1970-05-30 — Page 14

育教儒頁二第張四第日六廿月四年戌庚曆 WAH KIU YAT PO

報日僑莘

六期星 日十三月节年〇七九一公年九十五國民華中

罗僑

曰くく

(iv) Weight of hydrogen inside the balloon

19500 x 0,1

1.95 kg.wt

$#&%$#$%$#$%635

(v) Weight of hydrogen remained in the cylinder

Hooke's Law states that "strain

英中會考物理科答案

-5-1.95

3.05 kg.Hip?

Weight of partly emptied cylinder

18+ 3.05

21.05 kg.ta

(vi) applying Boyle's Law,

(100) (500)

500P

500P + 19500

30500 305/5

61.

If the elastic limit of the spring is notę

exceeded, the law kelde the proportion. An increase of force (105-85) kg.wt. con-) presses the spring for 4.4/3 00:

Let z ha the original length of the spring,

105 20

strese"

堅道英文書院撰答

Suggested Answers

for

PHYSICS (Conventional)!

Hong Kong Cert. of Education Exa

(Solution)

“Generai PHYSICS

(*) Archimedes' Principle'

દ

When a body is wholly or partially immersed in a fluid it experiences an upthrüst squal to the weight of the fluid displaced.

When a piece ofwood or other material of den- sity less than that of water is placed in water it sinks until the weight of water dis- placed is just equal to its own weight. It then floats, we

The same case is true for any object that is less dense than the fluid that surrounds it. Thus, the Archimedes! Principle is modified- to the Law of Flotation -

A floating body dieplaces its own weight of the fluid in which it floats.

The brass weight is suspended from the hook of the balance and obtain the weight of the brass in air (W). It is then lowered into a beaker of water. It weighs apparently W It is again lowered into a beaker of liquid and its weight is then recorded,

W3.

Upthrust of water on the brass weight

(H1 = 2) 801 w

Wt. of water dispd.,

ཐབ་

The displaced water has the same volume no

the brass weight which is: (K,

Specific gravity of brass is (

Upthrust of liquid on the brass weight

weight of liquid dispd, (Ng - Ng) zm.wt.

The brass weight displaces the same volume of liquid as that of water.

Specific gravity of liquid =

(0) Weight of empty balloon - 2 kg.wt.

The cylinder holds 500 litres of hydrogen Pressure inside the cylinder = 100 atm. Wts of hydrogen and cylinder - 23 kg. wt. Density of hydrogen at 1 atq,- 0.1 gm. per If temperature is kept constant,

PV

1 atm. }

lass/Density

100 atm.:

The pressure of the nyarogen remaining in cylinder is 61 atmospheres.

(a) Mechanical Advantage

Another term for mechanical advantage is Foros Ratio.

It is derined as the ratio of the load of the machine to the effort,

Velocity Ratio

Load

It is defined as the distance moved by the effort to the distance moved by the load. distance moved by effort distance moved by load

Efficienc of a machine

(b).

Eri

Erf.

Outout work

Input work;

Load x dist, moved by load:

Effort X dist. moved by effort).

EXA

(H/E) X (7/8)

M.A/V-R.

The screw has to release AB on before the

pring falls out of the jawB.;

which is 2022

pitches or 205 turn

Section Ba

HEAT

(a) By means of converving Molenwiaz energy into} heat, the apparatus is used to perform an experiment and calculate a very rough value, for the mechanical equivalent of heat.

Before performing the experienant, the follow ing readings should be: observed;

Initial temperature of mercury

mass of mercury

Specific heat of mercury.

Thermal capacity of glass

Water equivalent of

Effective length of the tube

Local gravity

gm

hom.

•6 cm/sec2

The temperature t deg. C of the content is taken Immediately before the experiment atarts and the tube is then inverted 100 times or n times in order to obtain a measureble temperature rise.. The final temperature t deg C of the mercury is noted and a value for J. may now be found as fol Lows.

Heat producea - m

Heat warms up

spring

Work doen n

x (tt

calories

tube - T (t - t ) calories gxhxn-nghn ergs

Mechanical equivalent of heat work required to

WOTK in erro

heat in calories

imghn

produce 1 cal,

(mS+

(mS + T)(t

ergs/calorie.

joules/calories

Ir is small (very small) as compare with mS, the equation becomes

joules/calorie

100 x 0.1.

10 m par lau

(1) The screw vice consists of two parts I

the screw and the lever

For the part of the soTOM: Pitch of screw = 0.5.0 om. Velocity ratio of the screw

For he leveri

2TR

.p.

2 x 22 x 21 0.5 x 7

{-4 x 22 x 3

Velocity ratio of the lever

Velocity of the combination

30 x 22:

40

9x20.1

V.R.. eff.

(b) Two factors affecting the boiling point of a li

liquids

External pressure.

The boiling point of a substance is defined as the temperature at which its saturation vapour pressure becomes equal to the ex- ternal atmospheric pressure. If the the external pressure increases, the boiling paint will consequently be raised, that is boiling point of a liquid will be lowered under reduced pressure.

Purity.

Impurities in solution affects the boiling point. It will be raised accordingly with the impurities consists.

Nater equivalent of the kettle= 500 x 0.1

50 gm

Rate of heating ➡

( 450 + 50 ) x (100 −40)

(1) Weight of 500 litres of hydrogen ay

ta 5000 gm or 5 kg.wt.

(11) Height of empty cylinder - 23 – 5 Kg.wt.

18 kg wte

When the balloon and its loading 10: JUST T start rising, the upthrust should be

(2 +5 +18) Kg.wt,

By Archimedies Principle, the upthrust equals to the weight of fluid it displaces, Therefore, the weight or air displaced

volume of air diapo, I density

of air. 25000 gm.wt.

Mechanius) Advantage of the combination.

99 x 2050.

45.

Tolume of air displaced

25,000

500 x 60

Therefore, a force of 1 kg/wt. szarted at the end of the handle will introduce a force of 45 kg.wt, at the jaw.

20000 litres

(11) Whe handle turns 2 revolution and the screw

will advance for two pitches or 1 cm.

By the similar triangles shown, an advance of 1 cm on the screw will press the screw jaw for 44/30 om.

5,000 cal/min

To heat (evaporate) all water away, it

540 x 450 calories

Time taken to bail away the water

540 x 450

40.5 min 6000

Volume of the cylinder itself is 500 Litres (iii)Volume of the balloon - 20000 – 500

19500 litres