算四第張七第一日九十月一十年寅丙居夏
1987中學會考 試題預習專欄
物理(十三)
MILL & DALE PRESS
Revision Exercises
The for:
Cet.Exam(1987)
PHYSICS (13)
4,6 Moment and Parallel
forces
(A) Moment:
. The moment for torque) of a force about a point (or sa azia) is the product. of the force and the perpendicular distance of its line of antion from the point for the sxia),
It clockwise moment. is taken as positive, then anticlockwise moment is taken as negative, and vise veras. And the
resultant moment can Tbe found by adding
moments algebricully.
Examp
Moment
about 0-0 Moment of Fa
about 0.
Moment of F, about 0-
Resultant noneut
(B) Parallel Forces
(a)Resultant of like
parallel forces
R-F
The position of x can be calculated ea follows
Moment ofg about: O-moment of P about 0:
4moment of F, about 0 O-FxAOsinĐ-2* Busine
Note: R is always nearer
to the larger force of the two; fac (b)Benultant of.
parallel
forces
RF
WAH KIU YAT PO $1.81% =
The position of 0 cán be found as followa Moment of E about
Dmoment of F, about 0
+noment of Fg about '0 O=F」xAOsinO-F,80sing.
F1*40' FQX80;
Note: R is always on the
outside of the line Joining the forces and nearer to the larger force of the tvo..
(C) Couple
A couple is formed by two equal parallel forces in opposite direction. A couple cannot replaced by single resultant force,. it produces rotational motion but no transla- tional motion. The turning effect of a couple, depends on the megnitude of the moment of the couple. Moment of a couple (about ANY point:in) the plant of the forces mmagnitude at one (of
the forces): -perpendicular
distance between the forces
Example 1:
A light, rod subjected
to four forces is shown
Pin the diagram.
(a) Is it possible to
replace the forces by a single force?
(b) Find the resultant
moment..
Solutions
(a) No, it is not possible
for the forces produce. turning effect only, (b) Take moment about A.
The resultant moment = (4) (1)~(5)(2)+(6)(3).
(Ane.)
the resultant moment about any point (say, a,) is 5Nm clockwisely.
Example 2
In the figure, a wheel of masa 100kg and 10 m in diameter rests on a horizontal surface.
(a) What horizontal force
applied at the axis would be necessary to enable it to surmount a step of Am? (b) What is the direction
and magnitude of the minimum force exerted.
to the wheel so that the wheel will be
pulled up the step?
Solution:
(a)
(D)
100g
QA=5m; OB=5~4~1 19. AB/5--1-
-1899 u
Tuke moment ahous盆 ∙Fx0B = (100) (g)XAB
| Px(1)= 100(10)x4-899-
4899N
The force required is" 4.899N
100g
地
(AUB.)
The minimum force Fi required should be tangent to the wheel as shown in the diagram.
Solution:
160g
六期星
(a) Since the ladder is 'in equilibrium, therefore.
the resultant moment
about A is zero.
(160g}x3=X; x8=0°
GOON
the normal
(
reaction; at the
apper end 19600N
2:0
N600N.
the frictional force, in 600N,
(c) Zyy =
-160g
0
Again, take moment
about
D
FAC = (100) (g)×AB FX10 (100)(10) x.
(4.899)
tanê
F 489.9N (Ans.)
Equilibrium of a body under.coplanar forces:
If a body美食购 equilibrium, it follows that (1)The resultant force
on the body is zero or the sum of their
components along any direction is
zero.
ZFX20 and £Fy=0}
(2) The resultant
moment of the foreci about any point (üraxis) is zero.
(Note: This shows the body will have no retational acceleration).
Note: Also, we recall that when there are... three forces acting -ợn a body and being in equilibrium, they must be concurrent,-Hence we.may apply the principle of triangle of forces to solve the problem (see Concurrent Forces)
Example 1:
A uniform Ladder of mass 160 kg, and length. 10 m is placed with its upper end against a smooth vertical wall. If the lower end of the ladder is 6 from the foot of the wall, find... “
(a) the normal reaction
acting on the upper end of the ladder, (b) the frictional force
between the ladder and the horizontal floor, and girl (c) the resultant force acting on the lower
end of the ladder,
的兄門,繞指,將會的
您,夜教家出 進口
據媒作當導腥,撒行
壓調問間
160g-1600N.
1600+16000
1708.8 N
1600 :600 -2.6667
(AND)
The resultant at the foot of the ladder fa 1708, 8N making 69027* with thei horizontal
APPENDIX:
ledr.
Of Gravity and Stability-
The centre of gravity of a rigid body is the point of application of the resultant force duo to gravitational force on it, or the point where the total weight appears to
For a system of coplaber weights, the position of the c. given by
mixi
Smiyi
and
y
Smi
| Stability.
A body which is in equilibrium can be in a state of stable. equilibrium, unstable equilibrium or neutral equilibrium, when the body in: diaplaced. slightly....
(a) Stable equilibrium; it returna to its original position. (i,e, its cuguia raised)
(b) Unatable equilibrium:
It moves farther a
ia lovered)
(c) Neutral equilibrium?
it remains stahle "in] its displaced position (ive. Its ©.g.is/ neither raised nor
Lowered)
a)stable
日十二月二十年六八九一年五十七國民華中 邨新門屯育教
(c)neutral
equilibrium
經濟(十三)
·MILE & DALE PRESS.
Revision Exercises
Befor
Cert Exam. (1987)
ECONOMICS (13)
MARKETS
1. Perfect Competition.
Perfect competition cxists in a thesretil- al:environment.in which the followings conditions are fulfilled:
(a)large number of
buyers and sellers -
With' a' large nuüber of bagers and sellers the transaction made by any buyers and sellers will be very small. This means that each of them cannot. exent his personal influence on the rket. price of the product by increasing or decreasing its purchase or output. Thus in perfect A competition firms must take the price at which they are to sell as fixed and the
demand for the product: of an individual firm ..is.perfectly, elnatic.
(A)Komogeneous st
product
Under a perfectly competitive market situation, the product of: any one:aeller 相机相 be identical with the product of any other aeller. This means that buyers do not have preference over the product of que firm 10 another and if a fir raised its pricey its sales would fall to zeromete
(c)free: mobility of
TeBource
-Factor of productioni aust be perfectly mobile. (d)perfect: knowledge.of
the market
All buyers and sellers rinst`posse68- full information of the commodity if the market is to be a perfectly competitive
(e)free entry by new
firms
There should not be any restriction for the e, autry of new.
fimis into the market
(b)unstable. II. Imperfect competitïun
equilibrina equilibrium
法此形
In real life
的近了然
非生
肉嚴等
conditions of perfect
competition cannot
always be met.
Whenever one or more
of the above, require- ments are not satis- fied, the market is. described as an inperfect one, Imperfect competition can be classified into three types according to its degree of imperfection. They
are monopolistic
competition, oligopo And monopoly. (a)monopolistic
competition
... This has three
fundamental characteristics: (1)there is a suffi-
ciently large number of sellers so that
each can act independently of the other firms. (2)products from various
sellers are "differentiated. (3)market entry of new firms is relatively
In monopolistic competition, individual sellers can have influence over the market price. This is because the product. of one firm is not regarded as a perfect substitute of another
An example of monopolistic competition is to be found in the retail. trade. Differentiation may take the form of
(for instance, one shop being nore conveniently situated for some people), the general attitude of the shop-keeper, and the extent of the.. service available.
Actually, there are
differentiation such as actual physical difference, location, service consideration reliability, personal attitude, etc. In addition, price may affect the choice" of products since many buyers prefer these
ellers who, offer a; lower price. Since differentiation of product is essentially created by: branding and packaging, Marketing activities play a major role in monopolistic. competition în attracting customers.
(b)oligopoly
This is a form of imperfect competition in which there are. only a few producers as compared to perfect, competition where there is a furge number of producers. Moreover, entry of
new firms is difficult.
A general feature of
oligopoly is that
the price policy ofval]
firm is influenced by. that of its
competitors. This can be subdivided into perfect and imperfect oligopoly.
(1)perfect oligopoly
In perfect oligopoly, the commodity is homo- geneous and consumers- have no perference for: the product of one firm as compared with nother. A price eut by one firm will lead
by other producers.. Hexice; the largest firm will usually become the price. leader.
(2)imperfect oligopoly
In this situation, some degree of differentiation exists between the products of different firms. Even a slight differentiation in the product will make one product not a perfect substitute for`. anothers Under these conditions, competition tends to
firms may alloy wa赌场
cutting strategies.
(c)monopoly
In the strictest meaning of the teru,. monopoly occurs only when there is one producer of a commodity for which there is ito...... substitute, a situation often referred to as absolute monopoly. This is quite rare in rent life to have only. a single producer of a product and even rarer to have no substitute. for the product et all Thus the tera wonopoly is often used. to describe forms of very imperfect competition as in the following sïtations: (1)when one firmoru::
group of fire working together cunt cal a large proportion of the total supply of commodity. For
instance, most of the world's supply of rubber comes frocit Malayal
(2)when it is wasteful to
duplicate a given. service as in the case of public utilities, (3)patent r
patent is a grant. to the inventor of a new ngeline or idea, giving him a monopoly ofitsuan fore certain period. )others a producers monopoly In the home: market may be protected from foreign competition by means
of a tariff ong
importa, thus giving rise to a monopoly situation.
查調出作别分會方官與員議區
屯門元朗新邨
泊停法非車貨門屯
異差現出果結查調
|查調次一另行進手携劃計方雙
個車
扣一近安員
示:
車
毀天生表示,屯門區內業 貨車停油於屋邨附近,現時,以 - 每當晚上返囘家財,便連帶將 車司機為生的公屋居多,因此一 蝴蝶邨及友愛邨泳池附近較嚴重
議,雙方合作進行另一次調查, 果不符,有關當局正考慮嚴氏提 所得結果顯示貨車與旅遊巴士停 消位需求,與有關當焜之調查結
由屯門區議員嚴天生完成的 區內非法停泊資車調查報告,其
版
出刊六期星逢
的
位於屯門政府合署的私家車台法泊車位有餘,却苦了貴車司機。
八查
拓展處的統計,區内共有一千一 個,其餘是無私人公司提供的泊 劃中的貨車停诂位是七百四十三 百六十九個貨車停泊車位,而計
一百五十及七百四十七部,而據 近期徒珒非法停泊数目 , 只有一
嚴氏表示,當局成立的「屯區的公民 麥,而所得繕果,則財示屋邨邊圍邨巡師舉行「反高舉行,「綜合性的宜。 員有地政處、警方及政務處等代一月初,在元朗邨及水下午三時元朗3籃球場八字。 門設粱停車場,工作研究小組成另外,十二月底至年5月三日(星期六)六二三或Q———七九四 將報告進交當局。湾,年入成爲有责任愛爾社 「好好鄰居天才大 年服務中心職員聯絡, 示,晚間有二千多部車及旅遊 物少年填色比賽,,從而提高居民對這問有興趣參加以上签 據联氏在年初所做的調查緻食對象而設的「反高空 空姉粉圓片資料展覽]傳節注。. *匯演[將會在「九八七 查詢電話 —— 七六四
車位及將來提供的車位共四百餘
人口增加消費力强市場吸納量大
屯門商業單位供應
預料明年大量増加
法停泊的情况亦是嚴重。 防止虐待兒童會在屋邨發問卷
與大街等。而現時合法貸 停泊地點是三聖邨及舊屯門碼頭。. 此外,私人屋邨又設立屋邨巴士按近居民上落級,而旅遊巴士在區內非 數字顯示, 一形式,發展至學童分派到不同的屋邨,因此,旅遊巴的需求相繼培訓, 酵强饰象。王救世蝴蝶督青少至十時牌。 茲局的調查則只以公共屋邨在調查範園,故此,兩者之間的報告結果出市區將有十大萬平平呎人口。 嚴氏指出,現時合法的停車地點,只設在三聖邨及舊屯門碼頭等。舖供應不斷增加,由八地方,建立社羣關係,晚餐,抽獎等。
·現差異。他希望當局他與他合作進行台一次調查,以僚解真實情來,望的商舖面積供應,與此蝴蝶邨青年閣 七日(星期六),
可
急增至大千三百五十八 天同溼的聖誕期間〔十 了牛肉干,更受不法份子勒素,收取停泊費,此外,他反指出,風量的瑪的投資是信心的反映 ( 远非法停泊踩車及旅遊工数量不少,以至讓璡徒有機可乘,作其行劫,預料商漣供應在明年洞,而這批公屋及私人二月二十七日)舉行開 黃龍,據嚴氏稱,他本人之調查是包括私人屋邨公共屋邨及私人樓宇,而統計調查顯示,八七年 成,由於會引入大量的開幕活動。舞會乎成了 當另外,營民表示,屯門區內學校分佈時,由初期的塞印目始足足的同時,市場吸約量亦有辦聖艇餐舞會發熱時間,晚上八時
用,但仍感到不足,該等街道是華街、新安街,洪麻路、青楊路及三彩消費力亦在不斷膨顺,私人樓宇獲批動工興炮 连價飲品,供青年五 一,增幅甚遠一點三倍,
一年的四萬八千平方米立
戴氏認陽,由於停泊車位不足,司機迫於非法泊車,不單經常收到力的新拓展地,屯門從千七百二十七個單位,「青年閣」正莚告 、嚴氏指出:當局雖然同意開設工廠區內幾條街作晚間貸車的停泊之一,屯門人口急劇增加 千一百三十六個單位,「青年閣茶座」,挺供 八十年代以來,屯門 熱評間有聚集
被商界認為是有生產後的數量亦從八四年的一談心。 會大量增加,亦據新的 根宇,將於明年陸續落幕,以聖誕餐舞會作爲 4 生活知與體驗, 地至十一萬二千平方米特別於中心設立「青西迎區內青年參加, 收費:每位三十元
日期:十二月二十
出青少年中心 內容:控會、自
地點:救世軍蝴蝶
調查兒童生活情况
會屯門區分會,在區內 的情况,防止虐待兒童五一一二三聯絡。 另外,會茆利署。 長關心子女及兒童生活 李座,可能三——七五 屯門區分會,在友愛邨 鋪的吸納量亦顯著提高 爲了解區內一般家兒童資料,及有意舉辦已答懋防止虐待兒童會. 統計數字反映出商
·定咩梭發出單位作爲合,由八一年的二萬五千
恢復上揚。. 放綏,但八七年以後將 年商舖供應是進入一個渴、電殿、飛鏢等設施 查詢電話 ——八七七 。於星期五、六並設有三〇〇... 1 青年之資訊傳遞,及電與青年閣之各項活動。
攤位設體育路
平方米調高至八五年的一
作爲日後工作指標
元朗年宵市塲
一月廿二至廿九擺賣。
撲問卷調查ㄆ野人,元朗區蓖机有額高達一點四倍,而對於職書給於 六萬七千平方米,期間
况,作爲日後展開工作
【進行,預算發出的問卷。.....
辦展覽及綜合表演
,但隨着人口增多,預料
未來兩年需求將告急
面, 泳於市通區
預區
原料
位即對朗地本市,
百年二觀趙面體貼 ̇年政逕
度日年投誰之有,度局有雖
至 管亦儁行 路該的,個然
1.市包 中人 元應 元已多距
工場舉 學專 ·朗點 朗於月離
公將 年日 用
將行 使
由。,逍共設衖訊但
·
堀過城 水·池元
求在 會响華朗
路體設滋
,門行此等有有至 為力
駛約 ·路將正:人外。路 起 使公 聯
入特 政會·如專
·卸低·交司 趁燈
都按往蓋用為馬貨限 通安香光
物度所排 宜
地屑之夷由全包
- 祗裝飲品摊位兩 反新界路政部要求,在
希 丝有關當局參攷。
·士乘客安全有一些意見:現向公眾發表, 。此外,不幸悲劇發生後,我們怎操汲取教程度的。 ,或有關政府部門及教育 嚴莖,危宮市民的安全,將會包括三大項目,八四年港府未有批江公 米乘四米。 民發生的巴士大車禍事件,提出對怎樣保障巴 公衆的意見,俾使巴士公司及政府當局铹敦:確的實驗的話,也應在分析時,計算其誤差 藝界人士,對於防止虐待;更損毁市容,因此而以元朗區各小學學生 陸,但八五年郑批出三區域市政署將提供國內1 福:區內有關人士若有動,乃法高超的人會上亦難位共十
·意協助是次問卷調查, 空諾物的情况,越來越高空出物運動內容 樓宇的墛刘最爲明顯, 個,每個攤位面積爲三 枱位租約特别明了 公屋居民約有七成,因服務中心於本月至明年 品,以至能改善居民黃 雖然八四年間港府在屯 年宵市場將由一月康路等。 王. 對巴士乘客安全的主要影响因妳有四方 卓禍事件育例,我們認爲,調查的過程必須 事件的調查報告,不但是市民的知情權利, 面...車輛、乘客、路面反琡愛我們認爲使用,客觀準確,才能找出席發真相。我們並非交 對傷難者的交代,更重要的是,透過調查公 將的嚴格審核;運作中的足夠監奈;出事後 通專家,但就憑常識,也覺得車禍發生後戲 開的做法,各界關注團體及各界人士可互相 別,總結經驗;有盐重要課題。以最近屯門 編輯先生:我們聯署團體,對最近屯門 區議會可獲多些資料參攷,而更全面地反映:我們認爲,當局若不能再設法進行一次較犁 *良,採用抽樣調查方式 共屋邨反高空擲物運動|勸發居民不可從高處囝 數量急跌,但八五年邦本年度的年宵市堀攤位球入行人專用道而損壞 此是次满畫將集中在公一月向聯合舉行元朗公室撼物的不正藤意識,門批出的住宅模宇興建廿二日至廿九日驚貫,將領此外,為避免車輛 據悉,屯門區現時差會基督教元朗青少年 望提鄫居民關注這種幣 表的統計數字亦顯示,位競投亦已舉行。在體育路,馬田路及安 便拋擲物件落街,以改 大樱咸升,顯示未來雨 數目,濕貨攤位共一百通道門:葉蓋欄杆及通 同時,有關的調查報告必須公開。公開
|止汽車駛入年安市場範
的情縻。但這年來,依然不斷: 能;司機的訓練及平日的操作時, 巴士出廠前的標準,使用中的效 套頗為複雜的審核程序如:監管 “雖然政府奥巴士公司均有一
團體就巴士
大車禍
可行的改善方案。 交流,溝通意見,從而找出實際
呼籲加强監察制度
·白白筱性。我們呼縮767
使不幸者的生命不會
;但若果我們能吸取教訓,總算
悲劇發生,大家都感到遺憾
有嚴重巴士意外發生,不禁使人懷疑,現存,日在現場進行的實地檢驗,並非不能改進的,政府一加强監察制度、公開調查
-的監察制度足夠嗎?有效嗎?.. 我們認爲,政府應考慮加强現存的監察 養,實質畫樂,員工運作等的進展報告, .交報告,皇交路綫發展計劃,應包括維修係
·化區議會的角色,透過要求巴士公司定期品 交通投訴組的監察功能;在地區方面,可强 制度;此外,政府可加强交通諮詢委員會及
當時是以沙包放在車廂中代替乘客;同
·報告」以防止類似的悲劇重演。
客觀地,試驗時的巴士便比較穩定得多。 時離心力所產生的姑威破壞作用使無從讀示 段距離;同時因沙包均被固定,巴士急轉爾 試驗時該事的重心與事發時實際懷况便有一 辦事處、朱偉彬、吳明欽區議員 時沙包均被固定於有關位置中;這樣一來,會,匯點屯門辦事處、蔡蔣月蘭 聯著團體 屯門關在農生傣
吳明欽辦事處。 楊笋光區議員辦事處、區域市政 辦事處、吳惠祖區議員辦事處、
效廣法刊明香三 本
刊告律登合港月九報力廣龐讀印柯必社必學必家必工
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