TUTORIAL ANSWERS 410-440

 

 
 


Your Answer to Q410.  Sorry, your answer is not correct. Work requires a force acting through a distance.

Help: Fundamentals of Sound, Sec. 10-A.

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

Return to Question 410.

 

 

Hint for Question 410:  Work requires a force acting through a distance; a distance counts only if it is in the direction of the force. Assume that the skater is just gliding without pushing off. Does that make a difference?
 
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Your Answer to Q410. Congratulations, your answer is correct.

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Correct Answer to Question 410:  d) is the correct answer. Gravity is doing work on the book since it is falling through a distance while acted by a force equal to the weight of the book. The child is pulling the truck, which causes a force along the distance pulled. If the skater is not pushing off, but is just coasting, he is doing no work. (if you assumed he is pushing off and gaining kinetic energy, then he would also be doing work, however the problem says he is just gliding.) The only force he is exerting is upward to hold his partner in the air. That force is vertical, but the distance moved in the problem is the horizontal displacement of the pair. So the force does not act in the direction of the distance moved and no work is done by the force. Note that the pair has kinetic energy, but that the kinetic energy and the potential energy of the partner are not changing, because he is doing no work on her. When he first lifted her over his head he certainly did work and she gained the potential energy that she now has.

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Your Answer to Q420.  Sorry, your answer is not correct. Work is done when a force acts through a distance.

Help: Fundamentals of Sound, Sec. 10-A.

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 420:  The distance that counts is that which is along the applied force. What workers are pushing through a distance that is along their force?
 
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Your Answer to Q420. Congratulations, your answer is correct.

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Correct Answer to Question 420:  b) is correct because the workers 2 and 4 are pushing perpendicular to the displacement of the piano and are doing no work. Workers 1 and 3 are pushing along the direction of the displacement, doing work, and increasing the piano's kinetic or potential energy.

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Your Answer to Q425.  Sorry, your answer is not correct. Will the book continue to gain velocity as it falls further?

Help: Fundamentals of Sound, Sec. 10-A.

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.

Return to Question 425.

 

 

Hint for Question 425:  Note that the book is still a certain distance from the floor. Gravity is still pulling on it.
 
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Your Answer to Q425. Congratulations, your answer is correct.

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

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Correct Answer to Question 425:  The book has lost some potential energy by falling half way to the floor, but it is still has some distance to fall so that it still has some potential energy. The answer is c). As it falls farther, gravity continues to do work on it so it gains more kinetic energy and loses even more potetial energy. It will not have lost all of its potential energy until just before it hits the floor where it will have converted all of its potential energy to kinetic energy.

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Your Answer to Q430.  Sorry, your answer is not correct. A diagram of the trajectory of the diver might help. Actually, there is a way you might have drawn your diagram that might make your choice of answer correct after all!

Help: Fundamentals of Sound, Sec. 10-A.

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 430:  If she is moving forward at the highest point she still does have kinetic energy. The correct answer depends on the trajectory you assumed for the diver.

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Your Answer to Q430. Congratulations, your answer is correct. (That is, it is correct if you have chosen the same trajectory as we show in the correct answer!)

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

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Correct Answer to Question 430:  See the two possible trajectories in the figure. In the A trajectory the diver goes almost straight up before she starts down. At the top she has no velocity—she has stopped momentarily. She thus has no kinetic energy and all her energy is potential. She is accelerating and gravity will soon convert her potential energy into kinetic energy. In this case the answer is b). In trajectory B, she is still going forward at the highest point of her dive because she initially jumped forward as well as up. So at that point she has both kinetic energy and potential energy. In this case the answer would be c). Sorry if we said you were wrong if you used trajectory B!

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Your Answer to Q435.  Sorry, your answer is not correct.  What does the ball do when it reaches the bottom?

Help: Fundamentals of Sound, Sec. 10-A.

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.

Return to Question 435.

 

 

Hint for Question 435: The ball continues to roll when it reaches the bottom.

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Your Answer to 435. Congratulations, your answer is correct.

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

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Correct Answer to Question 435:  The ball continues to roll when it reaches the bottom. It has converted all of its potential energy that it had at the top of the ramp into kinetic energy so it continues moving on with that energy. The answer is a).

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Your Answer to Q440.  Sorry, your answer is not correct. How does the string store potential energy?

Help: Fundamentals of Sound, Sec. 10-C.

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 440:  The string stores potential energy elastically by being stretched. It will release this by bouncing back to its unstretched state.
 
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Your Answer to Q440. Congratulations, your answer is correct.

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

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Correct Answer to Question 440:  In a) and c) the string is stretched so that it has some potential energy—much like a stretched spring. At a) the string is momentarily at rest (somewhat like the diver at the top of her dive in trajectory A of Question 430).  In c) the string is again stretched somewhat, but it is also moving, and so has both potential and kinetic energies. At the extreme point of the string's motion downward (not shown) it would again have no KE and its energy would be all PE. At b) the string is not stretched at all, but is is moving as fast as it ever does. So at b) it has all kinetic energy and no potential energy. (Note we have neglected gravity in our discussion because it plays such small role in changing the potential or kinetic energy of the string. You could think of the string as vibrating horizontally so that gravity plays on role at all. )

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