99 mc Q 14

[H]窮人

呢題唔識
睇左solution 都唔明
佢好似當左reciever 係stationary ....(or 我理解錯?

piyopiyo

咁岩無果題係手,上傳吧-.-...

aildux

S1 and S2 are stationary(they are not receiver)

This is a stationary wave and total four nodes between S1 and S2(including S1 and S2).

(1) is incorrect because S1 and S2 are positions of node.

(2) is incorrect because there is two points between S1 and S2 (excluding S1 and S2) which is always stationary.

(3) is correct the separation between two nearest antinodes between S1 and S2 is equals to wavelength/2 (as there is two points between S1 and S2 which is always stationary).

x-Stephen-x

你(1) ,(2) 定 (3) 唔明???都係計path difference only...

[H]窮人

第14 啊,,唔係13

piyopiyo

其實ans係咩-.-...
係咪D
不過我本書放左係locker...-.-...
用果條總結左ge equation又好似solve唔到
結果用relative speed撞-.-...

[H]窮人

你錯左啦..係a .....我都唔知點解...
最少有2年mc出過呢題

包包 klau1001

From the observer's point of view,
velocity of the ambulance = u2-u1
velocity of sound wave in air = v-u1
velocity of the car = 0

Due to Doppler effect,
入1=((v-u1)-(u2-u1))入/(v-u1+0) = (v-u2)入 / (v-u1)
入2=((v+u1)+(u2-u1))入/(v-u1-0) = (v+u2)入 / (v-u1)

therefore 入1/入2 = (v-u2)/(v+u2)
ans is (a)

=================================
可能是這樣吧?
我學校wave果課完全未開始教...
以上全是我單靠relative motion + eqt of doppler effect(from wiki) 推出來的...
wrong concept的話請插...

piyopiyo

我之前都係做到你咁,係唯獨一part
如果你係用 f'= f(v+vr)/(v+vs)
入'=入(v+vs)/(v+vr)
咁我就唔明點解,入2,果度個分母係 v-u1 -.-...
果個應該係velocity of the observer relative to the medium
但overtake左之後, 傳去car度ge sound同car應該相反方向...點解個relative velocity唔係v+u1 -.-...

包包 klau1001

我之前都係做到你咁,係唯獨一part
如果你係用 f'= f(v+vr)/(v+vs)
入'=入(v+vs)/(v+vr)
咁我就唔明點解,入2,果度個分母係 v-u1 -.-...
果個應該係velocity of the observer relative to the medium
但overtake左之後 ...
piyopiyo 發表於 8-4-2009 06:25

印象中,
計算兩個物體間的relative velocity的時候,
應該是統一用減的
因為velocity是vector
----------v--------->          --u1-->       ====>   -----v-u1------>
--u1-->                <----------v--------- ====>   <----------v-u1-------------

before: velocity of sound = v-u1 ,speed of sound =|v|-|u1|
after: velocity of sound = v-u1 ,speed of sound = v-u1 = |v| -(-|u1|) = |v|+|u1|