# 查看完整版本 : CE Physics - Force

mj19592009 2012-7-17 10:50 PM

## CE Physics - Force

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

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

W係 acted on man by the earth，

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

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 ?

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

mj19592009 2012-7-17 10:59 PM

[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.

Thanks

mj19592009 2012-7-17 11:12 PM

[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 ?

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]

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]

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]

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 ?

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 ?

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]

[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]

[[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]

[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

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

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，

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，

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]

mj19592009 2012-7-18 04:02 PM

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]

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]

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]

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]