results in extra rebound which causes the wave to bounce back the way it came but upside down. See Animation I-16. The correct answer is e).

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Your Answer to Q22. Sorry, your answer is not correct. Make sure you understand the meaning of wave velocity.

Help: Fundamentals of Sound, Sec. 1-C.

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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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Your Answer to Q22. Congratulations, your answer is correct.

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Hint for Question 22: The speed or magnitude of the velocity is the distance traveled divided by the time to travel that distance. Note that because of the reflection, the pulse has traveled twice the length of the rope.

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Correct Answer to Question 22: The distance traveled in 3 seconds is 30 feet because it reflected and came back to where it started. The wave velocity or speed is thus 30 ft/3 s = 10 ft/s. The correct answer is b).

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Your Answer to Q24. Did you answer 4 Hertz? If not, check that you understand what frequency means.

Help: Fundamentals of Sound, Sec. 1-D.

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 24: Frequency is the number of cycles per second made by an oscillating object. In this case the cycle is one motion down and back up to the starting point.

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Correct Answer to Question 24:
Frequency is the number of cycles in one second. The bob makes 40 cycles in 10 seconds.
Thus the frequency is 40 cycles/10 seconds = 4 cycles per second or 4 Hertz.

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Your Answer to Q 25. Sorry, your answer is not correct.

Help: Fundamentals of Sound, Sec. 1-J.

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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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Your Answer to Q25. Congratulations, your answer is correct. You are really clever; this is a tough problem.

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Hint for Question 25: Recall that a wave hitting the end of a rope that is tied down will reflect upside-down and a wave hitting the loose end of a rope will reflect right-side-up. A wave hitting a thinner rope is much like hitting a loose end. Also ask yourself how fast a pulse will move on the thin rope relative to the thick rope.

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Correct Answer to Question 25: The correct answer is (E). A thick rope has much more inertia than a thin rope; the thick rope's end is not held down very strongly and the pulse reflects back right-side-up as if it had hit a loose end. The pulse that goes on to the thin rope now moves faster because the wave velocity is less on a rope with less inertia. Thus the pulse must be further along to the right than the reflected pulse is to the left. Note also that, because wave velocity is greater on the thin rope, the pulse gets spread out.

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