physics!

小霞的fans · 發表於 5/6/2009 06:43 PM

本帖最後由小霞的fans 於 4/6/2009 11:52 PM 編輯

an object is acted on by three forces,F1,F2 and F3 in suitable directions so that it remains at rest. which of the followng combination 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.1and 3 only

我唔明點解1果個可以\= =

同埋究竟咩係average resistve force. .
唔係太明白要計咩...

sy · 發表於 5/6/2009 06:58 PM

compolent

Nidoq · 發表於 5/6/2009 10:27 PM

本帖最後由 Nidoq 於 6/6/2009 06:16 PM 編輯

//gram rule: (直角時用)
圖錯了囧

這個rule忘了英文名~_~: (這個如果問resultant force的數字會係格仔紙問)

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sapphire · 發表於 5/6/2009 10:39 PM

本帖最後由 sapphire 於 5/6/2009 10:45 PM 編輯

//gram rule: (直角時用)

登錄/註冊後可看大圖

這個rule忘了英文名~_~: (這個如果問resultant force的數字會係格仔紙問)
http://i104.photobucket.com/albums/m189/marbl ...
Nidoq 發表於 5/6/2009 10:27 PM

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這個example有問題吧
會得出兩個沒有balance的5N force
如果要resultant force = 0的話, 3N的force應該倒轉direction才對......

下面的那個好像是叫tip to tip method ?
早就忘得一乾二淨了......

小霞的fans · 發表於 5/6/2009 10:41 PM

//gram rule: (直角時用)

登錄/註冊後可看大圖

這個rule忘了英文名~_~: (這個如果問resultant force的數字會係格仔紙問)
http://i104.photobucket.com/albums/m189/marbl ...
Nidoq 發表於 5/6/2009 02:27 AM

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咁樣net force會=0 咩??
我就係諗唔到點balance佢地/3\

Ivanhy92 · 發表於 5/6/2009 10:48 PM

本帖最後由 arararchchch 於 5/6/2009 10:52 PM 編輯

1# 小霞的fans

1果個係得既...

(直角三角形)

3^2+4^2=5^2

用Geogebra整既圖:

登錄/註冊後可看大圖

小霞的fans · 發表於 5/6/2009 11:18 PM

1# 小霞的fans

1果個係得既...

(直角三角形)

3^2+4^2=5^2

用Geogebra整既圖:

登錄/註冊後可看大圖

arararchchch 發表於 5/6/2009 02:48 AM

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我明啦^^ 唔該..仲有1題..metal既溫度轉變點知?

A metal block is first immersed in boiling water for some time. The block is then
transferred to a cup of cold water. After a while, the temperature of the water is
measured.
20. The result of the experiment is as follows:
Mass of metal block = 0.8 kg
Mass of water in the cup = 0.3 kg
Initial temperature of water in the cup = 23°C
Final temperature of water in the cup = 38°C
Find the specific heat capacity of the metal (in J kg–1 K–1).
(Given : Specific heat capacity of water = 4200 J kg–1 K–1.)
A. 236
B. 381
C. 622
D. 953

Ivanhy92 · 發表於 5/6/2009 11:34 PM

本帖最後由 arararchchch 於 5/6/2009 11:50 PM 編輯

我明啦^^ 唔該..仲有1題..metal既溫度轉變點知?

A metal block is first immersed in boiling water for some time. The block is then
transferred to a cup of cold water. After a while, the temperatur ...
小霞的fans 發表於 5-6-2009 23:18

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Let c Jkg^-1K^-1 be the s.h.c. of the metal.

Initial temperature of metal = 100oC
Energy transfer完之後溫度係一樣,題目沒提供水既Latent heat of vaporization...
暗示水冇Change in state...(唔係正統解釋,但係做卷就係咁,出下絕招)

(解釋)
Boiling water => 100oC 水 ,木分子開始有change in PE
Gain in PE in water 轉移左去 KE in metal
所以水唔會蒸發,keep住100oC
到左100oC,Energy transfer就會停....

Final temperature of metal = 38oC

Energy gained by water = Energy lost by metal
0.8(100-38)c=(0.3)(4200)(38-23)

小霞的fans · 發表於 5/6/2009 11:39 PM

Let c Jkg^-1K^-1 be the s.h.c. of the metal.

Initial temperature of metal = 100oC
Energy transfer完之後溫度係一樣

Final temperature of metal = 38oC

Energy gained by water = Energy lost b ...
arararchchch 發表於 5/6/2009 03:34 AM

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..其實一放入boiling water就代表已經到左100度??

Ivanhy92 · 發表於 5/6/2009 11:48 PM

..其實一放入boiling water就代表已經到左100度??
小霞的fans 發表於 5-6-2009 23:39

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係...

Boiling water => 100oC 水 ,木分子開始有change in PE
Gain in PE in water 轉移左去 KE in metal
所以水唔會蒸發,keep住100oC
到左100oC,Energy transfer就會停....

(水分子就Gain PE,最終蒸發走)

應該係咁

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