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 http://www.nakuz.com/bbs/images/common/back.gif
咁既話個u2 變左冇關係 ... 本帖最後由 包包 klau1001 於 8-4-2009 13:47 編輯
咁既話個u2 變左冇關係 ...
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 本帖最後由 piyopiyo 於 8-4-2009 21:48 編輯
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 http://www4.nakuz.com/bbs/images/common/back.gif
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[] 樓主唔差在post埋個solution出泥睇下邊度有野 本帖最後由 包包 klau1001 於 9-4-2009 16:13 編輯
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[]
piyopiyo 發表於 8-4-2009 18:24 http://www.nakuz.com/bbs/images/common/back.gif
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) 本帖最後由 piyopiyo 於 9-4-2009 17:47 編輯
包包扣果然係phy神人,可謂觀察入微,完全將doppler effect融匯貫通[]
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[]
頁:
1
[2]