physics!

小霞的fans

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

compolent

Nidoq

//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

//gram rule: (直角時用)

                               
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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

//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

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

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

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

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

..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