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[Phy] 2 Mechnanics Question [hard]

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2 Mechnanics Question [hard]

Can anyone show me the correct calculations/explanations for the questions?
Q 30 and 32

Thanks!
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第一題係咪OUT C?
第2題係D...

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唔知.....本野係for呢個syl...

點解32 係d???

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F - f = mBa > 0
F > f

f = mAa

(F - f) / f = mBa/mAa = mB/mA

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唔係幾明 why F - f = mBa > 0...

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引用:
原帖由 jason200584 於 2010-4-18 01:50 PM 發表
唔知.....本野係for呢個syl...

點解32 係d???
the horizontal change of momentum would not affect the vertical velocity

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回覆 #6 MCWOODY 的帖子

however, does Jack have any mass?

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these two questions seem to be strange( or maybe my concepts are not clear ...)

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32 i think it is C...
by the conservation of momentum....

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no...the answer for Q30 is C and Q32 is D....

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I have seen some CE questions which said that a plastic lump is fallen on the trolley and the trolley will change it velocity
is it the different situation?

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Only vertical force is acting on carts and they are on tracks, so carts cant accelerate upwards or downwards (action and reaction pairs) .As a result,  they are moving with same velocity (horizontal / vertical) .

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Lets see if I've get the concept correctly..
For 30, F is used to push both two blocks with a total mass of the blocks
f is the friction force caused by F and the rough surface of B to push A with a smaller mass but with same acceleration.
By F=ma, f<F
Then similarly for choice 2, F is for both A and B therefore the correct one should be f/F = A/(A+B)
For choice 3, F is for both A and B, f is for A thereby (F-f) is the force pushing B. Regard that they've same acceleration.
(F-f)/f = B/A

For 32, initially three of them(Trolley A, B and Jack) have the same velocity, By converse of momentum,
We can draw a conclusion that Jack have same horizontal velocity and thereby that even the mass changes, velocity is unlikely to be affected.

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但係如果 jack 同 blocks 一樣重=m
當jack 一係開始既時候, jack同block a 既momentum= (2m)v
但當佢jump過去block b既時候, 咁momentum 咪=mv (jack 走左.. 無左個m)

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q32,咩momentum唔係唔會轉既咩?horizontial net force都冇轉wo?
咁如果個b個m increase,個speed咪會decrease lor@@

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