三期星 日十月五年二七九一层公年一十六國民華中資教儒華
頁二第張六第 日七廿月三年子壬罄夏
WAH KIU YAT PO
報日僑
ing horizontal forces balanced
Pece
(4)
1972
Considering vertical forces balanced:
英文科 (廿八)
中文中學會考試題預習專欄
許藩芳
+Prino
(2)
1912英文中學會考試題預習專欄
堅道英文邊院主編
Since the block is at the point of moving
F-limiting friction
物理科
(廿八)
From (1) & (3)
(4)
Physics (28)
from (2) & (4)
50. (a) Laws of friction
(i) Friction is the force which
axists between surfaces in contact. It always acts in a direction to oppose the faces from sliding over one another.
-BUT-
(ii) The magnitude of the friction
is, up to a certain limit,
exactly equal to the force which tends to produce motion.
(ii) The frictional force cannot
increase indefriately with the applied force. The maximum
amount is called the limiting friction.
(iv) The magnitude of the limiting
friction is a constant for a pair of surfaces. It is inde- pendent of the area of contact. but is directly proportional to the normal reaction: The constant of proportionality i is called the coeficient of static friction. It depends... on the nature of the surfaces of contact.
(v) When motion takes place, the
friction still opposes the motion. It is independent or the velocity and is propostional to the normal reaction, but the constant of proportionality ig slightly less than the coeffici- ent of static friction. This
onstant is the coefficient of dynamic or kinkle friction, (b) (1) When the applied force in zeru.
(1.8, no force is applied) friction = 0.
(ii) when the block 18 not moving,
the frictional force is exactly aqual to the applied force,
• friction Fg (Law (111)]
(111) when the block is at the point
of moving, the applied force is equal to the maximum or limiting friction. [Law (iii)
Friction
(iv)]
Limiting friction
(Normal reaction)
0.8 mg
(iv) When the block is moving, the
fuctional force still exists
& opposes the motion. Ita value is proportional to the normal reaction and the cons- tant of Proportionality is the coefficient of kinetic friction.
Friction - Normal read- tion)
0.5 mg.
(e) The acceleration of the block
is given by Newton's 2nd Law. of motion.
Unbalanced force masa x accelera-
tion
Unbalanced foros
Applied force
tion
WWW
-0.5 mg.
(d) There are altogether, four forces
acting on the block in Fig. (2)) VIZ (1) force of gravity (1.8.
ite weight, W)
(2) The normal reaction from
the ground, R.
(3) The applied force F.
The friction force F
51. (3) Resonance is the phenomenon which
occurs when a system is vilcating with its natural fregency as a result of impulses received from some vibrating sources.
The velocity of sound in air at room temperature can be obtained expenimertally by performing an experiment on the resonance tube,
ne resonance tube
water reserv
tubing
A turning fork of known Trequency
18 first get into vibration only strik- ing it on a soft pad and brought near the open end of the resmice tube as shown. The length of the air column le adjusted by raising and lowering the water reservior is obtain resonance. A very short length of air is used to started with to ensure resonance with the fundamental mode. The first reson- ince length is measured and racorded. The water reservoir is then further low-
red to obtain the second resonance length.
M
sonance length
sebong resonance
eza corraotion
length.
Second resonano
The following equations can then be written
be
ENGLISH (28)
model Answers (27)
(a) gravel (b) luminous. c) precious (d) petition
auction
f negotiation
refund rejected lease
penalty
re was almost blinded. The diamond was taken to the Collection Depot.
It would be sold by public. auction.
5. He could get 80%.
"He celebrated his wedding.
The Collector of Income Tax and the state government agent from the sales tax department.N YOUR
It was because nis petition had been rejected..
9. He advised Rogoal to see a lawyer with a view to t obtaining a refund of the, sales tax.
10. He hoped that ne would not
dig for trouble any more.
are you going; as praying: have seldom heard; gave
had intended: has just come: has made
should like; 1s raini
have lost.
would write; saw
were, would you do
would mind; came
an leaving; have all grown? gone feel; should like: was born
8. are
see you.
en oving, have ev
were having collapsed; was; took; have seen; is getting 10. Do you remember have vou
forgotten
Exercise 26
Re-write the following sentences as directed,
He left the town a few days Mago
(Use since")
All the lights in the city
went out because the electric
Remembering
273
new volocity 33
273
300
330
W280
330 $300
new wavelength
3.00 280
Up the end correction can be nagubed to be temperature-indendent.
from equation
330
300
0.002
(2)
x300/280.
new first resonance
length
330 #300.
300 280
0.202 m.
(2) - (1) *** >« 2(b) - Ip)
Hence tained, Knowing the frequency, f, the velocity of round in air at the temperature of the experi ment is give by Be
light plant failed.
(use "failure")
All small plant grows with
such force that it can
split a great stone.
(Use as to sulit"}
It was impossible for unem
to climb over the wall.
(Start with, they)
When we got home it was already 1 o'clock..
(Use "It was not")
As soon as the rain stopped the children went out to play.
(Use "than").
A child seldom tells a lie.
(Start with "Seldom")
A great wall surrounds the house.N
(Start with "There 15)
You need not answer at once.
(Use, "did not")
10. I do not fear this felloW
(Use "afraid")
Multiple Choice
1. I turned w nis proposal, which was so unfair to me. 1. up 2. iny
4. on 5. down
He prided himself. own success,
for
1) on 2. at 3. in
4. toward 5. for
Not a single egg
the beginning of this month.
1. is leid.
2.
had been laid
3. has been lain
4. has been laid
5. was laid
since
The art of printing is said
in China many years 220. to invent
to be invented
being invented
have been invented
5. to have been invented he never used to pay me a visit,
1. dia he 2. didn't he
3. would he 4. wouldn't he 5. won't he
This is wi animal
leopard,A
1 it in called.
2 is called ".
3. being called
4, to be called
called
He was accused.
fire to the house
1
Setting
for 2. because or 3. owing to 4. of 5 wit
(未完轉入第六張第三頁,
10cm.
mercury
(c) Referring to the previous notatio
0.273
0.823
3.00. H
Then - U.273
0.023
3007
C
0.002
Questions for this week, 521(a) State Archimedea! Frinciple
A piece or cork or volume 20 a 0. floats in water with of its volume unmersed. A piece of steel of 5.0 8.0 is tied to the piece. of cork, as shown, to make Just sink.
atmosphenie pressure
* 76cm of Hg.
A uniform tube open at both ends is bent into a symmetrical U-tube, Mercury of 5.0. 13.6 is poured into the tube, and when the mercury level is 10 cm from BOT.H ende, one end is sealed to trap a length or air. A liquid of unknown S.G. is poured into the open arm until it is completely filled. If the length of the air trapped decreases to 9.5 cm., find the S.C. of the liquid, (given that the pressure of the atmosphere is equivam lent to 76 cm
of mercury). 53.(a) What are the conditions which must
be satisfied by a systém of forces. to be in équilibrium.
(b) Using a board and a block or wood
as shown, describe how the coef- fiorent of static friction between the surfaces of contact can be. obtained experimentally. Give the necessary precautious.
block of Wood
(3)
Faine
From
)+(2) X= 2(0.623 - 0,273):
(0.823- x 0.273)/2
0,002
from (3) V = 300 x 1.1 g
¿nawera V330m
330 m
002
(a) If the temperature r1890
to 27°C, the velgeit
(c)
sound incr6a888.
teel
Water
(1) the masa
of the piece
of cork
of cork
(ii) the mass of the pleas of steel.
(iii) the tension
in the string (Deneity of water = 1 gm/0.9.
1000 Kgm
uniform Ladder of weight 20 kg and length to a is resting on a rough ground sgainst a smooth wall at an angle of tan (5/7) to the horizontal. A man of weight 50kg. ascends the ladder. If the coef- ficient of static friction between
cient the ground and the ladder is 0.9, find the greatest length along the ladder, which the man can reach with out causing the ladder, to alíp.