# 查看完整版本 : 力學問題

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]

R-W=ma 咩???

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

weight係點計? W=mg

-W係因為個force向下

當上係正,咁應該係-Fnet=ma

[[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

R-W      = ma

R-W 已經包括左一個DIRECTION

NET FORCE=sum of the force嘛

R-W已經包左DIRECTION

g哩個時候先係-10

vector=scalar + direction:smile_27::smile_27:再用番R-W講
weight=mg 向下

R=normal force 向上

[[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

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