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發表於 4-4-2009 00:23:23
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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.) |
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