1969-12-10 — Page 20

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70英文中學會考試題預習專欄

道英文書院主編

物理科

FORCES

A force is defined in terms of the effect that

it can have on a body.

A force may change

(1) The state of rest of a body

(2) The speed of a body

(3) The direction of motion of

R

body

A force may not change the above mentioned

states, but it may tend to change them,

Therefore, a definition of force is concluded as:- )

A force is that which changes or tends to change, a body's state of rest or of uniform motion in a straight line.

VBS of force

Forces can be classified as FUSH or FULL. Some particular types appear so often that they are given special namos:

(a) gravitational forces

(b) upthrust of fluid (u)

frictional forces (f)

weights (W)

(d) normal contact forces — reaction (R)

(e) tension forces -

nsion ()

To solve problem involving forces acting on a body, a free-body diagram or sketch should be drawn, to give a general view of the problem."

Example

郭日橋

Newton's Third Law of Motion

To every force there is an equal and opposite

reaction force.

These laws are not to be subjected to attempted proof by derivation from defining concepts. They are rather to be considered as laws of nature and the threshold of many deviations of other relationships, as well as the key to the solution of many problems.

The first law of Newton is the justification for the procedure followed in equilibrium probleme set the resultant of all the forces acting on a body equal to zero. The third law also explains the fact! that for every force acting on the body, there is a corresponding one exerted by the body on something:

The second law, however, is more general than the first law. It tells not only what the forces acting on a body add up to when the body is in equilibrium, but also what they add up to in general, In mathematical language the second law states that:

where is the force, m is the mass of The Day, and a is the acceleration caused by it. k in the formule is a proportionality factor. It associated with the units in which Fm and a are expressed. (dynes)

m(gm) x 1(om/sec/sec)

(poundal) » (15) x a (ft/sec/600)

Σ (newton) = m (kg) i'a (m/seo/sec)

(To be continued)

SOLUTIONS

甥響,十月二十年,六九一零公生八十五國民中

接第五張第二頁)

中文中學會考試題預習專欄 英文科(六) 王淑方

15) He treated me like a fool and mocked

everything I said.

(A) at. (D) for

(B) in

(C) to

(E) from

ANSWERS TO PREVIOUS EXERCISES

EXERCISE 5 (1) (B) (5) (A) (9) (P) (13)(A)

(2)(E)

(6) (s):

(10)(Q)

(14)(A)

(7)·(Q) (11)(B) (15)(C)

88

(D)

(8) (s) (12)(0)

EXERCISE

(1). (B)

(5) (A) (9) (P)

38

100+

88

(10)(R)

EXERCISE (1) (B) {(5) (D) (9) (F)

(2) (C) (Q)

(13)(B)

(P)

(11)(D) (15)(A)

(3) (A)

(4) (B) (B) (S) (12)

acts at angle,

The required force is 11

tan

ft. from B.

The wire frame is erected vertically as in Fig. The forces acting down the vertical frames, are

box

is the pull of the Barth on the box

Pis the push of the man on the box

B is the reaction or pash of the farth on the box.

The force-system is in equilibrium.

RY + P.

CG:

equal, both being 15 1b.t.

Therefore the centre of gravity of whole frame work lies on AA.

12

(FLA)

lump

D

Tis the tension of the string or the pull of

the string on the lump

Wis the weight or the pull of the Earth on the

Lump

Vis the upthrust or push the liquid on

lump

Newton's First Law of Motion: states that

Every body continues in its state of rest or of uniform motion in a straight line unless compelled by some external force to act otherwine.

This Law may

also be statoo des

body will move with constant velocity when there is no resultant force act on

Newton'a Second Law of Motion

A moving body possesses momentum. The momentum of a body is the product of the mase and the velocity of the body..

Momentum mv

Newton's Second Law states that the rate of ohange of momentum of a body is directly proportional to the resultant force on the body and is in the same direction and sense as the resultant force.

Regultant force

Tinal momentum initial momentum.

time taken

u)/e

Ma

Thus, the second law may be defined as

If a resultant force acts on a given body acceleration in the direction of the force and proportional in magnitude to it is acquired by body.

(6)

The series of diagrams give a self-evident to the location of the centre of gravity of the folded plate.

When folded, the plate is still symmetrical. The 0.0. must lie on the line of symmetry as in (b). When it is turned to position (c) C.G. of BB A A and 30DA are 0 and G'. Both portion have equal weights, The point of balance is in the middle of CG', We may conclude, therefore, the centre of gravity of the folded plate is 1 ft. from AD and

from CU.

1060437*

lo$4:37*

Let X and Y be the horizontal and vertical component of the force applied on the bar to. maintain equilibrium, and assuming that the force açta at a point ft. from B.

X-10 sin 37.9.

6 16.th:

0; Y = 2 + 10

2+ 101b

7136

(Fig.B)

The wire frame is put as in fig. 3.*

As the weights on both sides are in ratio

or 3, the C.G. lies on BB cutting the frame

into 1 to 3, that is to là ft.

The C.G. is on the intersection of AA' and BB That is 1 ft. from the 3-12 arm and 1 ft

from the 3-6 arm-

1.225

The bus begins to topple

(5)

its tilted

G.G. is then just above the point of turnin

h is the height of C.G. (In fig. a)

1225 cot 30°.

2.12 m.

If it is tilted 45. (The chain on the.

taken away). The weight, W of the bus tends to turn the bus clockwise.

Wisin(45

30%

The force, E, acting at 0.0, tends to keep the bus in equilibrium by offering a moment

FLcos(45 30

Zoos 15

Wisin 15°

Mtan

8000 tun

21

Ans (1) The centre of gravity of the bus is 2.12

metres above the base.

(ii) The horizontal force acting at 0,0, is

2343 ke.wtim

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