1979-04-17 — Page 27

育教經濟 頁三第張七第

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·64(N)

1979 中學會考試題預習專欄: 物理 (廿八)

64-(8)(10)

the block will ac

downwards with

lerate

明德出版社有榮家提供資料

PHYSICS (28)

acceleration 2 ma

Since the planck is in

equilibrium, the resultant

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11.

·記錄。往年由於

月以來的高

自一九七四年十

人三美元,剃下

到

mass of the man

75 kg

mass of the planck

50 kg

1120 N

Στ

Let the pulling force

exi

by the string-

I

= 1120 N (Ang.)

The reaction at the

the

Sinc

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華僑經濟

CORRECTIONS

PHYSICS (26)

SECTION B

4(b) What conclusion on the

relationship between

volume and temperature

can you draw from the graph

PHYSICS (27)

SECTION C

Figure

Figure

Metal sphere

with drawn

Figure 10

Figure 1F

**********

*******

Suggested solutions to

Revision Test

SECTION A - MIRCHANICS

BRAND

PROPERTIES OF MATTER.

(a) Let a acceleration of

the block

6)

true weight.

the block

tension in the

spring

(iii) If the elevator moves

upward with uniform

speed, then

80

the reading of the

balance is 80 N.

(iv) If the elevator moves downward with uniform

speed

then

m.s

80 N

ས

the reading of the

balance is 80 N.

(v) If the cable of the

elevator breaks, then

the block accelerates

downwards with

acceleration 10 ms

Therefore, the reading of

the balance

(b)(1)

10kg

5kg

Let the force between

the blocka

acceleration of the

blocks

1.5N.

∙10kg.

15

5kg

ne mass of the block

As shown in the abo

force diagrams,

R5 a From (1) and (2),

R 5 (Ans.)

the

force between.

the blocks is 5 N

(2)

of forces acting on the

planck is zero.

ng Mg

750 – 500

1250

Also, the sum of moment

about any point must be

zero.

Take moment about P; R2(6) -B2 (6)

mg(5)' – Mg(3) (75)(10)(5).

875. N.

(50)(10)(3).

(Ans.)

From

1250"

1250 - 875. 375 N (Ang.)

(11) Suppose the planck will

tilt when the man is

x meters from B. Hence,

the reaction at P.

Take moment about Q

Mg

Mg(3) ► mg(QB = (50)(10)(3) – (75)(10)(3-x)

(Ans.). When the man is

meter from B, the planck will tilt.

As shown in the above force

liagram

weight of the ladder 200(10)

vein the diagram,

wall is 1120 -N.

(ii) As

the ladder is about to

slip when the man is x

meters from the lower end B

Take moment about B,

Sum of clockwise moments

about B

= \'(x co×9) + W(5 cose)

800xcoat + (2000) (5)cosẽ

(800% 10000) ense

Sum of anti-clockwise

moments about B

• R1 (AC) B

(1120)(8)

8960

8960 - (800x10000)cose

(300x+10000) (-7) - 480x46000.

2960

6,16

the ladder is about to

alip when the men is

6.10 meters from the

lower end,

3.(a) The factors that affect

the efficiency of a single

string pulley system are:

1. Friction; the efficiency

Lowers when friction

increases.

eight of pulley and string; the efficiency

lowers when the weight

of pulleys and string

increases.

The efficiency lowers if the portions of the

string are not vertical. (b)(i) The required velocity

(Ang.) ratio is 5.

(11) The upthrust of water

the cube B

the weight of

water displaced

(1000)(10)(10)

(Ans.)

Therefore

Effort

10

10

80%

2 x 10

Let the acceleration of

the cube bea,

apply Newton's 2nd law

B) mga ma

where E

- mass of the cube

(2.5 x 1000)(10)

2.5 x 10 kg

+105 ) - ( 2,5x10* } (10)

(2.5 x 10′)a

(Ans.)

(iv) The cube will finally

stay at the water surface

with part of its volume

immersedi

Let the volume of the

cube immersed in water

be Vi hence

mg-0 Where is the upthrust

ted on the cube.

510)(10)

20x105

5x10

BULB (1000)(g)(V*)

(1000)(g)(Y') = 0.5x105.

Therefore, the cube finally

stay the water surface

with half of it's volume

Immersed.

(v) Inorder to push the

whole cube out of water, the pulling force exerted on the black must at least

equal to the weight of the

block in aj

Hence

the pulling force

Take the upward direction

as positive,

As shown in the force

diagram

ma

ng

me

100m(2.5

10)

8 kg

the true weight

the block

~ (8)-(10)

mg

8)

N (Ans.)

(if) If the balance reads

64 N

then

2000 N

10kg 5kg

15N

ght of the man (80)(10)

Similarly

800 N

•(3) (4)

Trictional force at

the lower end

From (3 and (4),

R 10

(Ans.)

2,(a) Let the reactions at P

and

and

be R

respectively.

(i) As shown in the

normal reaction the upper end normal reaction,

the lower end

Στο

force disgram below;

W WA

2,000 800

2800 K

3m

2m

3m

slip

Since the ladder is about

therefore

1

(0.4)(2800)

(iii)

•Effo

Dig

2.5 x 10 N

Let the effort required

Again

2.5 x 103

105

80

2 = 6.25×10′′N

the additional force

required.

6,25x10

1.25x10

5x104

(Aus)