phy vector[YU]爺叫到就入la

piyopiyo

For 3 non-parallel forces to be in equilibrium,they must
其中一個選擇係
There are lines of action passing through one point.
即係點[[tk_30]] 咁係唔係
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2.
有架車係頂衝落泥
Q係前面個坡ge頂,亦可以睇成為一個circle of radius 10m ge頂部(圖中個circle係假的,條track係係Q成一個circle上部咁落去)
vertical distance between Q and R is 24m
問係R ge max speed so that the cart can complete the track safely
我咁做
mg-R=mv^2/r...max v when R = 0,,,如果唔係就會做到circular motion衝出條track
v^2=gr
KE at Q = KE at R - PE QR
1/2(100) + (24)(10) = (v^2)/2
580=v^2
v=24.1ms-1
其實呢題係mc
A.18.4  B.22.4  C.24.1  D.26.1
但有幾個人都話做到18.4...[[tk_21]]

aildux

幾個人計到18.4 m s^-1系因為佢地用

2mg-R=mv^2/r

v^2=2gr

然後又計錯變成

2gr/2+24(10)=v^2

或者本來系

gr/2+24(10)=v^2/2  兩邊乘2,唔記得將PE乘2, 結果變成

gr+24(10)=v^2

正確應該系

gr/2+24(10)=v^2/2

話俾佢地知咁樣系錯, 18.4 m s^-1連Q都去唔到仲點complete the track safely

去Q最少21.9 m s^-1

Puppet

Static equilibrium
1. no rotation -> forces are concurrent.
2. no net force.

piyopiyo

其實唔係好明
點解force一定要concurrent先唔會轉
好似有枝打橫ge棍,mg另外加2個non-parallel force,就算唔係act on同一點但都可以透過調較force ge位置做到equilibrium wor[[tk_02]]

Zend

其實唔係好明
點解force一定要concurrent先唔會轉
好似有枝打橫ge棍,mg另外加2個non-parallel force,就算唔係act on同一點但都可以透過調較force ge位置做到equilibrium wor[[tk_02]] ...
piyopiyo 發表於 13-1-2009 15:52

e.g.?

piyopiyo

還是我誤會了concurrent的意思?[[tk_21]]

Zend

example plx...

piyopiyo

見圖
mg = 100N
2邊斜上30度100N ge 力...打橫打直都無net force
2個打斜ge力同mg ge distance都一樣
咁又無moment
equilibrium[[tk_02]]

Zend

見圖
mg = 100N
2邊斜上30度100N ge 力...打橫打直都無net force
2個打斜ge力同mg ge distance都一樣
咁又無moment
equilibrium[[tk_02]]
piyopiyo 發表於 13-1-2009 17:48

你果兩個上斜的force, direction 係相同定相反?

相同: , THEN, 應該有net force (assume mg is balanced)
相反, mg 點balance?(assume the two forces have the same magnitude)

可能我唔知你幅圖係點,因為你無比圖我-.-

都係找人答你啦,我D physics 忘記左唔少

piyopiyo

唔記得上圖,補番