momentum 一問 (URGENT)(10分)

momentum conserved 唔等於no change in momentum
but我依要介定你個no change in momentum係指兩件物件的其中一件...而唔係整體黎講..
例如8kg的R球以2ms^-1向右...4kg的Q球以1ms^-1向左
撞埋一齊後兩球都向右
by conservation of momentum
m(R)u(R)+m(Q)u(Q)=(m(R)+m(Q))v
take右為正...
8x2-4x1=(8+4)v
v=1ms^-1(呢個係正so都係向右)
咁momentum change in R呢就係:mv-mu=8x1-8x2=-8kgms^-1
就咁睇...Q都似冇charge in momentum 因為佢u係1ms^-1 而v又係
but momentum係有direction..
so momentum change in Q:mv-mu=4x1-4x(-1)=8kgms^-1
so 如果你就咁睇R or Q..咁就有change in momentum..
整體呢..就冇change in momentum啦...因為momentum conserved
即係change in momentum inR +change in momentum inQ=0
so..睇你指咩..

2008-03-11 18:19:18 補充：
你個問題問得幾好......一個2kg ball以2ms^-1向右撞向牆.....當佢會以-1ms^-1返返轉頭...即係1ms^-1向左...
咁你就可能以為佢個momentum唔 conserved 啦wo...
but千其唔好唔記得wall都係一件物件..而佢又連住地球..so個mass就等於wall+earth個mass
m(b)u(b)+m(w+e)u(w+e)=m(b)v(b)+m(w+e)v(w+e)
2x2+0=2x(-1)+m(w+e)v(w+e)

2008-03-11 18:19:23 補充：
(極極極大的mass)v=6
v=極極極極極細...somomentum都係conserved
至於個ball change in momentum就係mv-mu=2(-1)-2(2)=-6kgms^-1
so still有change in momentum

唔等於！！！


１．
momentum conserved係指所有撞之前同撞之後的總momentum都係保持不變

係物理學上，momentum必定係conserved


２．
change in momentum係撞之前同撞之後的momentum改變

這是對的.momentum conserved 等於no change in momentum
因為intial momentum=final momentum
| intial momentum-final momentum |=0
so change in momentum=0

2008-03-13 00:49:37 補充：
回答者在第一個例子中，只考慮一件物件的momemtum,當然the change of it's momentum is zero,but mometume 已經不conserved 了，如果一件物件

2008-03-13 00:53:21 補充：
這是對的.momentum conserved 等於no change in momentum
因為intial momentum=final momentum
| intial momentum-final momentum |=0
so change in momentum=0

我們考慮momentum 的時候應考慮整個系統的momeutm 吧。如果只考慮一件物件的momentum的話，momeutm 就不 conserved,所以才會結論為"momentum conserved 唔等於no change in momentum"

2008-03-15 22:08:18 補充：
我們考慮momentum 的時候應考慮整個系統的momeutm 吧
如果整個系統的momentum conserved,咁整個係統等於no change in momentum吧
其實問題問得好唔清楚
究竟是整個系統還是其中一件物件?
intial momentum=final momentum | intial momentum-final momentum |=0 so change in momentum=0??