1973-05-24 — Page 18

真二第張五第日二十月四年壬癸屬

WAH KIU YAT PO

報日僑華

四期星

日四廿月五年三七九一曆公年二十六國民華中育教僑華

the bullet.

Then 0 V-ft

and

The answer is T.

214 0.

If the depth penetrated is then

22. Applying the formula

v2 = 2(\)x

100 100

100.

100

vt

The answer is F..

in

40. The beat frequency is

1)400

100

=

2 x

7.m

1973英文中學會考試題預習專欄

WZ4 X NBA E UM

物理科(卅一)

Physics (31)

Solution to revision exercise 1.

1. C. One complete oscillation is from A to B to C and then from C through B back to A,

2. E

6. E

7. In fig Let S be the reaction of the wall & be the angle between the reaction R and the horizontal

Or X =

The answer is C.

At the end of the 3rd second, the speed is V 2 x 3 = 6 m/s This is the speed at the beginning of the 4th second. The distance travelled in the 4th second is

The answer is B.

Let I be the tension in the string and a be the accelerat- ion of the system. Suppose the 1 kg-mase is moving downward.

For the 4 kg-mass, the equat- ion of motion is

2m.

Then

= R sind

and

Taking moment about A.

W x 1 = S x √3

Hence

tan

and ∞ = 300

The answer is therefore R.

8. Since the wooden block is just moved up the inclined plane, we have

4 kgf

3 sin 45°

M(4 sin 45° + cor 45°)

Note that the normal reaction is now equal to the normal component of the wooden block plus the component of the 4-kgf force normal to the plane.

Hence

зм

The answer is T.

When the wooden block is at. the point of toppling, the vertical line through the C.G. of the block must pass through the edge of the wooden block as shown in fig 3.

WSING

Taking moment about A

Wsino x 4 = Woo

tane =

The answer is

10. If = 0.6 is the coefficient

of static friction between the wooden block and the inclined plane then at the point of slipping, Wsine -MWCODO'

tane' = 0.6 = -

Since

inclination of the plane

> 롤

Therefore slipping occurs first.

11. Let f be the retardation of

T= 48

(1)

For the kg-mass, 5

(2)

Substitute (1) into (2)

KET

The answer is. P.

13. As shown in fig

16.

18

R- mg - má

R = m(g+a) = 50

750 N

The answer is B.

Let V be the recoil velocity. By the principle of conservat- ion of linear momentum,

0.004 x 500

·5 V

V = 0.4 m/s. in the direction opposite to the bullet.

Since

Then 32-140-100-

or

280°C.

The unknown temp

280 273. 553 K.

The answer is

23. C.

24. The coefficient of linear

expansion of brass

0.000078 /°K.

The area of the hole is

100(1+ 2 x 0.000078 x 100)

101.56 cm

The coefficient of real expansion is

54 =

26. When the 20 1.6 x 1000|

is horizontal,

the pressure of air trapped inside the tube is atmospheric when the tube is vertical, the pressure of the air column is 75 + 15 = 90 cm. of mercury By Boyle's law,

30 x 75 =

- 90 x l l

= 25 cm.

The answer is P.

Applying → = constan

300

2P(1.5)

T

627°C.

The answer is R.

28. Let S be the S.

liquid.

of the

10 x S x 10 = 2 x 10 x

S-4200 J/kg/°K.

The answer is Q.

29. S.H. of metal

(0.05 + 0.15) x 4200 x 0.5 x 80

105 J/kg/°C.

The answer is P.

100%

80M.A.

100

30.

MA

31. R.H.

52 effort

effort = 13 kgf.

The answer is R.

Let S be the R.D. of the mixture. By conservation of mass

45

4 + 3 x 2 10:

The answer is:

When the plate is at the point of falling,

H+3adg H+ a

X

1000g

Where H

atmospheric pressure in depth of water and

d

density of oil.

Then d

333 kg/m3

The answer is

Q.

19. Let be the length of the

rod and be the length submerged.

Then A x 10 = b A x 8

The answer is P

20. Let a be the depth of water,

Then

a.A.1 + (-a)A.10 = f.A.8

l- a - l - Z l = Zl

The required ratio is

100%

50%

The answer is Q.

102 100

The answer is S.

Revision exercise 2. (on electri- city, magnetism modern physics and light).

When an ebonite rod is rubbed with fur, the rod becomes negatively charged because

rubbing produces negative charges on the rod and the fur.

B. rubbing causes air molecules

between the rod and the fur to become charged.

D.

rubbing attracts negative charges to the place of contact.

Some electrons in the fur are transferred to the rod..

some protons in the rod are. transferred to the fur

A charged rod is used to charge an electroscope. Which of the following will give the elect- roscope a charge of the same kind as on the rod?

I. Touch the cap with the rod II. Bring the rod near the cap,

I.

earth the cap, remove the earth connection and then remove the rod.:

Put a needle on the cap and bring the rod near the cap.

B. II. C. III

D. I & IT E I & III

A neutral sharp pointed conduct -ing rod held near the cap of an electroscope charged posit- ively

A. increases the divergence of

the leaves.

B. will become negatively

charged.

C. will become positively

charged and then repelled by the cap.

D. causes the leaves to collapse

has no effect on the diver gence of the leaves.

When the north pole of a bar, magnet is held near one end X of a bar of unknown nature attraction occurs. What can you say about the nature of X ? A. X is magnetic but remains

unmagnetised.

B. X is a south pole. CX is a north pole

D. X is magnetised.

E. No definite conclusion.

The heat supplied in one hour i's

15

Iron is used as an electro magnet because

4 x 5 x 103 x3600

2 x 36 x 10"

The volume of gas required is

106

x36x10)

3.6 x 10

The answer is

33E.

36.

34.

- √E YRT

T remains constant implies that velocity remains constant The answer is P.

37. T.

38. The note an octave above the

given note is of frequency 25, The frequency which d vide the octave into 2 equal in- tervals is 2f.

The answer is T.

39. For a closed pipe, the fre-

quency of the 1st overtone 18

For a half-open pipe, the 1st overtone frequency is

3V

f2

www

For an open pipe, the 1st. overtone frequency is

£3 = X

The ratio of frequencies is

AG

nit is cheaper.

B. it conducts electricity. C. it can be magnetised.

D. it can be easily magnetised

and demagnetised..

E. it can be magnetised electri

cally and remains magnetic for a long time.

The magnetism of the earth may be considered as due to an imaginary bar magnet inside the earth which

P. lies along the equator Q. lies along the axis of

rotation of the earth.

R. is at a small angle to the

axis of rotation with the N-pole in the northern hemisphere.

S. is at a small angle to the

axis of rotation with the S-pole in the northern hemisphere.

T. coincides with the axis of

rotation with the S-pole in the northern hemisphere.

7. The ratio of the equivalent

resistances of 3 resistors sach of ohms first connected in parallel and then in series

is

9:

R. 9. r2

S.

19

T

1

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