REFERENCE LIBRARY
24DEC 1969 CITY HALL
育教倴華頁三第張七第日六十月一十年产产麻夏 WAH-KIU TAT P 菲日橋
LO英文中學會考試題預習專欄
文書院主編
物理科
(八)
PHYSICS (8)
WORK, ENERGY, AND POWER
When a foro acts on a body with the point of application of the force being displaced, then work is said to be done. Is is measured by the product of the applied force and the displacement of the point of application in the same direction and senses the force.
work done
(applied force) (distance
: moved" in direction and sense.
of force)
Fevir
work done by force
time taken in doing work
sre P is power in weten
Wis work done in joules and
t is the time taken in sed.
1 watt is the rate of working when one foule of work is dans in 1 second.
1 horas-power is equal to 550 ft-lb.wt./seo and 1#
therefore a gravitational unit of power.
Example 1
Ablook of wood starting from reat at the top of an inclined plane slides with a minimum of friction.
A block of wood starts from rest at the top
alides down a frictionless inclined plane. The block 1@ 60 ft above the ground before sliding. How fast is it travelling when it reaches the base.
Facos
教
基敎書院慶祝校慶
式由教育可簡寧伉儷主持 該校室內運動場同日舉行剪綵儀
中野中三甲 小野足球比賽排 要節日助興,全校王琦
·各班也表演各樸椅乐游 #KITEER-RE | 中三班都在晚會上演出
乙,中苏對中四
,
花
少校友嘅校參加是有 在返日下午作賽,更有 【典,全校洋婆游戲樂
【校奧良於;斅臂司簡邠蒞臨袞講及周校窆內運動傷剪綵,而掌夫人邪 :六种别》香港基敦護院於龍(女二)日在的校大陸眾孤行糊校四週年
二日四十月二十年九六九一屬公年八十五國民莖中
|壽山中學
·慶祝校慶
#
•AGEM
RETE
• • #2
FA
BUK
滿務。 退為 監辦 司上司
裴育司在致詞時,富许錫校之成就,并籖游本港富有之商人谈榔與 教育司又稱;興辦一個學校,德要龐大的金餘,又須阱咕優良的安
標準商科外文校
・府市を行を必文試中心
(RE) KAN
┅按現痳有望五十餘間:薇育家,科學實驗室,圖書舘等多所,由必文學會香港區老 發展計劃。今年暑假,擬建天台運動搖,此外,能聽致育設備及而對龍3 31五號香港標準商科外 文學校,飛近建筑
TENGERE
請宇篊海心必學3
[港阼習,各班除於是日晚行,校嶷專號襞, 中五、一查詢。 SKYEKEZER PELEECKENE VZOEKTKOP 校第十九周年校寳,勁開各頊活動,中六班出軌之校外學生,可到接 7日爲香港秀山中泰创 聯晚會外,並分別展 中苦,方參加該項
where W is the work done in erge, joules, ft-pdl
of ft-lb.wt.
P is the fóres applied in dynes. newtone.
pda or 1b.wt.
● is the displacement in om., metre, and feet is the angle between the force and the
isplacement
The o... system of unit of work is the erg. One erg is the work done by a fores of 1 dyns when it moves a distance of 1 cm in the direction and sense of the force..
The m.t.s, system of unit for work is the jouls, One joule is the work done by a force of 1 newton when it moves a distance of 1 metre in the direction and sense of the force,
The 2.p.8. system of unit for work is the ft-pal, ne ft-poundal is the work done by a force of poundal when it moves a distanon of 1 foot in the direction and sense of the force.
Work Units in both Absolute and Oravitational may be tabulated as s
Since time is not specifioslly mentioned in thin problen, the energy method is suggested.
Total energy pocesses at the top -
Total energy pocésses at the foot
By conservation of energy, mgh
2gh 12(32)(60) 162 ft/s00
ENERGY
Absolute
fest-poundal (It-pdl) the work done moving a one pdl force "through a distance
of one foot
orge the work done
moving a one- poundal force through a distance of one cm.
Joules the work done in moving a one-newton force through a distance one metre.
Gravitational
foot-pound weight (ft.lb.wt;) is the work done in moving, one pound weight force through a distance of one foot:
on-gunt, the work done in moving a one- gram weight force through a distance of one centimetre.
-kilogram weight. "the work done in
moving a one-kg.«t. force through a distance of ons matre.
Energy is a concept defined as the capac
of doing work,
Units of energy as per that of work.
Because there are many different reasons why a body may be able to do work, energy are classified as I
kinetic energy,
gravitational potential ener
elastic potential energy,
chemical potential energy,
heat energy
radiant energy
(g) light energy,
(h) sound energy,
magnetio potential energy,
(a) electrical energy..
Example z
The suspended bob of a simple pendulum or pendulum length 3 ft. is pulled aside so that the string makes in angle of 10 degrees with the.. vertical. The bob is then released, Tind the speed of the bob as it passes through its lowest position..
Solution
This problem is readily solved by energy method, However, it may be solved by force analysis, But it is time consuming.
Top of Swing.
Bottom of Swing
Energy Conservation
(接第七張第二頁)
亵際游出压
中文中學會考試題預習專欄
英文科 (八)
王淑方
(DIRECT)
He asked, "Can John speak English?" (INDIRECT)
He asked if (whether) Joahn could speak English. (DIRECT)
She
asked "Will you help mer
(INDIRECT)
She asked me if (whether) I would help hei
(b) CHANGES WHEN THE YERE IS NUT A SPECIAL
(DIRECT QUESTION)
She asked, "Do you know Mr. Smith?" (INDIRECT QUESTION) A
She asked me if (whether) I knew Mr. Smith, (DIRECT)
He asked, "Did they all do the exercise wrong (INDIRECT)
· He asked (wondered) at (whetner) they nam siz done the exercise wrongly.
(DIRECT)
My father asked. "Does Peter speak French?" (INDIRECT)
My father asked me if (whether) reter spoke French.
(o) CHANGES WHEN THE DIRECT QUESTION BEGINS WITH AN
INTERROGATIVE.
(DIRECT QUESTION)
She asked, "What is her name?". (INDIRECT QUESTION)
She asked me what her name was. (DIRECT)
"Why did you go there?" she asked
(INDIRECT)
She asked me why I had gone there. (DIRECT)
He asked, "What did one
(INDIRECT)
He asked what sne had said.
Here we concern with KINETIC ENERGY and POTENTIAL ENERGY (gravitational potential energy) only,
Potential energy is the energy due to position, whereas kinetic energy is due to motion, More precisely, kinetic energy is defined by the relatio
and potential energy is the energy stored in a body by virtue of work having been done on it by a so-called conservative force, or one whose value does not depend upon velocity, but rather upon position.
ngh
Conservation of Energy
In all mechanical processes the sum of the potential and kinetic energy remains constant for the system concerned. If there is work done against.... friction in the process, this is not included in
the above-mentioned summation because the force of friotion is not a conservative, but rather a dissipative force, which therefore cannot be Rssociated with potential energy. Another statement s the principle of conservation of energy is that, excluding such forces as friction, the gain in kius $10 energy of a system must equal the loss of potential. energy in the same system during any process.
POWER
When a xoras aces work, its rate of working 18- called its power.
Example 3
3 — 3 aos o
3-3 (0.985)
2.96
√2(32)(0.04) 1.6 ft/sec,
The ballistic pendulum is a device which makes possible the determination of the velocity of a projectile which is fired into a blook supported an simple pendulum. A bullet of 20 grams mass is fired. into a block of 500 grama so supported from a 100 cord, causing it to swing through an angle of 10. degrees with the vertical, Calculate the velam the bullet.
fa0cm
20gm:
M=5009M
By the Principle of momentum,
of
By the Principle of energy conservation.
妻
2gh
√28ll - Loc8 20°
therefore
-
coa ☺)
520 20
A2 (980) (100) (1 - cos
Адв
26 1960 (1.5)
1400 cm/sac
The initial velocity or the bullet is 1400
Exercise 8
14 car weighing half a ton accelerates uniformly
from rast, taking 30 sec, to attain a speed of 40 miles per hour, Neglecting friction losseg,cal- culate the horsepower of the engine when the speed is (i) 20 n.p.h. and (ii) 30 w.p.h.
A simple pendulum of length 1 metre is vibrating with an amplitude of 30 degrees. What is the velocity of the pendulum bob at its lowest point?
A BULLET OF 4U gn 19 shot into a 20 kg wooden block which is suspended by a 2-metres string. The bullet remains in the block and the amplitude of swing is measured to be 30 geared. Calculate (1) the speed of the bullet before hitting, (ii) the lost of energy of the bullet and (iii) the resistance of wood if the penetration iB 2 cm.
Solutions fem EXERCISES 7 and 8 will be given on next isskS',
No comments yet.
Private notes are available after approval.