本帖最後由 bearwing 於 3-4-2009 21:51 編輯
You say momentum will be not conserved only if there a time lag to allow the external force to change its velocity (thus its momentum), then that means you can use conservation of momentum at any instant t->0. But, it seems not true...
Refer to piyopiyo, if u say tension will affect the ball, then why u cannot say gravity will affect the particle? (頂樓果題)
沖沖下涼 想到點解了
與其用上高個definition, 轉下definition 更好
Momentum will conserve only if all of the components in the system are in the same relative frame.
頂樓果題, 未爆前同爆左之後既particle 都係under gravity, 所以係same frame
下低果題, 個sphere A net force =0 (tension + gravity) 但B net force =/=0 (gravity)
Hence, cannot use conservation of momentum
沖沖下涼 想到點解了
與其用上高個definition, 轉下definition 更好
Momentum will conserve only if all of the components in the system are in the same relative frame.
頂樓果題, 未爆前同爆左之後既particl ...
bearwing 發表於 3-4-2009 22:08 http://nakuz.com/bbs/images/common/back.gif
why the first statement is true?
why the first statement is true?
Zend 發表於 3-4-2009 22:17 http://www4.nakuz.com/bbs/images/common/back.gif
直覺
中國人重視主觀直覺及關聯感應
我係中國人
no rigorous proof ?
11# bearwing
how does it seem not true?
我的直覺講我知係咁
要proof 既話 我唔係Newton 同Einstein ,屈唔到出黎-.-..
本帖最後由 Puppet 於 4-4-2009 00:39 編輯
"For an instant to be an instant, the total momentum of a system must be conserved in a certain instant."
It seems untrue because, it can be deduced from this statement that, the system will have a constant momentum when acted upon by an external net force.
We know this deduction is wrong from our experience. There are 2 possible reasons.
1. The statement is wrong. However, it follows that the total momentum changes in a certain instant. This is contradictory.
2. The deduction is wrong. The deduction assumes that the whole motion is given by the sum of all instantaneous motions. This assumption has no experimental ground.
After all, because the split takes place at an instant (the particle is with zero vertical momentum), you can apply the conservation of moemtum law for that event in that instant.
(Actually, it is not necessary to think that "可能是因為發生時間太短(分開前後)".
The reason is that there is only one instant of the splitting. There is only one instant in which the particle becomes 2 halves. You can have the state of "the 2 halves are about to separate", but in this state, the splitting does NOT occur yet.
When we accept the term "the initial speed" of one part, it means this speed is just after the split. There is no point to talk about "發生時間太短" because the term "initial speed" refer to the speed just after the split. The speed after some time lag > 0 is no long called the initial speed.)
本帖最後由 bearwing 於 4-4-2009 01:36 編輯
收返先, 我搞錯少少野
"For an instant to be an instant, the total momentum of a system must be conserved in a certain instant."
It seems untrue because, it can be deduced from this statement that, the system will have a co ...
Puppet 發表於 4-4-2009 00:23 http://www.nakuz.com/bbs/images/common/back.gif
即是說"分離是瞬時的",
以及"對於瞬時的碰撞/分離來說momentum of a system always conserve"?
因此,樓主的果題time of impact = 0 所以 momentum conserve?
而之後果題因為time of impact > 0,所以如果將果兩個當係一個system的話,
tension = external force,vertical momentum 就唔conserve?
我這樣說對嗎?