CE 2010 phy mc

2010-08-22 7:03 am

See the links below
http://i820.photobucket.com/albums/zz127/forupload_2010/35.jpg
http://i820.photobucket.com/albums/zz127/forupload_2010/4.jpg

Answers
Q31 is B
Q36 is A
Q40 is A

Questions are below:

For Q31, Suppose the tension in the string is T and the mass of the ball is m, why should we resolve T into 2 components but the mg to solve this question?
If the question is asking for the acceleration of the ball, is it right to resolve mg into 2 components this time?

For Q36, why statement(1) is correct but statement(2) is incorrect? What does the word 'at rest' mean? I just know that particles at crests or troughs are momentarily at rest and move with highest speed when passing through the equilibrium.From the graph, destructive interference occurs at position A, I want to know whether Particle A is at equilibrium or at rest. Or at equilibrium=at rest??

ForQ40, why statement(3) is incorrect? Sound waves can reach the points along the line OQ. This shows the wave property diffraction. Right?

Thanks!

If anyone has the time, please help me to solve the physics problems stated in the link below, I really have trouble with those mc.

http://hk.knowledge.yahoo.com/question/question?qid=7010081801781

Thanks X100

更新1:

I almost get what you mean,but I still get some problems on the questions.Please help! For Q31, If the question is asking for the acceleration of the ball, is it right to resolve mg into 2 components this time?

更新2:

For Q36, particle A is always at rest. Can I say particle A is always at equilibrium (position) instead of saying it always at rest? ForQ40, to prove whether sound is a wave, is it enough to show that sound has either the diffraction property or interference property. Or both must be satisfied?? Thx

回答 (1)

天同

2010-08-22 9:01 am

✔ 最佳答案

Q31. It is a general practice to resolve forces into vertical and horizontal components. Since mg is already in the (downward) vertical direction, you don't have to resolve it further.

Q36. Statement (1) is correct. Particle at position A is always at rest, it is NOT momentarily at rest. That is, it doesn't not vibrate at all. The reason is apparent, destructive interference always occurs at position A. The wave from source S1 and wave from source S2 are always in anti-phase at that position.

Statement (2) is wrong. Although constructive interference occurs at position B, the particle at that position may not always be at a crest. As shown on the givem diagram, after a short interval of time, when the crests from the two waves move away, the particle at B is no longer at a crest.
You could see that after half a period, when the two troughs from the two waves meet at position B, the particle there will be at a trough.

Q40. The experiment is insufficient to prove that sound is a wave. Sound from the two louspeakers travels directly to any point along the line PQ. There is no diffraction or interference effect observed by using the sound level meter.

2010-08-23 12:38:56 補充：
Q31: As said, the weight of the ball is already vertically downward, and the acceleration is horizontally forward. There is no point to resolve the weight again. Should you resolve it, what direction would you choose?

2010-08-23 12:41:21 補充：
Q36: The measnings are the same. Particle A is always at rest at the equilibrium position.
Q40: Waves should have BOTH diffrarion and interference properties.

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