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物理科
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PHYSICS (1)
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The author has been responsible for this column for more than three years. The
repetition of the contents has become apparently monotonous and dry to the readers and the writer himself. This year, objective type of question is introduced and this column will be produced in a new form. A fresh topic is discussed each Wednesday and examples are given immediately. Special attention is paid to the meanings of formulae, which are simply shorthand expressions batween physical concepts. These formulae are. illustrated by sample problems workad out in moma detall through logical approach and caraful analysis. The purpose is to help students in setting up and tackle the problema. It is not claimed that all the problems in this. column are original, many problems owe their origin to recent examinations of the HKCEF and GCE
VECTORS
The resolution of vaotora.
Law of consine becomes cumbersome if more than Swo vectors are to be added. The resolution method. cust be employed. Each vector is resolved into two components, rectangular components one in the horizontal direction and the other is the vertical direction.
Forces
·Fsine
Fisiner
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ECO英文中學會考試題預習專欄
化學科 (一)
QUESTIONS FOR NEXT WEEK
What do we mean by atomic weight, atomic number
and equivalent weight? Give (a) the slectrical charges on the ione formed in solution by elements of atomic number 16 and 20 respectively; (b) the electronic formula of the hydride of an element E of atomic number 17, what type of compound is it?
A solution of 0.125 gm. of an alum
K2SO •M2 (90) ̧•24H, 0, gave 0.117 gm, of barium
(412)
sulphate on precipitation by excess barium chloride solution. Calculate the atomic weight or]
the element M
VECTORS' AND SCALARS
Quantities in Physics which require a specification of direction as well as magnitude aza called vector quantities or vectors. In contrast to vestora, a scalef quantity does not require a specification of direction. It has magnitude only
REPRESENTATION OF VECTORS
Vaotor quantities can be represented by directed arrows. The length of the arrow representa to scals the magnitude or the amount of the quantity; the direction of the arrow, measures an angle with respect to a given direction, represents the direction of the quantity.
RESULTANT OF VECTORS
Areator which can replace two or more vectors that act simultaneously at a point and can producs the same effect as they do is called the resultant veotor, or simply the resultant..
ADDITION OF VECTORS
Unlike scalar quantities, vectors cannot add algebraically. Within the precision of measurement. by rulers and protractors, vaotora ban be added graphically by:
(1) Triangle of vectors,
(2) Farallelogram of vectors, or
(3) Polygon of vectors.
For two vectors acting simultaneausly at a point!]
(A) in same direction
algebraically;
they can be added
(B) in opposite direction
other.
they subtrzot sach
of
F. COBO
-components of forces,
008-2009
Bin 0, + Fine-b
_)2 + (_)2
2 - √(3)2 + (8)2
The angle which the resultant makes with the horizonta
Z
Vootor aspect of forcas
ΣΕ
800
Foroe has a directional characteristic. It is treated as a vector, Problems on for ose are solvad. either graphically or by resolution method. It is clear that the 'study of forces becomes a geometry and trigonometry apply, since arithmetic and algebra are insufficient to deal with vector quantities.
Example 1
Determine graphically the magnitude and the direction of the resultant of a 40 lb-wt force moting east, a 40 lb.-et force acting south, a 60 1b-wt foros acting southwest, and an 30 lb-wt forcs] moting northwest
Denoribe an industrial process whereby sodium bicarbonate and sodium carbonate are manufactured. Indicate any important physical principle involved,
2.34 gm of a mixture of washing soda and baking soda required 30 ml of 0.7 N hydrochloric acid. for complete neutralisation. What are the percentage composition of each of the salts in the mixture?
Describe the preparation of hydrogen sulphide in * state of purity, State and explain the resotiona of hydrogen sulphide with (a) sino sulphate solution, (b) dilute nitric acid, (o) concentrated ! sulphuric acid, (d) bromine. Mention the principles involved in these reactions
What is a homologous series? Uss paraffine, olefins, alcohols and aoida to illustrate your interpretation.
An organic acid on analysis is found to have the pezosatages of composition of C - 57.70, H-3,71, 0.38.59. The silver salt of the acid gave 56.8% of silver on ignition and the vapour density of the ethyl ester was found to be 111. What are the molecular formula and the basicity of the acid?
(NOTE: Provide any physical constants that you "consider relevent in your calculatione)"
(c) at an angle cannot be added algebraically, The resultant is determined graphically by Parallelogram method as follows.
(1) Draw two directed arrows in scale to the
vectors quantities..
(2) They should be in the same direction as
specified, and adjacent to each other.
(3) Complete the parallelogram of the ventoza. (4) Draw the diagonal from the point of
intersection.
34 56
Measure tha length of the diagonal. This diagonal represents the resultant both in mangitude and direction.
SCAL
Bena
16.
ib.wt
Salbut
4010.WT.
W
68
the Law of Cosin com
32 + 42 + 2(3)(4) oom 120°
9+
16 +24 (-0.2)
12
13.
3.6
4 min 60°
0.962 74°
For several concurrent vectora,
The triangle method is utilized to find the resultant of three concurrent vectors and the polygon method is for the resultant of more than three concurrent vectors. In these methode, the firat vector is laid off from the original point, using a suitable scale. The second vector is then laid off from the head of the first vactor. The third is from the head of the second, and so on for all the vectors head to tail. The resultant. is that vector which, starting from the original point, closes the polygon.
The law of Cosines
The resultant of two concurrent vectors at an angle can be calculated by the cosine formula.
+2AB cos →
نا
Example
Vector 4 of 3 units acts at an angle of 60 degrees with vector B of 4 units. Find the magnitude and direction of AB and B - A
For A B
The B vector points in the opposite direction an vector B, making an angle between A and -B equal. :120 dggraeni
(For
unita
acting down towards the right at
from the horizontal
-Aveotor points in opposite direction of M
B makes 120° with -▲ also. This will be
symmetrical with A- B. The magnitude as also 5.6 units. It direotion is 74 degrees up from the horisontal.
Exercise
wha on
A body 10 acted on by two forces, a und make an angle of 90 degrees, with such other. The magnitude of a 10 20 lb.wt. and that of B is 30 lb.wt. What is the magnitude and the direction of the resultant?..
ib. block is pushed along a horizontal
by a stick which makes an angle of 60 Msurface
degraan with the horisontal. Assuming the fores of the stick acts along the dirsation of the stick and amounts to 2 lb.ut., what is the resultaat vertical foron acting on the block?
3. Three forces A, B, and C not ok a given point".
and make angles of 120 degrees with one another.] If the magnitude of each foros is 10 newtons, what is the magnitude of the resultant foros?
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