ရူပေဗဒဘာသာရပ္တြင္
အမွတ္ျပည့္ရႏိုင္ရန္ အခန္းတိုင္းကို ေၾကေၾကညက္ညက္ နားလည္ရပါမည္။ တစ္ခန္းစီအတြက္
(၁) Definitions/laws မ်ားကို ျပဌာန္းစာအုပ္ပါအတိုင္း
က်က္မွတ္ရပါမည္။
(၂)
စာတစ္ေၾကာင္းခ်င္း၊ စာတစ္လံုးခ်င္းကို နားလည္ေအာင္ ဖတ္ထားၿပီး Fill in the Blanks, True/False ႏွင့္ Quiz မ်ားေျဖႏိုင္ေအာင္ ျပင္ဆင္ရပါမည္။
(၃) Formula derivation မ်ားကို ေလ့က်င့္ရပါမည္။
(၄) Questions
and Problems မ်ားကို ေလ့က်င့္ရပါမည္။
(၅)
သရုပ္ေဖၚပံုမ်ားကို ေလ့က်င့္ရပါမည္။
(၁) က်က္မွတ္ရန္ Definitions/laws မ်ား
1.
Work Done: Work may be defined as
the product of force applied and displacement.
W = F cos q d, [where W = the work
done, d = distance moved,
q = the angle between
force and displacement, F = applied force]
2.
Power: The rate of doing work is defined as
power.
P = W/t; [where P = power, W = work
done, t = time]
3.
Watt: The unit of power equal to a rate of
energy transfer (or work done) of 1 joule per second.
4.
1 watt: If the work done in 1 second is
1 joule, the power is 1 watt.
5.
Simple Machines: There are
three types of Simple Machines. They are (i) the lever (e.g, a crowbar), (ii)
the inclined plane (e.g, a screw jack) (iii) the hydraulic press (e.g, brake
system of a car).
6.
Hydraulic System: A system that transfers force from
place to place using fluids.
7.
Lever: An appliance which is pivoted about
some point, and which generates a turning effect when a force is applied at
some point other than the pivot.
8.
Machine: An appliance that enables work to be
done.
9.
Mechanical Advantage (MA): The
mechanical advantage of the machine is defined as the ratio of a load (W) to an
effort (P).
10. Velocity Ratio (VR): The ratio of the distance per second
moved steadily by the effort to that of the load is called the velocity ratio
of the machine.
11. Efficiency: The ratio of output work to input
work is defined as the efficiency of the machine.
12. Input work: The work supplied by the effort is called input work.
13. Output work: The work done on the load is called output work.
14. Perfect Machine: A perfect machine is one for which output work is equal to
input work or whose efficiency is 100 %.
15. Elasticity: The ability to retain the original form is called
elasticity.
16. Elastic Limit: There is a limit beyond which if the spring or any other
elastic object is stretched, it will not return to its original form.
Such a limit is called the elastic limit.
17. Hooke’s Law: As long as the elastic limit of a
body is not exceeded, the strain produced is (directly) proportional to the
stress causing it.
In symbols, F µ x (or) F = k x; (k = constant)
where F = applied force or stress
(N), x = elongation or strain (m)
18. Stress: The force applied to an elastic body is called stress.
19. Strain: The elongation produced by an elastic body is called strain.
(2) Fill in the blanks.
1.
The unit of power in SI system is the _________.
2.
In the British system the unit of power is _________________________.
3.
Power is equal to the product of force and ___________.
4.
Machines transmit _________ from one point to the next.
5.
Large machines are made up of __________ machines.
6.
In lifting a large load, a ___________ is employed.
7.
The work done by the effort is called _______ work.
8.
The work done on the load is called _______ work.
9.
The efficiency of a machine is always less than ____________.
10. Hooke called the applied force the
______.
11. Hooke called the elongation the
_______.
Ans: (1) watt (2) foot-pound per second (3) velocity (4)
force(s) (5) simple
(6) machine (7) input (8) output (9) 1
(OR) 100 % (10) stress (11) strain
(3) Say True or False
1.
Power is a fundamental concept of physics.
2.
Power is a scalar quantity.
3.
The VR is usually much greater than 1.
4.
VR is greater than MA.
5.
In using a machine, the small effort applied will have to move through a
large distance.
6.
Efficiency is dimensionless (unitless).
7.
In practice, output work always exceeds input work.
8.
It is possible, in practice, to build a perfect machine.
9.
Elastic limits of different bodies are different.
10. Springs, threads and rubber bands
have elastic property.
Ans: (1) False (2) True (3) True (4)
True (5) True (6) True (7) False (8) False (9) True (10) True
(4) Quiz:
1. Explain why power is a useful concept
in practical works.
Ans: Car engines, water pumps, refrigerators, air conditioners and electric
bulbs, fluorescent tubes, etc. are classified according to their rated powers. Thus power is a useful concept in
practical works.
2. Which is more advantageous: to pay
wages according to the amount of work done or according to power?
Ans: Power can give only the rate of work done. The wage depends on amount
of work done.
To
pay wages according to the amount of work done is more advantageous.
(5) Calculation အတြက္ Formula မ်ား
1. Work Done
W = Fs = mgh
W = work (J), s
= distance (m), F = force (N), m = mass (kg), h= height (m)
2. Power
P = W/t = Fv
W = the work
done, P = power (W), t = time (s), F = force, v = constant velocity (m/s)
3. Mechanical
Advantage
mechanical advantage (MA) = load (W) /[effort(P)]
4. Velocity Ratio
velocity ratio (VR) = (distance
moved by effort)/ (distance moved by load)
5. Efficiency
efficiency = e = (Wout/Win)
´
100 % = (MA/VR) ´
100 %
6. Hooke’s Law
F = k x
F = applied force or stress
(N), x = elongation or strain (m), k = constant (N/m)
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