phy energy問題

小霞的fans

WORK DONE = Fs 呢 ..

個F係APPLIED FORCE定係NET FORCE 呀 . .

如果有一個推力係80N推一個箱 .. FRICTION係20N .. 佢行左5米 ..
咁WORK DONE係80X5 定 (80-20)X5 呀 ?

另外呢　MOMENTUM果個 MaUa+MbUb+MaVa+MbVb 即係momentum will be conserved

當a同b係2個trolley啦. . u係initial velocity .. v係final velocity . .

阿sir一個向前既時候另一個就會向後 . .

咁呢個同action and reaction force 有咩分別 ?

超夢夢的夢境

原帖由 小霞的fans 於 5/3/2009 06:18 PM 發表

                               
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WORK DONE = Fs 呢 ..

個F係APPLIED FORCE定係NET FORCE 呀 . .

如果有一個推力係80N推一個箱 .. FRICTION係20N .. 佢行左5米 ..
咁WORK DONE係80X5 定 (80-20)X5 呀 ?

另外呢　MOMENTUM果個 MaUa+MbUb+ ...

1. Net Force

2. Action and reaction 同 momentum 究竟有乜關係呢?
考慮冇mass defect o既時候
F = m*dv/dt = d/dt (mv)
force 係 rate of change of momentum

Consider two bodies
=>)
因為 action and reaction pairs o既關係,
rate of change of momentum of two bodies are equal in magnitude and opposite in direction
將兩個 rate of change 加埋就係零
即係 sum of momentum of two bodies is constant
即係 momentum is conserved

<=)
momentum is conserved
sum of momentum of two bodies is constant
rate of change of momentum of two bodies are equal in magnitude and opposite in direction
If one body exerts a force on another, there is an equal and opposite force exerted on the first body by the second

其實兩樣野係冇分別 =w=
insight 唔錯
有錯請狠插

有 mass defect o既話,我完全唔識...


p.s.係 MaUa+MbUb = MaVa+MbVb

[ 本帖最後由 超夢夢的夢境 於 5/3/2009 07:16 PM 編輯 ]

abc666

1.如果是friction的話
total work done by「有一個推力」是80N X 5m =400J
60 x 5 =300J轉成KE
20 X 5 =100J轉成internal energy(due to friction)

超夢夢的夢境

原帖由 abc666 於 5/3/2009 08:04 PM 發表

                               
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1.如果是friction的話
total work done by「有一個推力」是80N X 5m =400J
60 x 5 =300J轉成KE
20 X 5 =100J轉成internal energy(due to friction)

啊~竟然把 internal energy 忘了
Kinetic energy 是看 Net force
可是有 work done against friction...

abc666

再補充一點:
答之前看清楚問什麼work done

total work done by the applied force, the total work done by the man,etc
-->applied force x s=80Nx5=400J

gain in kinetic energy of the box
-->Net force x s=60Nx5=300J=

work done against friction,increase in internal energy,heat released,etc.
--->friction x s=20 X 5 =100J