雜科問題～不定期更新～(拖

ice · 發表於 3/4/2010 11:21 PM

本帖最後由 IceDragon 於 9/4/2010 09:39 PM 編輯

An object is acted by three forces F1, F2 and F3 in suitable directions such that it remains at rest. Which of the following combinations of the magnitude of the forces is/are possible?

   F1    F2    F3
(1) 3N    4N    5N
(2) 3N    4N    7N
(3) 3N    5N    9N

A. (2) only
B. (3) only
C. (1) and (2) only
D. (1) and (3) only

呢題完全冇頭緒由邊度入手做= =
求指教
Thx~

unlock-YM · 發表於 4/4/2010 12:02 AM

C.
(1)F1向上，F2向右，F3向左下。
(2)F1,F2同一方向，F3對面方向。
(3)冇可能balance到。

ice · 發表於 4/4/2010 12:19 AM

唔該哂
我明(2)點解岩啦
咁(1)係咪用Pyth. Theorem做?

想問多題
A brick X is placed on horizontal surface and another brick Y, which is made of the same material, is placed on top of X. The width and height of Y are equal to that of X, but its length is half that of X.

Let P1=pressure exerted by Y on X,
P2=pressure exerted by the bricks on the surface
Find P1 : P2

A. 1:1
B. 1:2
C. 1:3
D. 2:3

當X既mass係2m
Y既mass係m
咁P1就係m
P2就係m+2m=3m
但點解答案唔係1:3 orz

unlock-YM · 發表於 4/4/2010 12:27 AM

本帖最後由 unlock-YM 於 4/4/2010 12:50 AM 編輯

理論上只要 F1+F2>=F3 就係possible, where F1,F2<F3。

Pressure = Force/Area

ice · 發表於 4/4/2010 12:49 AM

理論上只要 F1+F2<=F3 就係possible, where F1,F2<F3。

Pressure = Force/Area
unlock-YM 發表於 4/4/2010 12:27 AM

登錄/註冊後可看大圖

咦
咁以(3)黎講
3+5<9
咁點解係impossible?

第二題我都係唔明點計= =
個force應該係搵唔到?
咁點計P1 : P2?

唔該哂你解答我既問題~

unlock-YM · 發表於 4/4/2010 12:51 AM

咦
咁以(3)黎講
3+5
IceDragon 發表於 4/4/2010 12:49 AM

登錄/註冊後可看大圖

因為我打錯左。

P1係mg/A，P2係3mg/2A。
你畫個圖睇睇。

renz · 發表於 4/4/2010 02:19 AM

pressure out C?

ice · 發表於 4/4/2010 02:26 AM

我真係好煩= =
岩岩做完有題bio唔明

A student found that the leaves of a plant growing in the laboratory had turned yellow. Which of the following is the least possible explanation for this observation?

A. shortage of nitrate in the soil
B. shortage of magnesium in the soil
C. insufficient light supply to the plant
D. insufficient carbon dioxide supply to the plant

我淨係知Mg會影響chlorophyll
其他果D都唔係好清楚= =

Thx

fish · 發表於 4/4/2010 05:46 AM

本帖最後由 fish 於 4/4/2010 05:48 AM 編輯

我真係好煩= =
岩岩做完有題bio唔明

A student found that the leaves of a plant growing in the laboratory had turned yellow. Which of the following is the least possible explanation for this observa ...
IceDragon 發表於 4/4/2010 02:26

登錄/註冊後可看大圖

2005年 43題(?)
Chlorophyll molecules contain nitrogen and magnesium. N除了是chlorophyll的部分外, 亦是制造protein的主要材料, 沒proteins的話不能做任何biosynthesis.
Chlorophyll biosynthesis generally requires light. 試想想韭黃的種植方法就知了

Insufficient CO2 also leads to yellowing of leaves, but the effects are not as direct/ immediate as N, Mg, and light.
嗯, 有點含糊, 不過我不想對你們說太多完全out syl. 的biosynthetic pathways

總之answer 是D -w-

[GW]赤沙 · 發表於 4/4/2010 06:29 PM

Pressure out c 的
如果是應屆CE生就不用浪費時問了
Phy MC (3) 的問題
由於F1 + F2 才等於 8N , 少於9N，即是即使 F1和F2 are both in opposite direction of F3 ,there will still be a net force of 1N. In this case, F3 can never be balanced.

這種題目,如 F1+F2 < F3 , 或 |F1-F2| > F3 ,即不可能處於 Equilibrium State.

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