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育教僑華真三貫張七第日四初全军戎庚栲夏 WAH KIU YAT PO 日僑
郭日僑華
WE
日九月二年○七九一公年九十五個民
If the machine is frictionless. its efficiency
is 100%.
박
LO英文中學會考試題預習專欄
坚道英文書院主編
物理科
(十四)
PHYSICS (14)
Solutions for exercise 12
In a windlass.
·Velocity Ratio =
length of orank sta
radius of dzua
The mechanical advantage is then equalato
the velocity ratio, which is 40 The effort required to raise the load is 50 lb.wt. Therefore the load is 50 x 40
2000 lb.wt.
If the machine has an efficiency of 60%, (the offe iciency is 0.6), the mechanical advantage is not
gual to 40. It is 24. With an effort of 50 lb. #t, it raisea a load of 50 x 24υ==1290 lb.wt.
FLUID CONCERT (Cont.)
Other methods of measurement of Density of Spacirio Gravities of liquid.
U-tipsa (Balanced column of liquid)
Example 14 – 1
▲ U-tube with its lishe vertical is about half filled with water. Paraffin oil of specific gravity 0.8 is then poured into one on the limbs to a depth of..5 da. How far will the water rine in the other limb?
Therefore,
End, sinGS
In the diagram As representa the original water level. BD is the column of oil 5 cm long, and C is the final water level in the other limb Since the tube may be assumed of unifore bore, the length of water coluez aues be con- stant, and therefors AC - BD. Being in the same liquid and on the same level, the pres- suren at D ́and F are equal, so that the column of water CP exerts the same pISBEVIS. sa the column of oil-END.. #P x 15 x 0.8
- 2 Om.
Assmeri. The watey will rise 2 om, in the limb
of the U-SABRI
6.
112/F
Kechanical advantage= load/effort
where E as the effort an ib.
The efficiency of a machine is equal to
Mechaniozl Advantage
velocity ratio.
112/5 x 100
* 100%
112 x 100
6 x 85
For liquids do not mir when put together, {@) 18 employed. 011 has been poured onto water which initially is about half-way up each side of the tubs. Eurements of the heights of the liquid column 2 and 3, ara essentiel for the calculation
Example 145
A meter rule supported by a knife-edge at i ite mid-point has a piece of metal hung at one end and a weight capable of sliding along the rule. It is found that the rule balances horizontally when
When the the weight is 48 cm from the knife-edge. piece of metal is totally immersed in water, the weight has to be moved 6 cm nearer to the knife- edge to restore balance. Calculate the specific g gravity of the metal.
Answer.
22.0 lb.wt.
The effort applied to the crank 12 raising the load is 22,0 1b.wt.
The connection of the
Block and Tackle system -
ia as shown in diagram,
having a velocity ratio
€
FOUR
The load is 400 lb.wt. while the effort is only.
90 1b.wt. Even though the machine is friction- less, the effort of 90 1b.wt. fails to raise
the load. Such a connec- tion as shown in diagram
A is of no use,
The connection should
be modified as in diagram. having a velocity ratic of 5. The efficiency of this pulley system is
400/90
x 100% =
89%
90/bx
Diagram B
Diagram A
where
74, is the specific gravity
B is the atmospheric pressure
is the hộight of oil colman hy is the height of water coluan
For liquida which do mix up when put together, (b) 10 employed. Some zeroury has been used to separ- ate the two columns of liquid.
If I and H, ara tha dorresponding heights of alcohol and water which balance one another, and and dare the corresponding densities of the
then the prossuze balazos is expressed by
whe
the specific gravi of alcohol
B is the atacapuario: pressure
I is the height of alcohol dolana is the height of water colum
Eare's apperamus is an "izver796 U-tube in wh which the balanced coldens are sucked up from boskora containing them.
50cm
Let the volume of the meta be Voic.
and its speċific gravity
he weight be M
Taking monen Me
50 x Va 48M
and the maB8
(1)
By Archimedes' Principle, when the
metal is immersed in water, its loas in weight
Vem. and its weight in water (V© − V) gm,
Taking moments again at 0, -
(48 - 6)M ...
Divide equation (2) by (1)
Vä
(2)
Answer: The specific gravity of the metal ¤
Bxersos 14
14-1 A hydrometer consisting or a bulb of volume.
5 c.c. Burmounted by a cylindrical stem of diameter 5 man.. sinks in water to a depth of 2 cm above the bulb. To what depth will it sink in a liquid of specific gravity 0. 0.95?
12-3
The Velocity Ratio of this pulley system is
Mechanical Advantage: 18 Load/Effort
·300·
Efficiency of the
machine is the rat
of H.A./V.R.
100%
Answer:
x 100 = do
300 x 100
3 x 80
125 lb.t.
The required effort in 125 lb.t.
Distance moved by effort - 10 ft. Distance moved by load. 3 fa
Velocity Ratio of machine
Efficiency of machine
tage velcosity Ratio
Mechanical advantage of machine - 0,6 x 40
24
and h, are the corresponding heights of liquid and of water which balance one another, and -end d are the corresponding densities of the quide, then the pressure balance is expressed by
Tao specifio gravis of the liquid is found.
ARCUTEENUS PRINCIPLE
When a body is wholly or partially immersed in a fluid, the pressure or thrunt exerted on the upper and lower curface of the body do not balance out to soro because those on the lower suface exceed those on the upper surface as a result of the difs ference in pressure or thrust with fluid level,
is difference in force constitutes a buoyant form ee, whose value is the weight of the displaced fluid, This is Archimedag! Principle. Obviously if the specific gravity of the liquid is less than that of the body immersed in it, the body will wink so as to be completely immersed. On the other hand if the specific gravity of the liquid is greater than that of the body, the latter will sinklonly to such a depth that the weight of the displaced liquid will equal to the weight of the body. That is, it #ill float with partly immersed,
Fadhlea Prosedures.
When dealing with buoyance problems it is recommended that a diagram be drawn, that the body in question be isolated, and that the weight of the displaced liquid, the weight of the body it- self and all the vaætionl forces apting on it ba considered. If the body is in equilibrium, all upward foross will balance mil downward foross.
14
The weight in air of a piece of cork obtain- ed with a reliable balance using brass weights is 45.75 g. What is the true pass of the cork??%
Density of brass, Density of cork
Density of air
8.5 gm per 0.0. 0.25 gm per c.c.
-0.00129 ga per c.c.
In a connected vessel as shown, the dianet- ers of the two limbe are 4:1, Mercury is
put in the vessel and then water is poured onto it until becomes 70 cm. What are the changes in liquid level on both limba?
A solid copper ball floats in a basin of oil and mercury as shown. How much of it im in oil? in mercury?
Density of cil 0.9 go/c.u. Deneity of copper - 8.9 gm/cic. Density of mercury... - 13.6/0.0.
Volume of ball
2014
10.0.0.