頁二第張六第2 日一廿月二十年個已番覚 WAH KIU YAT PO
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170英文中學會考試題預習專欄
堅道英文書院玉編
物理科
(十三)
PHYSICS (13)
MACHINES (cont.)
報日僑罩
Hore an advantage of metrio unite le found. Since the density of water is for all practical
purposes 1 gm/c.o., the numerical value of density
in metric units is the same as that of the spacific
gravity of a given substanos. This is not trus in
三期星 日八廿月一年〇七九一靨公年九十五國民華中 育僑犂
British system. Because, one, oubio ft. of water weighs
62.5 lb., it specifio gravity will not be, numerically the same, as that of density.
For problem purposes it should be realized that
the weight of a substance can be expressed asi
W = 0g - DVg (5.0.) DVE
where W is the weight in absolute unite
3.0. is the specific gravity of the
substance
Dia the density of water.
Vis the volume of the substance e is the acceleration of gravity.
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在過去三年來,
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明日頒獎典禮
七七安装
a
point
must also
The
p at the surface,
actual pressure at such take into account the pressure such as atmospherio pressure.
- nag - Po
Atmospheric pressure
Example 12-2
A Bozow-jack bas a sorey of pitch in. turne by a handle having an arm 2 75. 10ng. If the efficiency of the screw-jack ja 40 per cent, what is the greatest load which can be raised on it by a man capable of exerting an effort of 50 lb.uk. on the end of the -handle?
Soluzion
Efficiency of a machine
Mechanical A
velocity ratio
Measurement of relative density of specific gavity of a liquid
by Specific Gravity Bottle
by Archimedes' Principle (in latter part of the
Byllabus)
by hydrometer (in latter part of the syllabus
For a liquid, the procedure of the measurement ie self-explanatory in the following series of diagrams s
The presence of air molecules, in random motion, causes the atmospheric pressure. The atmosphere becomes less dense with increasing haight above the Earth's surface. The pressure of the atmosphere is greatest at the Earth's surface. Its normal value is approximately 14.7 lb./sq.in. This corresponds to a pressure at the base of a column of water 34 ft. op a column of mercury 76 cm. high.
* 100%
Load
Machaniaal Advantage =
Effort
LORE
50
Velocity Ratio
Distance moved by the effort in 1 COVOLUTion
Distance noved by the load
empty
Full with water
full with liquid
21 x 2 x 13
*
M
pitch of screw
2 x 2 12
0.5
* 95 TI
Therefore,
Load/50
40 -
x 100
36 π
Load - 36 77 x 2
- 6033 lb.Nt.
Exercise 12
1. A windlass with an efficiency of 85 per cent-is
used for raising a load of 1 swt. If the diameter of the drum on which the rope in wound is 6 in. and the length of the crank arm is 18 in., what force must be applied to the crank arm to raise the
2. By using a block and tackle a 400 lb. piano san
be lifted by a force of 90 lb. If each block has two pulleys, show by a diagram how they must be connected.
3. What effort is required to raise 300 lb.wt. ́by means of a combination of pulleys shown in diagram if efficiency of the system is do%?
EFFORT
LOAD
Pascal'e Principle
When the pressure on any part of a confined fluid (liquid) is increased, the pressure on every other part of the fluid in increased also by the same, amount. This means in the case of a liquid in communicating vessels that a force applied to a surface of the liquid where the area of cross section is small will be multiplied at some other place where the area of cross section is large.
specific gravity of the liquid
•
mass of liquid
mase of same volume of water
Tm2
-
For sand or fine pastical of solid.
This multiplication of force by the transmissi➡ bility of pressure aqually throughout a liquid, in direct proportion to the ratio of the areas. ft accounts for the action of hydraulic presses, brakes, and similar devices in which small forces overcome Large forces.
empty (,)
full of water (#2)
BUHO Holid (13) Specific Gravity of solid
sold + some water (=4)
mass of solid mass of mame volume of water
m.)
*
р
Exumple 13-1
A rectangular cistern 6 ft x 3ft is filled to depth of 2 ft with water. On top of the water is a layer of oil 3 ft deep. The specific gravity of the oil is 0.6. What is the absolute pressure at the bottom, and what is the total thrust exerted on the bottom of the ais tax?
- 3 ft
011
S.0.0.6
P2
4. With a certain machine the applied force moves
10 ft. to raise a load of 3 in. Determine (a) the velocity ratio and (b) the mechanical advantage if the efficiency is 60%. What load can be lifted by an applied force of 50 lb.wt. (o) if there is no friction. (d) if the efficiency is 60%?
Donaity
MATTER AND FLUID CONCEPTS
Density of a substance is defined as mass volume.
Density -
Mase of substance_____ volume of substanca
Der unit
*
D =
What measure we have to take if
the solid is soluble in water?
Concepte and Principles Applicable to Fluids at Rest
The term FLUID includes both liquid and gas, Líquid is incompressible while gas is compressibla.
Pressure
Pressure should not be confused with force. Pressure is not a kind of forge. It is defined as the force acting normally to an area over which the force is distributed.
A
h2- 2 ft
WATER
Area - 6 x 8 ft
Pressure at the bottom =
2d
whers d is the density of oil
P1
+
P
0.6 x 62.4 1b/cu.ft.
ie the pressure at the depth of the
line of discrimination between oil and water
d, is the density of water which 13
W
62.4 lb./cu.ft.
is the atmospheric pressure
14.7 x 144 lb./ft."
-
Units: The density of solids and liquias usually
is expressed in gram per c.c. in metric units and pounds per cubic foot in British system.
In practicel work, the weight per unit volum❤ leads to a second concept called weight density,
W.
D W/V. This must not be confused with true density in the analyses of problem situations.
Specific Gravity
where p 18 the pressure expressed in dynes/sa.co...
eto.
F is the normal foroe zoting on the area зn
dynes, etc.
A is the area where force distributes, in
Bouare cm.. etc.
The pressure has no direction end is a scalar quantity.
* 2 * 62.4 + 32 + 3 x 0.6 x 62.4 x 32. 14.7
x 144 x 32 pal/ft.Ž
- 2 x 62.4 + 3 x 62.4 x 0.6 14.7. 144
•
124.8 112.3 + 2120
2357 lb./sq.ft.
It is often convenient to compare the density of a substance to that of a standard substance, usually water. Specific Gravity of a substance is. defined as the ratio of the density of that substance. to the density of water. It is obviously a dimensionless quantity (a quantity without unit).
איתי
-
Pressure of liquid
AN a aspra a Beneath the surface of or uniform density there exists
liquid weight of the liquid. This can be expressed as *
a sera due to the
D - nag
the total thrust on the bottom is due to the liquidə and the atmosphere
Chrust
Pad
(to be
2357 x 6 x 8 113,000 lb.wt.
be continued)