頁四第張六第 日九十月九年子壬瑟毁
WAH-KIU YAT PO
郭日橘茶
三期星
日五十月十年二七九一屦公年一十六國民華中
育教僑華
1973英文中學會考試題預習專欄
1973英文中學會考試題預習專欄。
WIN AV P 12 t W
化學科
v) three (or more) vectors
Method T. Repetition of parallelogram of vectors
1973
中文中學會考試題預習專欄
英文科(一)
ENGLISH (1)
余有祿
CHEMISTRY
Preface
This is written for students preparing for the Hong Kong Certi ficate of Education Examination and help them to know how to revise. and how to answer the questions.
Multiple choice items and Convensonal questions will be set to cover the whole syllabuses.
ine of matte
The struct
Atom and molecule
An atom is the smallest parti cle of an element which can take part in a chemical change.
The atomic weight of an element is the radio of the weight of its atom to one- twelfth of the weight of an atom of carbon.
Atomic weight
weight of 1 atom of el
-12 x weight of 1 atom of
carbon
A molecule is the smallest particle of an element or con- pound which can exist in a free 'state.
The molecular weight of an -element or compound is the number
of times one molecule of its 29 heavy as one-twelfth of an atom of carbon
Molecular weight-
weight of 1 molecule of substand
x weight of 1 atom of carbon
Chemical and physical changes
Chemical changes: New substances. with different properties are produced, There is usually evolu- tion of heat (sometime absorption of heat, and sometimes(as in Combustion) of light; the change is not easily reversed by mere change of external conditions w➡ less the system is in a state of equilibrium, the weight of a given material usually changes when it : is convented into the new substance
Examples: (1) when en aron pen-knife is immersed in blue copper sulphate solution the iron Comes coated with a layer of red
copper.
Cu So
Te SO + Cu
YAM K NESTE NA
物理科
PHYSICS (1)
Scalars and Vectors
Physical quantities are classified either as scalars or vectors
A scalar quantity is one which has magnitude by no direction.
Examples: time
speed Volume
area
density mass
A vector quantity is one which possesses both magnitude and di rection.
Examples: velocity
acceleration
force weight
Representation of Vectors
Vectors are often represented by straight lines drawn to scale with the length of the line re- presenting the magnitude, the direction and sense are indicated. by the arrow-head. Vectors Addition
Concurrent vectors:
14 unita
B (3 units)
(1)
same directiur
A + B = 7 units
̈A + B =
ii) opposite in direction
jinit
That is, they are added alge- braically.
But this is not true when the vectors are at angles. (iii) at right
The resultant R
3+ 4 The resultant vector is measured by the length of the diagonal of the rectangle, or calculated by Pythagonous. Theorem as
at any
9.
Method II Polygon of
vectors.
The sides of the polygon are drawn parallel to each of these vectors and their leng ths must be in scale, However
the order of which to be dram first is of no importance. So far as each of them is use once and the arrow-head of one touches the tail of the other.
The line that clases the paly gon is the resultant
The parallelogram of forces and the triangle of forces are modified from the above
Components of Vectors
Any one vector may be resolved into thousands of pairs of com- ponents. Only the rectangular components are useful. They are the horizontal components b. the vertical components
If A is the vector, its horizontal component vertical component
A cose- A sine
When more than three vectors act at a point, it is always advisable to resolved them into x- and y- Components. These components are on the same directions. They can be added algebraically as:
and
Ax
The resultant s
Beginning from to-day's issue, our paper will provide the readers, in this column every Wednesday, with some exercises on English, aimed at helping those who are preparing for the Hong Kong Certificate of Examination (Chinese in 1973.
A. Rewrite the sentences without changing the meaning, according to forme asked.
Example: Kary promised that she would'
not tell anyone my secret. (Use to tell")
nary promised not to all anyone my secret;
"Mary, will you come to see me to-morrow?" John asked. (Use. Hif"
John's father said that he should
alk too much, (Uao "to talk")
Jack was crossing the road. knocked him down. (Combine beginning with "orossing"}
1 car has many (Use "consist
John was ill, He did not go to school. (Use "because of")
I hope I can see you soon,
(Use "look forward")
They worked as long as
(una"until)
8. They could not complete the work in
time
(Use "suoceed")
(11) Men the reactive metal magnesium is heated in air it burns with a blinding light, leaving a white powder (magnesium oxide). This is quite unlike the beheviour of platinum on heating.
2Mg + 0,
2MgO
Physical changes: No new substance is produced; if anything more than mere haating or cooling is involued, it is usually merely a change of state (melting, evapo- fation); apart from latent heat. absorbed or evolued in change of
state, and heat supplied to or removed from a body, there is no marked evolution or absonption of heat? the change is easily reversed by appropriate alteration in the external conditions; the weight of the material remains unchanged.
Examples: (1) Candle wax contained in a baker is heated. Sulphur con- tained in test-tube is heated.
(iii) making a piece of iron into an eletromagnet.
Elements mixtures compounds
An element is a substɛnce which cannot, by any mown chemical process, be split up into two or no more simpler substances,
A Compound is a substance. made up of two or more elements chemin cally combined together.
R is measured by the length of the diagonal of of the parallelogram drawn from the point they inter- sected. It may also be calpulated by
RAB +2ABCos
Parallelogram of Vectors: If two vectors acting at á point are represented both in magnitude and direction by the adjacent sides of a parallelogram, their reslt ant will be represented both in magnitude and dir- ection by the diagonal of the parallelogram drawn from the point,
A mixture may consist or any number of elements or compounds, or elements and compounds, mixed together, and combined.
not chemically
Distinction between Mixtures and compounds
Mixtures
(1)
The component parts may be present in any proportions
(2) Have variable properties depending on composition, but closely related to the individual properties of the component parts
Exercise for this week
1. Use the method of rectangular
resolution to find the result- ant of the following set of forces and the angle it makes with the horizontal: 200 kgf along the x-axis to- wards the right; 300 kgf at 60° above the x-axis to the right; 100 kgf at 45° above the x-axis to the left; and 200 kgf vertically downwards. A 5-kg weight suspended from
3 m cord is pulled to one- side by a horizontal force F so that the cord makes an an- gle of 30o with the vertical. What is the value of the force.
and what is the tension of the, cord?
(3) There is no heat change dur ing formation.
(4) The component parts are e easily separated by methods
which do not involve any chemical changes.
Compennäя
(1) Formed Trom elements by chemical combination in fixed proportions only,
- (2) Have-fixed properties quite unlike those of the component elements.
10.
She saw her clasenate She was very pleased.
the dance
did not let him play the (Use "stopped")
rite a Letter to your school. principal to explain why you could
attend scheež last week, explaining that your parents are at present on holiday abroad.
(3) There is often a heat change during formation,
(4) It may involve a complicated chemical change f to separate the component elements.
Dalton's Atomic Theory:
The main points of Dalton's Atomic Theory were:
(1) Elements are made up of atoms which are indestructible.
(2) Al1 atoms of the same element have the same weight,
, and are similar in size and shape.
(3) Compounds atoms are formed by the union of elementary atoms in simple proportions,
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