1970-01-28 — Page 22

頁二第張六第2 日一廿月二十年個已番覚 WAH KIU YAT PO

%

170英文中學會考試題預習專欄

堅道英文書院玉編

物理科

(十三)

PHYSICS (13)

MACHINES (cont.)

報日僑罩

Hore an advantage of metrio unite le found. Since the density of water is for all practical

purposes 1 gm/c.o., the numerical value of density

in metric units is the same as that of the spacific

gravity of a given substanos. This is not trus in

三期星 日八廿月一年〇七九一靨公年九十五國民華中 育僑犂

British system. Because, one, oubio ft. of water weighs

62.5 lb., it specifio gravity will not be, numerically the same, as that of density.

For problem purposes it should be realized that

the weight of a substance can be expressed asi

W = 0g - DVg (5.0.) DVE

where W is the weight in absolute unite

3.0. is the specific gravity of the

substance

Dia the density of water.

Vis the volume of the substance e is the acceleration of gravity.

A

一沖就讀,清明過

很不在地戮高之私立

夫家政憑投√體,

鮮雞

詞事九學。

不中彩

58.

MILPEAN-K

願

名举办中就之光生人 學鵡萊生配台,當屬些 IEC-NEE-PREKNS

政府 政府資助中興內二百七十個,花年大 出,將可飛得機會油入|增加資助小學學位11 杓有一宫四下省龙岩之一字,顯示在抛去年深 RENT*

PES • VELACH 中韦岸位叫百分之十五路;一九六七年只有一 [成中陽,位之供不應求中學位。一九六八年 揭搂政府分派股那有一一千五官叫什 一般衆。由於登妍升中號,大約有14三千四百 MENE RESTARTNER SEX<BNEREN | IR< • KENKIR 一敎宵可显取粱限制总机 升中装之生,超過 在外中國內取資助中生計,以黃糊口。 CERES-NE-EYRO (E)K ENT-KKEK** 萬名。

在過去三年來,

增加二千二百餘個 過去三年來資助中學學位僅

萬四千中學學位 五萬升中生競爭

(新新、勇

明日頒獎典禮

七七安装

a

point

must also

The

p at the surface,

actual pressure at such take into account the pressure such as atmospherio pressure.

- nag - Po

Atmospheric pressure

Example 12-2

A Bozow-jack bas a sorey of pitch in. turne by a handle having an arm 2 75. 10ng. If the efficiency of the screw-jack ja 40 per cent, what is the greatest load which can be raised on it by a man capable of exerting an effort of 50 lb.uk. on the end of the -handle?

Soluzion

Efficiency of a machine

Mechanical A

velocity ratio

Measurement of relative density of specific gavity of a liquid

by Specific Gravity Bottle

by Archimedes' Principle (in latter part of the

Byllabus)

by hydrometer (in latter part of the syllabus

For a liquid, the procedure of the measurement ie self-explanatory in the following series of diagrams s

The presence of air molecules, in random motion, causes the atmospheric pressure. The atmosphere becomes less dense with increasing haight above the Earth's surface. The pressure of the atmosphere is greatest at the Earth's surface. Its normal value is approximately 14.7 lb./sq.in. This corresponds to a pressure at the base of a column of water 34 ft. op a column of mercury 76 cm. high.

* 100%

Load

Machaniaal Advantage =

Effort

LORE

50

Velocity Ratio

Distance moved by the effort in 1 COVOLUTion

Distance noved by the load

empty

Full with water

full with liquid

21 x 2 x 13

*

M

pitch of screw

2 x 2 12

0.5

* 95 TI

Therefore,

Load/50

40 -

x 100

36 π

Load - 36 77 x 2

- 6033 lb.Nt.

Exercise 12

1. A windlass with an efficiency of 85 per cent-is

used for raising a load of 1 swt. If the diameter of the drum on which the rope in wound is 6 in. and the length of the crank arm is 18 in., what force must be applied to the crank arm to raise the

2. By using a block and tackle a 400 lb. piano san

be lifted by a force of 90 lb. If each block has two pulleys, show by a diagram how they must be connected.

3. What effort is required to raise 300 lb.wt. ́by means of a combination of pulleys shown in diagram if efficiency of the system is do%?

EFFORT

LOAD

Pascal'e Principle

When the pressure on any part of a confined fluid (liquid) is increased, the pressure on every other part of the fluid in increased also by the same, amount. This means in the case of a liquid in communicating vessels that a force applied to a surface of the liquid where the area of cross section is small will be multiplied at some other place where the area of cross section is large.

specific gravity of the liquid

•

mass of liquid

mase of same volume of water

Tm2

-

For sand or fine pastical of solid.

This multiplication of force by the transmissi➡ bility of pressure aqually throughout a liquid, in direct proportion to the ratio of the areas. ft accounts for the action of hydraulic presses, brakes, and similar devices in which small forces overcome Large forces.

empty (,)

full of water (#2)

BUHO Holid (13) Specific Gravity of solid

sold + some water (=4)

mass of solid mass of mame volume of water

m.)

*

р

Exumple 13-1

A rectangular cistern 6 ft x 3ft is filled to depth of 2 ft with water. On top of the water is a layer of oil 3 ft deep. The specific gravity of the oil is 0.6. What is the absolute pressure at the bottom, and what is the total thrust exerted on the bottom of the ais tax?

- 3 ft

011

S.0.0.6

P2

4. With a certain machine the applied force moves

10 ft. to raise a load of 3 in. Determine (a) the velocity ratio and (b) the mechanical advantage if the efficiency is 60%. What load can be lifted by an applied force of 50 lb.wt. (o) if there is no friction. (d) if the efficiency is 60%?

Donaity

MATTER AND FLUID CONCEPTS

Density of a substance is defined as mass volume.

Density -

Mase of substance_____ volume of substanca

Der unit

*

D =

What measure we have to take if

the solid is soluble in water?

Concepte and Principles Applicable to Fluids at Rest

The term FLUID includes both liquid and gas, Líquid is incompressible while gas is compressibla.

Pressure

Pressure should not be confused with force. Pressure is not a kind of forge. It is defined as the force acting normally to an area over which the force is distributed.

A

h2- 2 ft

WATER

Area - 6 x 8 ft

Pressure at the bottom =

2d

whers d is the density of oil

P1

+

P

0.6 x 62.4 1b/cu.ft.

ie the pressure at the depth of the

line of discrimination between oil and water

d, is the density of water which 13

W

62.4 lb./cu.ft.

is the atmospheric pressure

14.7 x 144 lb./ft."

-

Units: The density of solids and liquias usually

is expressed in gram per c.c. in metric units and pounds per cubic foot in British system.

In practicel work, the weight per unit volum❤ leads to a second concept called weight density,

W.

D W/V. This must not be confused with true density in the analyses of problem situations.

Specific Gravity

where p 18 the pressure expressed in dynes/sa.co...

eto.

F is the normal foroe zoting on the area зn

dynes, etc.

A is the area where force distributes, in

Bouare cm.. etc.

The pressure has no direction end is a scalar quantity.

* 2 * 62.4 + 32 + 3 x 0.6 x 62.4 x 32. 14.7

x 144 x 32 pal/ft.Ž

- 2 x 62.4 + 3 x 62.4 x 0.6 14.7. 144

•

124.8 112.3 + 2120

2357 lb./sq.ft.

It is often convenient to compare the density of a substance to that of a standard substance, usually water. Specific Gravity of a substance is. defined as the ratio of the density of that substance. to the density of water. It is obviously a dimensionless quantity (a quantity without unit).

איתי

-

Pressure of liquid

AN a aspra a Beneath the surface of or uniform density there exists

liquid weight of the liquid. This can be expressed as *

a sera due to the

D - nag

the total thrust on the bottom is due to the liquidə and the atmosphere

Chrust

Pad

(to be

2357 x 6 x 8 113,000 lb.wt.

be continued)