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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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