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10201020 2013-10-31 12:55 AM

力學問題

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jmlo 2013-10-31 05:30 AM

回覆 1# 的帖子

On the left hand side of the equation, you set R - W, implying that W is pointing downward in the force diagram. In such case, g = 10 m/s^2.

10201020 2013-10-31 09:45 AM

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jmlo 2013-10-31 10:17 AM

回覆 3# 的帖子

If you define up as positive and down as negative, then you should not write R - W because in this case you are dealing with vector sum. You can think about the difference when you plug in +10 and -10 respectively to the equation. When g = +10 is used, the effect of W in R - W = R - 10m is to reduce the upward acceleration R. However, when g = -10 is used, R - W = R -(-10)m = R + 10m; the elevator is indeed being accelerated by the gravity, which makes no sense.

Therefore, make sure you don't confuse force diagram with numerical calculations. If you solve the question using force diagram, you should use vector sum (i.e., vector R + vector W) in which numbers should follow the definitions properly (i.e., +ve for up and -ve for down). If you solve the question algebraically (i.e., R - W = R - mg), then you should use positive values throughout since the effect of W is taken into account by the minus sign already.

[[i] 本帖最後由 jmlo 於 2013-10-31 01:24 PM 編輯 [/i]]

10201020 2013-10-31 02:39 PM

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dustbus 2013-10-31 08:49 PM

del

[[i] 本帖最後由 dustbus 於 2013-10-31 09:46 PM 編輯 [/i]]

10201020 2013-10-31 09:18 PM

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dustbus 2013-10-31 09:54 PM

[quote]原帖由 [i]10201020[/i] 於 2013-10-31 09:18 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=374796214&ptid=22618313][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]

個題目我100%已經完全還原,一粒字都無少

佢就係得中文版我先咁頭痕


架lift應該係向上運動, 然後越升越慢, 所以向上速慢
但既然向上運動,乜唔係應該

R-W=ma 咩???
向下運動先係 W-R=ma ???
thanks [/quote]
yes:smile_40::smile_40:我依家明條題了,岩岩個COMMENT DEL左,依家再解釋番

係向上運動,R-W=ma
但A要代負數
好啦你個問題係問點解G唔係負數,而係正數
咁weight係唔係向下?咁向下既話,地心吸力係咪向下Fnet=ma
而Fnet會同a平行咁去
weight係點計? W=mg
我覺得jmlo師兄係想講,R-W=ma
-W係因為個force向下

如果都係唔明
我簡單D舉例
將個波DUM上去
變Fnet=ma
  當上係正,咁應該係-Fnet=ma
咁再拆 -[m(-g)]=-mg
咁m=-m:smile_27::smile_27::smile_27:???

[[i] 本帖最後由 dustbus 於 2013-10-31 10:00 PM 編輯 [/i]]

10201020 2013-10-31 11:08 PM

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dustbus 2013-10-31 11:54 PM

冇問題LOL

你等龍哥PHY神上水:smile_27:

我岩岩可能1999,依家組織番

黎多場net force=ma
         R-W      = ma

R-W 已經包括左一個DIRECTION

NET FORCE=sum of the force嘛
咁應該係R+W
但考慮到佢既direction,先變係R-W
R-W已經包左DIRECTION
我用R+W做既話,會變成R+W=MA
g哩個時候先係-10



因為R+W=scalar,but not direction
vector=scalar + direction:smile_27::smile_27:再用番R-W講
weight=mg 向下

R=normal force 向上

咁向上-向下已經有方向,你仲有考慮多一次:smile_41:

[[i] 本帖最後由 dustbus 於 2013-11-1 12:20 AM 編輯 [/i]]

10201020 2013-11-1 12:44 AM

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↗亢龍無悔↙ 2013-11-1 10:50 AM

成篇compo 咁,我點會詳睇



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