99 mc Q 14

[H]窮人

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-->                    ...
包包 klau1001 發表於 8-4-2009 12:18

咁既話個u2 變左冇關係 ...

包包 klau1001

咁既話個u2 變左冇關係 ...
[H]窮人 發表於 8-4-2009 13:29

上面純粹講緊speed/velocity of sound relative to the car
u2只係會對sound wave的入同f有影響
唔會影響velocity of sound(<--depends on medium and velocity of observer)
所以v當然同u2(velocity of source)冇關係

piyopiyo

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-->                    ...
包包 klau1001 發表於 8-4-2009 12:18

如果上面個v係正,so relative velocity = v- u1
但下面果個v同上面個v相反方向,係咪應該轉做-v -.-...
咁如果全部係減咁入2 果度都應該v-u2[[tk_21]]

piyopiyo

樓主唔差在post埋個solution出泥睇下邊度有野

包包 klau1001

如果上面個v係正,so relative velocity = v- u1
但下面果個v同上面個v相反方向,係咪應該轉做-v -.-...
咁如果全部係減咁入2 果度都應該v-u2[[tk_21]]
piyopiyo 發表於 8-4-2009 18:24

oops,我果然是有點wrong concept,
好,重來
============================
before catching up,
From the observer's point of view,
magnitude of the velocity of the ambulance = u2-u1
magnitude of the velocity of sound wave in air = v-u1
velocity of the car = 0

Due to Doppler effect,
f1=(v-u1+0)f0/((v-u1)-(u2-u1)) = (v-u1)f0 / (v-u2)
入1=(v-u1)/f1 = (v-u2)/f0

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

Due to Doppler effect,
f2=(v+u1-0)f0/((v+u1)+(u2-u1)) = (v+u1)f0 / (v+u2)
入2=(v+u1)/f1 = (v+u2)/f1

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

piyopiyo

包包扣果然係phy神人,可謂觀察入微,完全將doppler effect融匯貫通[[tk_23]]
原來我錯在計入1果陣照用左v/f1泥計
無留意到其實呢個係一個relative ge case[[tk_21]]