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mj19592009 2012-7-17 10:50 PM

CE Physics - Force

最近幫人補習,做幾題 CE pastpaper 時,
有d唔記得,希望大家幫幫手:loveliness:

A man of weight W stands inside a lift which is moving upwards with a constant speed. If the force exerted by the floor on the man is R, which of the following statements is/are correct?

(1)R is greater than W in magnitude.
(2)R and W are in opposite directions.
(3)R and W form an action and reaction force according to Newton's third law.

(1)同(2)比較明顯,

R 同 W magnitude 一樣,(1)錯
R 同 W opposite directions,(2)0岩

但(3)我有小小搞唔清楚,睇 solution(下圖) ,

[img]http://i.imgur.com/uQ5uF.jpg[/img]

佢話 R 係 acted on man by the floor,
W係 acted on man by the earth,

其實以上我都明,即係(3)都錯

但佢加多個 F 同 E,就有 d 唔明,佢話 :

F 係 acted on floor by the man
E 係 acted on man by the earth

咁其實 F 點解會出現 ?
move with constant speed,W 唔係應該係唯一0既力咩 ?

E 又係乜? acted on man by the earth ?

太耐冇掂過 physics,有 d 生疏,希望大家幫幫手,thanks:smile_40:

[[i] 本帖最後由 mj19592009 於 2012-7-17 10:53 PM 編輯 [/i]]

mj19592009 2012-7-17 10:59 PM

仲有一題:
[IMG]http://i.imgur.com/mwRu3.jpg[/IMG]
The blocks X and Y of mass 0.2kg and 0.8kg respectively are suspended from a ceiling by two strings of negligible mass as shown. If the string connected to the ceiling is cut and the bodies allowed to fall, find the tension T1 and T2 in the two strings.

上面條繩斷左,T2應該係 0

咁 T1 又點計 ?

Thanks

mj19592009 2012-7-17 11:12 PM

最後兩題,有關 contact force

[img]http://i.imgur.com/5lEEV.jpg[/img]
Two blocks of equal mass are placed on a smooth table top as shown above. A constant push 12N is applied to block P so that the two blocks move towards the right together. The force acting on P by Q is?


[img]http://i.imgur.com/GIFp8.jpg[/img]
Three blocks equal massed are placed on a smooth horizontal surface as shown in the figure. A constant force F is applied to block A so that the three blocks move towards the left with the same acceleration. The net force acting on block B is ?

我 CE 果陣已經唔係太識計 contact force...
請大家賜教,Thanks:loveliness:

Zzlaz 2012-7-18 03:19 PM

[quote]原帖由 [i]mj19592009[/i] 於 2012-7-17 10:50 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=335957568&ptid=20009187][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
最近幫人補習,做幾題 CE pastpaper 時,
有d唔記得,希望大家幫幫手:loveliness:

A man of weight W stands inside a lift which is moving upwards with a constant speed. If the force exerted by the floo ... [/quote]
分清楚, 個人係靜止, 即係作用佢身上NET FORCE 係0

同力EXISTS IN PAIRS 無矛盾.

W 係地球拉人, REACTION 係人拉地球 (一般唔會畫出來)

R 係地面對人的力,  R' 係人對番地面的力

mj19592009 2012-7-18 03:26 PM

[quote]原帖由 [i]Zzlaz[/i] 於 2012-7-18 03:19 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=336029126&ptid=20009187][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]

分清楚, 個人係靜止, 即係作用佢身上NET FORCE 係0

同力EXISTS IN PAIRS 無矛盾.

W 係地球拉人, REACTION 係人拉地球 (一般唔會畫出來)

R 係地面對人的力,  R' 係人對番地面的力 [/quote]

Weight 果 part 係咪因為 gravitational force ?
兩邊都有力俾對方,而且 magnitude 一樣 ?

咁 R 係點解會出現 ?
因為 acted on the earth by the man 0既 gravitational force ?

地面同地球又有咩分別 ?

Zzlaz 2012-7-18 03:30 PM

[quote]原帖由 [i]mj19592009[/i] 於 2012-7-18 03:26 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=336029721&ptid=20009187][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]


Weight 果 part 係咪因為 gravitational force ?
兩邊都有力俾對方,而且 magnitude 一樣 ?

咁 R 係點解會出現 ?
因為 acted on the earth by the man 0既 gravitational force ?

地面同地球又有咩分別  ... [/quote]
你企地面, 仲有得加速咩?

一定有個力去BALANCE 番作用你身上的W, 方向相反, 大小相同.


地球比的係GRAVITATION

地面比, 你可以理解成TENSION 之類的東西, SUPPORTIVE FORCE

Zzlaz 2012-7-18 03:34 PM

[quote]原帖由 [i]mj19592009[/i] 於 2012-7-17 11:12 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=335960420&ptid=20009187][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
最後兩題,有關 contact force

[img]http://i.imgur.com/5lEEV.jpg[/img]
Two blocks of equal mass are placed on a smooth table top as shown above. A constant push 12N is applied to block P so that the two blocks ... [/quote]
12 - F = ma
F = ma

Solve F

net force on each block = 6N

12- F1 = ma
F1- F2 = ma
F2 = ma

Solve F1, F2

anyway, net force on each block should be the same, i.e. F= 4N

mj19592009 2012-7-18 03:36 PM

[quote]原帖由 [i]Zzlaz[/i] 於 2012-7-18 03:30 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=336030123&ptid=20009187][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]

你企地面, 仲有得加速咩?

一定有個力去BALANCE 番作用你身上的W, 方向相反, 大小相同.


地球比的係GRAVITATION

地面比, 你可以理解成TENSION 之類的東西, SUPPORTIVE FORCE [/quote]

明白,Thank you:loveliness:

第一題其實我係唔記得 d concept 想知多 d

但第二、三、四題真知唔從何入手,完全唔記得:smile_27:

[[i] 本帖最後由 mj19592009 於 2012-7-18 03:40 PM 編輯 [/i]]

Zzlaz 2012-7-18 03:39 PM

[quote]原帖由 [i]mj19592009[/i] 於 2012-7-17 10:59 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=335958711&ptid=20009187][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
仲有一題:
[img]http://i.imgur.com/mwRu3.jpg[/img]
The blocks X and Y of mass 0.2kg and 0.8kg respectively are suspended from a ceiling by two strings of negligible mass as shown. If the string connected to the ... [/quote]
initial case

T1 =2N
T2 =8 +T1 = 10N

if string is cut, then of course T2 =0 N,

2- T1 = 0.2a
T1+8 = 0.8a

Solve equation

mj19592009 2012-7-18 03:40 PM

[quote]原帖由 [i]Zzlaz[/i] 於 2012-7-18 03:34 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=336030478&ptid=20009187][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]

12 - F = ma
F = ma

Solve F

net force on each block = 6N

12- F1 = ma
F1- F2 = ma
F2 = ma

Solve F1, F2

anyway, net force on each block should be the same, i.e. F= 4N [/quote]

sorry
可唔可以講清楚 d F, F1, F2 係咩 ?
Thanks

Zzlaz 2012-7-18 03:47 PM

[quote]原帖由 [i]mj19592009[/i] 於 2012-7-18 03:40 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=336031067&ptid=20009187][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]


sorry
可唔可以講清楚 d F, F1, F2 係咩 ?
Thanks [/quote]corresponding to
Fa , Fb, Fc

acceleration direction  <----

to A   

12N<--- A -->Fa     (12-Fa = ma)

to B

Fa <----B ---> Fb    (Fa - Fb = ma)

to C

Fb <--- C      (Fb =ma)

mj19592009 2012-7-18 03:53 PM

Sorry,第三題唔係好明,

點解會有 F, F1, F2 三個力 ?

Zzlaz 2012-7-18 03:57 PM

[quote]原帖由 [i]mj19592009[/i] 於 2012-7-18 03:53 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=336032283&ptid=20009187][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
Sorry,第三題唔係好明,

點解會有 F, F1, F2 三個力 ? [/quote]
你當好似有條線係兩個OBJECT 中間連住.

這是internal force ma, if u consider the object as a whole.

mj19592009 2012-7-18 03:59 PM

[quote]原帖由 [i]Zzlaz[/i] 於 2012-7-18 03:57 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=336032682&ptid=20009187][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]

你當好似有條線係兩個OBJECT 中間連住.

這是internal force ma, if u consider the object as a whole. [/quote]

咁都係得兩個力姐 ...

mj19592009 2012-7-18 04:02 PM

仲有,#3 果度有兩題,唔係同一條數.

Zzlaz 2012-7-18 04:04 PM

[quote]原帖由 [i]mj19592009[/i] 於 2012-7-18 04:02 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=336033125&ptid=20009187][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]
仲有,#3 果度有兩題,唔係同一條數. [/quote]
答哂你~ 上面係上面(F), 下面係下面.(F1,F2)

只係引用果時唔會顯示成個POST

mj19592009 2012-7-18 04:07 PM

[quote]原帖由 [i]Zzlaz[/i] 於 2012-7-18 04:04 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=336033362&ptid=20009187][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]

答哂你~ 上面係上面(F), 下面係下面.(F1,F2)

只係引用果時唔會顯示成個POST [/quote]

但係第四題 A B C blocks 果題,

佢靜係俾個 F ,唔係 12N 喎

Zzlaz 2012-7-18 04:14 PM

[quote]原帖由 [i]mj19592009[/i] 於 2012-7-18 04:07 PM 發表 [url=http://www.discuss.com.hk/redirect.php?goto=findpost&pid=336033573&ptid=20009187][img]http://www.discuss.com.hk/images/common/back.gif[/img][/url]


但係第四題 A B C blocks 果題,

佢靜係俾個 F ,唔係 12N 喎 [/quote]

Change it to F~

[size=12px]F- F1 = ma[/size]
[size=12px]F1- F2 = ma[/size]
[size=12px]F2 = ma [/size]

[size=12px]then, F = 3ma , a = F/3m [/size]


[size=12px](easier way, consider the objects as a whole, F= 3ma, a =F/3m)[/size]


[size=12px]on A = ma = F/3[/size]
[size=12px]on B = ma[/size]
[size=12px]on C = ma...[/size]
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