1972-11-01 — Page 24

頁四第張六第二日六廿月九年子壬屏夏

1973英文中學會考試題預習專欄

* AA **R+9

物理科

PHYSICS

Suggested Answers:

Solution

()

The rectangular components: of individual forces are tabulated as follows:-

Forces X-component Y-comporent

Resu? t-

ant

Ans

2001bf

+300cos600 +300 sin60 -100c68 459 +100 sin47

0

-200

+279-1bf +180.51

(279)2 + (130.5)~

R = 308 lbf

tan

1130.5

308

250

The resultant is 308 1bf. at an angle.

above

the x-axis.

Solution

Procedure

1. Diagram.

2. Isolate the

body...

3. Indicate forces

on the body.

4. Tabulate the

componenta of forces.

Equate ZP

y

Solve the simult-

aneous equations.

Forces x-components y-components

If

-T cos 60°

+ T sin 60°

Where F is the force,

T is the tension of the

cord, and

W is the weight, 5k gf

system is in equilibrium, IF O

Therefore,

cos 60° Tsin 600

From 1 From

0.50

csc 60°

:77

2.89

ins

The tension of the cord is 7.77 kgt.

the force 18 2.09 kgt.

FORCE IN

EQUILIBRIUM

Resultant A resultant force is

a single force having the same? effect as two or more forces act- ang on a body.

Graphical Representation of a force

Any one force may be represented by a straight arrow drawn with its length in proportion to the nagnitude, the sense of ite ar row head for the direction; and the origin of the line for the point of application of the fore Concurrent forces

If forces act on the same point, or their lines of action when produced cut at a point, they are concurrent forces.

The resultant force of the sys- tem determines the motion of a body.

It may cause the body to

韩日僑羊

WAH KIU YAT PO

move laterally, or rotate the body. It may also experience no notion at all. That 19, there is no resultant and the system is said to be in equilibrium.

First Condition of Equilibrium The resultant force of the sys- tem must be zero.

ZF = 0. This also means that the sum of the x-components must be zero and the sum of the y-components must be zero for the body to be at rest or moving at a constant rates

IF

-0 and X

Problem proc

1. Diagram:

Summar

Read the question care- fully and draw a clear but simple diagram to visualize all the expli cit and inplicit factors.

Isolate the body

This is done for the pur- pose of a detailed consi- deration made on the for- ces acting on a portion. All forces should be clearly indicated on the diagram

Evaluate and tabulate the x- and y-components of all forces acting on the isolated body.

Equate Σ x

Solve the simultaneous equations.

three forces, non-parallel, are equilibrium, they must:

1. be on the same plane, 2. meet a point, or be con-

current when produced, satisfy the Triangle of Forces shich states that "If three forces acting: at a point are in equili- brium, they are represent ed in magnitude and direc- tion by the three sides of a triangle taken in order" Any one of them must be an equilibriant of the

ther tw

Lamis ineorem

The forces that satisiy the Tri angle of Forces can be calculated as follows: –

A

Non-concurrent Forces

If the forces acting on a body are not concurrent, the first condition of equilibrium becomes a necessary but not sufficient. condition of equilíbrium. It must satisfies another condition. The Second Condition of Eau111- brium

If a body is in equilibrium, not only must the forces acting on it when added up be zero,

but also the torques exerted by these forces with respect to any one axis when added up be zero. This is known as the second condition of equilibrium. ΣΕ

Problem Procedure:

Daigram.

Isolate the body, Indicate forces on it. Tabulate all forces com- ponents and brques. Equate

ΣΕ

0

0.

Solve the three simult- aneous equations.

Moments Equilibrium of Parallel Forces

The Moment of a Force or a tor- que, about any axis or a point is the turning effect of the force. on a body about that axis or point.

It is measured by the product of the force and the perpendicular distance from the tuming point to the line of action of the force.

三期星

(接第六張第一頁)

日一月一十年二七九一歴公年一十六圆民希中育教儷華

1913英文中學會考試題預習專欄

堅道楽書院主編

化學科

hus the weights of lead which

combine with a fixed weight (gm)

of oxygen are in the ratio

4 3:2, which is a simple ratio, Exercise

Part I: Multiple Choice questions

Which of the following substances would you expect to stay the same weight when heated in air?

A. Silver oxide

B. Ammonium nitrate

C. Magnesium

D. Gold

Copper

When a compound is formed two elements, the chemical properties

of the compound,

are an average between the properties. the two elements

not resemule those. of the elements: depend on the element which weighs the most depend on the states of the elements

depend on the element which is there is the greater volume

Which of following changes belong to physical change?

A. The evolution of gas fr

health salts placed in water

Solid iourue 19 heated gently in a test tube

C. Sodium nitrate and

ammonium chloride solutior are warmed together

Iron filings and powder sulphur are heated togethe E. The Solution of magnesium

in dilute sulphuric acid

4. The atmosphere is a mixture

of gases because

A. air contains nitrogen and

oxygen.

B. different gases in aıı can be separate by diffusion method

it is soluble in water" its composition does not vary

E the pressure of the

atmosphere varies from place to place

The Law of multiple proport ions is illustrated by

Principle of Moments.

If a body is in equilibrium, the sum of the anti-clockwise moments of forces about any point is eq tal to the sum of the clockwise- moments of forces about the same point.

Conditions of Equilibrium for Parallel Forces

The algebraic sum of the ces acting on the body in any given direction must be zero There will be no linear dist placement as there is no un- balanced force.

The algebraic sum of the mom- ents about any point or axis is zero. There will be no ro tation.

Couple

A couple consists of two equal but opposite forces at a distance apart. A couple produces rotation on a body.

The moment of a couple is equal

to the product of force and the distance between the two forces

F

Moment of the couple Centre of

ravi

Centre of gravity of a body is. the point of application of the resultant weight of that body.

The weights of the cons tituent particles of any body are parallel forces directed towards the cen-

A. Sodium chloride and sodium

Iodide

B. Nitrous Oxide and Nitric

oxide

Sulphuric acid and sulphrous acid

D. potassium chloride and

potassium chlorate

E. Oxygen and ozone Consider the

Statement If two different compound of copper and oxygen are formed it can be shown that the weights of copper combining with a fixed weight of oxygen are in the ratio 2:1 This an illustration of the law of

P. constant composition Q. Conservation of mass R. compound proportions combining weight's multiple proportions

In the atomic theory, Dolton stated that all atoms of the same element were alike.. Which of the following. statements would he rejected its

P. An element can form

allotropes

An element can form isomers

R An element can fore

isotopes

5. An element can have mora

than one valency

·T.

An element can have more than one equivalent. weight

Metal alloy one generally

P. A chemical compounds.

Q. A mixture.

R. Metallic ones

S. An element:

1. Can be found in nature

Give definitions of the law

of multiple proportions. Show how the following i analysis of three compounas of nitrogen illustrate the law of multiple proportions:⇒

A per cent per Nitrogen 63.65 Охудел

36.35

B

Per cent percent 46.68- 25.94 53.32 74.06 2. 5.85 gm. of sodium chloride is found 15 contain 2.30 gm. of sodium. 3 gm. of sodium, reacts with water to form sodium hydroxide, which is further added to hydrochloric acid, after neutrisation, and heaty, the weight of the residue(sodium chloride) is 7.63 gm, use these figures. to illustrate the law of constant composition.

tre one earth. Their resultant, the weight of the body, acts through a definite point which is independent of how the body is placed.

Exercise for this week

The above figure shows a square A and a triangle B. attach together. A of mass 300 g, and B of mass 100g. Given the dimensions as in figure. Find the position of the centre of

At what angle gravity of mass.

to the vertical will the side 00' be when the, figure is freely pivoted at

A train starts from rest from a station A and travels with uniform acceleration 50 cm/ sec/sec tro k 20 seconds. It is then travelling at uni- form velocity for another

seconds, and brakes are then applied so that a unifom betardation obtained and the train comes to rest in a fur- ther 10 seconds. Draw a vel City-time graph of the motion, Use the graph to calculate (a) the maximum velocity reached, (b) the retarda-. tion, and (c) the total dis tance travelled.