1980-05-26 — Page 22

而二第張六第二日三十月四年申庚膳

1980

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8

Physics

SECTION A

1. Initially,

speed=0.8ms

[3.0kg

=63004.J.

WAH KIU YAT PO

報日僑華

一期星

The graph of mass added to

the tube against the

Liquid-level reading is ns shown below.

1:50

160

(170)

180

Liquid level reading vție car

(11) According to the result

of part (a). M-fy-fx

Thus, the slope of the graph is equal to the density of the liquid

25-10 170-1585

Finally,

3.0 kg

Slope

-1.25 gen

1250 kgm

(a) Mass of the block m«3kg

Initial velocity

-0.8ms

Final velocity V ̧-Ons

at rest) The work done by the block in overcoming friction

loss in kinetic energy

-4(3)(0.8) -4(3)(0)2. =0.96J (Ans.)

(b) Work donesfrictional force

x distance travelled

Let the frictional force

0.96-f(2)

1-0.48N

(Ang.)

(c) Deceleration

0.48

=0.16ms.

acceleration of the

block a=-0.16ms

Time taken=

0-(0.8) -0.16

59.

(Ans.)

(iii)The test-tube must he

loaded initially in order to keep it floating upright vertically.

(a) The rate of rotation of

wheel C

27(12) 21(3)

4 revolutions per second

(Ans.)

Effort E÷80N:

Distance d moved by the effort through one revolution

=21(40) c

w£80=

fem

(take

Work done by the

effort

Ed

=(80) (2.4)

=192J

100800-63000

9=16

Ans: The temperature of

the mixture after another length of time T is 16°C.

The boiling point of a solution will be raised when solute is added Therefore the temperature of the mixture is alightl higher than 100) C when 15g of salt is added to the water.

(ii)The melting point of ice

will be lowered when salt

(impurities) is added. Therefore, the temperature of the mixture of ice and water is slightly lower than 0 C when salt is added.

(c) The liquid boils again,

because the boiling point of aliquid decreases if the external pressure is reducced.

15cm

2cm

Concave Mirron

r=2f=10cm

1-5cm

Real-is-positive New Cartesran

f5cm n-15cm.

-15cm

Apply

Apply

(Ans.)

one axle A turns (ii)When the

revolution, the axle D turns 4 revolutions. The vertical displacement h of the load for one revolution of A.

4 x 21(2)cm

48cm

0.48m

(take 1=3)

The gain in potential energy of the load for one revolution of A

* load x b

v-7.5cm

(ADS.)

日六廿月五年〇八九一厮公年九十六國民華中育教儁棄·

wavelength of the sound

decreases' and thus the

frequency of this sound.

increases.

For same density, same cross-section and same tension, the fundamentál frequency of a stretched wire is inversely proportional to the length of the wïre,

→250Hz

b=1:2:3

-(3)(250).

-125Hz (Ans.). the quency of Lg

is 125Hz.

ilarly,

(Ans.)

()(250)

83.33Hz

the frequency of

is 83.55HZ.

(ii) For same fundamental

frequency,

Length Tension

2:3° 4:9

(Ans.).

Thus,

V=-7.5cm

(Ans.).

T3>T2> T1

(c) f=250Hz

V-340ms

4;

The image is real and is 7.5cm in front of the mirror (Ans.),

Magnification=12-3

(14)

(Ans.)

(Ans

(d) Assume at time t, the

distance travelled by the block is s and the veloc of the block is V.

loss 10 K. E. Work done. in overcomeing friction

500 x 0.48 144J

(c) Efficiency of the machine

useful work output

x 100%

work input

144

x 100%

192

75%

(Ans.)

Size of the image

*2cm.

(Ans.)

în contact, the net

charges in the systön

(A and B) is

107°C+3x10 C=

Hence the nët charge

+2x10 °C, will be shared A and B. and therefore each of them carry. positive charges, Sluce like charges repel euch other, therefore, A and P will repel each other. (iv) If the charges, were

equally shared, each of them will carry.

x 10 c (08C (Ans.)

The graph is linear in the range 0-IV, therefore Ohm's law is obeyed in this range......

(11) The resistance of the...

the lamp changes.. temperature of the lamp changes due to heating effect of current).

Consider when the

voltmeter is turned on position 1. Hence if the animeter gives full scale deflection, the voltage

the so measured is ly current through R. is

10 mA.

102

Apply Ohm's Law

VIR

10mA

3=10

(R, $10).

B=90

(Ans.)

here shortest length of

the air column.

Similarly, when the

voltmeter is turned on, position 2.

10V

V=fx

OMA

10mA

-340-250(4L)

1×0.34m

(Ans.)

SECTION C

10-10

[R] [R2

+By+10)

my

-4(3)(0,8)2-(0.48)s

=0.96–0.485

K.E. -0.96-0,488

(1)

Thus, the graph of the

kinetic energy (K.F.) or

the block against the

distance s travelled by the

block is linear.

According to equation (1),

we obtain the graph

KLF. (in Joules)

0.96)

(d) The graph of the

efficiency of the machine against the load is sketched below: Efficiency

SECTION B

Load

(iii)Since sound is also a

kind of waves, therefore, iz can be reflected. Thus, the microphone should be placed at B' (.e. the position of the image of B) so that the" ticks of the watch could he most clearly heard.

(b).

The divergence of the gold-leaf decreases because electrons: released from the hot filament (due to thermionic emission) discharges the electroscope.

The surrounding air will be ionized and the electrons in the ionised air dischanges the electroscope. Thus, the divergence of the gold- leaf decreases,

The radiation will ionize the surrounding air, the gold-leaf electroscope will be gradually discharged. Thus, the divergence of the gold-leaf decreases.

8

current

charged

time

-5 x 10.

conlomb/ second

5 x 10A (Ans.) The force diagram is as

shown below.

(11)

B=900

(90+R2+10)

(Ans.)

Position 2 should be used. Position 1 is not

suitable because the internal resistance of the voltmeter at

position 1 is R2+10=1001⁄2• and is comparable to the resistance across PQ. On the other hand, the internal resistance of the voltmeter at

position 2 is R1 +Rq+

×1000A and is much higher than the resistance across Pil:

Induced e.n.fi

Time

(2,0)

(2.0)

S meter

Assume when the of mass M is added, "the volume of the test-tube. immersed in the liquid increased by V-y-x, e By principle of floatation weight of additional mass

=increase in bouyant force

Mg÷JgV

where is the density of the liquid.

(Ans.)

(a) Let Q-amount of heat

energy absorbed in the period T. -(336000)(0.3) =100800J

Since the mixture absorbs heat at a constant rate, therefore, the amount of heat absorbed after

another length of time T is still 1008003. Let the required final temperature of the mixture be C.

Heat absorbed by the mixture.

(mass of the mixture) (specific heat capacity.

of water) x (rise in temperature)

=(1.240.3) x (4200) x (9-0)

As shown in the figure, water is poured steadily into the cylinder. A "close pipe" of length L is formed. The wavelength of the sound, so produced is proportional to L. As the water level in cylinder rises, the

the

T»Tension in the thread W=Weight of the metal

sphere A F-Attractive force

exerted on A by B (iii)When A and B are brought

(iii

A D.C. generator is

required to charge a ca battery. Therefore, we have to replace the slij rings of the given generator by commutator.

modified

generator.

car battery