bearwing 發表於 3-4-2009 18:25:18

[phy]conservation of momentum

I know that momentum does not conserve when there is a net force acting on the system.
So does the below case obey conservation of momentum, pls explain in detail...

Zend 發表於 3-4-2009 19:10:36

關唔關2次做的WORK DONE 方向相反事?

bearwing 發表於 3-4-2009 19:40:40

關唔關2次做的WORK DONE 方向相反事?
Zend 發表於 3-4-2009 19:10 http://www4.nakuz.com/bbs/images/common/back.gif
唔係好明你講咩..

marking 係直接用conservation of momentum on vertical direction
但vertically 有gravity, 點解都用得?

Puppet 發表於 3-4-2009 20:03:14

Just before the splt and just after that, vertical momentum of the particle is conserved.
After some time, the gravitational force changes the vertical speed of each fragment, so total momentum of the two fragments is changed.

包包 klau1001 發表於 3-4-2009 20:08:36


唔係好明你講咩..

marking 係直接用conservation of momentum on vertical direction
但vertically 有gravity, 點解都用得?
bearwing 發表於 3-4-2009 19:40 http://www.nakuz.com/bbs/images/common/back.gif
類似explosion?
可能是因為發生時間太短(分開前後),
所以gravity的影響接近零?
之前不conserve的case(例如一個free fall的波)
都要一個非instantaneous的時間去計算吧..?

(不太知自己在說什麼=="")

bearwing 發表於 3-4-2009 20:10:37

本帖最後由 bearwing 於 3-4-2009 20:14 編輯

But why we cannot use conservation of momentum in direction joining line of action in this question?
We must consider horizontal momentum for this question

Puppet 發表於 3-4-2009 20:45:16

6# bearwing
唔明你想表達甚麼
你話邊題?

bearwing 發表於 3-4-2009 20:46:45

本帖最後由 bearwing 於 3-4-2009 20:48 編輯

6# bearwing
唔明你想表達甚麼
你話邊題?
Puppet 發表於 3-4-2009 20:45 http://www4.nakuz.com/bbs/images/common/back.gif
上高Q7 果題, we cannot use conservation of momentum "along line of action"
because there is an external force acting on vertical direction
但頂樓果題就用得

我唔明呢兩個case 有咩分別, 點解一個用得一個唔用得

piyopiyo 發表於 3-4-2009 21:35:35

本帖最後由 piyopiyo 於 3-4-2009 21:48 編輯

下面寫住
since velocity of B perpendicular to the line of is unaltered
u2=usinx...
唔係好明點解係unaltered-.-...將u打散做2個component,其中1個along the line of centre,另1個就perpendicular(u2)...而撞擊個force淨係along the line of centre,所以唔影響u2果個direction ge velocity?

piyopiyo 發表於 3-4-2009 21:37:24

本帖最後由 piyopiyo 於 3-4-2009 21:43 編輯


上高Q7 果題, we cannot use conservation of momentum "along line of action"
because there is an external force acting on vertical direction
但頂樓果題就用得

我唔明呢兩個case 有咩分別, 點解一個用得一 ...
bearwing 發表於 3-4-2009 20:46 http://www4.nakuz.com/bbs/images/common/back.gif
係唔係tension ar
2粒波中間個force就係lie on the line of centre,果個force係打斜ga wor
但A淨係move horizontally...-.-...
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