頁二第張六第 日四廿月十年子壬
WAR KIU YAT PO
邹日橋罩
三期星 日九廿月一十年二七九一歷公年一十六國民華中育教僑華
1 kilowatt (kw) equals to 1000, w.
1973中學入學試
試題預習專欄
育教僑華
1913英文中學會考試題預習專欄:
WINXX AMEVZ
物理科
PHYSICS (6)
(六)
WORK is done when a force acts on matter and produces motion.
In measuring work done, two fac- tors must be considered:
its displacement, anu the component of the for- ce in the direction of the displacement.
Therefore, the amount of work da W, eqaals the product of the fore component, F cose, and the disp placement, S
W = FScos ☺
where is the angle between the force (vector) and the displacement.
Unit of Work
A force of one newton, acting throught a distance of one me does one JOULE of work.. Joule is a newton meter. (SI unit) Work done by varying forces
When a force is used to stretch a spring, the magnitude of the for- ce increases as the spring getw Longer
The force required to Launch a space vehicle decreases in magni- tude as the altitude of the vehi- cle increases.
The above two examples indicate a variation of force magnitude. An easy way to find the amount Work done by a varying force is. by means of a graph. The horizon- tal axis denotes the distance, 5, knd the vertical axis the force that acts in the direction of – motion (F có90), The area under the curve line represents the work done by this varying force.
(in newton Force, Foose.
Distance; (in metres)
The problem of finding the total work done by a varying force is somewhat simpler in the case of stretching spring, provided that the force is constantly applied: the direction of the strectching. The force required to stretch a $pring depends on its stiffness, and the force is directly propor- tional to the amount of stretchin (Hooke's Law)
Consequently, the work done by a stretching spring is as followel
Force Faid
Distance, s
The area of the triangle under the graph exactly measures the work done.
POWER is defined as the time rate
of doing work.
Power
ends upon three factors: the displacement of an ob- ject,
the component of IoLce an the direction of the dis placement, and
the time required.
Fscose
which P represents power,
Wis work, and
t is time.
When work is measured in joules. and time in seconds, power is expressed in watts (w)
A watt is a joule per second.
ENERGY: potential energy an
kinetic energy.
Pote
Energy is defined as one capacity of doing work..
Units of energy, as per that of
work
Potential energy is the energy due to position, whereas kineti energy is due to motion of an. object,
In the following, we will deal quantitatively with these concept. and see how the various forms of energy are expressed in terma of work units.
The poetential energy acquired by an object equals the work done. against gravity or other forces to lace it in position. The for mula for calculating work, when the force acts in the same dir- ention as the displacement. is
Y = Fa
Therefore, E = Fa
In Titing an object, Fis the we ght, which from Newton's 2nd law of motion equals mg, and s is the vertical distance h thro ugh which it is lifted. so
B
mgh
Measurement of Kinetic mergy. At any particular instant, the velocity of a freely falling ect starting from rest, in term of acceleration of gravity and th listance travelled, is given by
Solve for h
An object possesses height of h
This energy will be converted to kinetic energy when falling down Since this kinectic energy equals to the potential energy,
Ex = my2
Although this equation for kinet ic energy was derived from the motion of a falling body, 10 can
pe shown that it applies to meton in sny direction or from any cElse
Machine
Uses: 1. Energy Transformation
Energy Transfer
3. Multiplication of MODĖ Multiplication of speed
5. Change of direction of
force
Machine works with an effort which does work on some part of the machine, and the machine exerts a force which does work on a load. It should be noted that no machis does work more than that is done on it. In fact, the moving parts of the machine consume some waste work. Thus, the idea of efficien- oy of a machine is introduced:-
Efficiency
Work Output. Work Input Work Output = load x distance
moved by bad
Work Input Effort x distance
moved by the
19
For en ideal machine, the effica iency is 100% and work cutput is equel to work input,
Work Output = Worx lup
S = fs,
The force ratio F/s is known as the Mechanical Advantage of the machine. It is defined as the ra tio of the load overcome to the
effort applied on
on the x machine..
The velocity Ratio, S/S is a mo- dified expression of distance ra tio. It is defined as the ratio. of the distance moved by the ef- fort to the distance moved by the Load
Relationship of Efficiency, Mecherical Advantage, and ocity. Ratio.
Efficiency Mechanical Advantage M./
Velocity Ratio
Work Qutput Work Inpu
AB
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智慧出有社主
英文科 (六)
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Exercise for this week
A car, of mass 1000 kgm without help from the engine, travels down a hill with a slope of 17 in 15 at 20 m/s. Assuming that the resistance is the same, find the power developed when
travels at 20 m/s along a
- level road. Suppose that, on
meeting a steeper part of the hill, of slope 1 in 5, the driver uses the brakes to keer the speed at 20 n/s, find the rate at which energy is being. cor verted into heat in the brake system.
With a certain manine the ap- plied force moves 10 Pt to rai- se a load of 3 in. Determine (a) the velocity ratio and (b) the mechanical advantage if, the efficiency is 60%. What load can be listed by an appli- ed force of 50 16, -(c) if there is no friction, (d) if the efficiency is 607? (c) A ball of mass 10 Eg 18- thrown vertically unpward with
kinetic energy of 1,000 joule At what height will its kinetic.
to
ferry.
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energy be half the initial va- lue? What is the maximum heimt it will reach?.
(1) Explain how 17 19 that the efficiency of a machine is -- always less than 100 per cen~ and why the effciency of a pul ley system is greater for mo- derately large loads than for small onesi
(c) A man sits in a light slanj attached to the lower block of E pulley Systen whose mechani- cel advantage is 3. What frac tion of his own weight must be exert v as an effort in order to lift himself?
A bullet of mass 10 g travely ling horizontally with a velo- city of 300m/s strikes a block of wood of mass 290 g which rests on a rough horizontal # floor. After impact the block and bullet move together and come to rest when the block has travelled a distance of m. Calculate the coefficient sliding friction between the block and the floor.
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