时间:2018-12-27 作者:英语课 分类:英语时差8,16


英语课
Potential energy in a crossbow When we think of the concept of energy, we often think of objects in motion: a car rolling down a hill or a jogger running up a hill. This kind of energy-in-motion, called kinetic 1 energy, isn’t the only kind of energy. Another type, which motionless objects can have, is called potential energy. Potential energy is stored energy; that is, energy that can be tapped later. To better understand the concept of potential energy, consider how a crossbow works. If you took a crossbow, pulled its string back, and cocked it so that the arrow remained in place until you decided 2 to shoot it, you would have created energy, energy that can be released by pulling the trigger. You have increased the potential energy of the crossbow. The amount of potential energy in the cocked crossbow is equal to the amount of work you put in it by pulling it back and cocking it. This potential energy becomes kinetic energy when you pull the trigger. With the release of the trigger, the potential energy of the cocked crossbow converts into kinetic energy as the arrow flies through the air. Except for a bit of energy lost to friction 3, the kinetic energy of the flying arrow is equal to the potential energy that was stored in the cocked crossbow. Of course, potential energy doesn’t only exist in crossbows. Other examples include a boulder 4 on the edge of a cliff, and gasoline in the tank of a car. In effect, potential energy is simply kinetic energy waiting to happen.
 

1 kinetic
adj.运动的;动力学的
  • There exist many sources of energy both potential and kinetic.存在着许多势能和动能的能源。
  • The kinetic theory of gases is the best known example.气体动力学理论就是最有名的例子。
2 decided
adj.决定了的,坚决的;明显的,明确的
  • This gave them a decided advantage over their opponents.这使他们比对手具有明显的优势。
  • There is a decided difference between British and Chinese way of greeting.英国人和中国人打招呼的方式有很明显的区别。
3 friction
n.摩擦,摩擦力
  • When Joan returned to work,the friction between them increased.琼回来工作后,他们之间的摩擦加剧了。
  • Friction acts on moving bodies and brings them to a stop.摩擦力作用于运动着的物体,并使其停止。
4 boulder
n.巨砾;卵石,圆石
  • We all heaved together and removed the boulder.大家一齐用劲,把大石头搬开了。
  • He stepped clear of the boulder.他从大石头后面走了出来。
学英语单词
alarm dismissal probability
antiskeptic
as warm as a toast
ASCR
attorney at law
automobile traffic
Balastya
ballast waterline
ballow
barium soap
betz
biasing spring
bitchier
brassica campestris chinensis purpuria
buckle folds(flexural folds)
cellular dolomite
centilitre
central sphere
chisel nail point
cilleral
coliindex
crotchlines
cyberutopians
cyrtohypnum sparsifolium
dated back
decision-making section
die like flies
differential astrometry
domestic transport
edelrid
eisenstaedts
electric filter
elsilimomab
engh
excess 3 binary coded decimal
fixed count check
fractional-order
God botherer
haires
have a touch of
heart-kidney imbalance
Heli-Grid packing
hochner
hoggets
howea belmoreana becc.
ingroup (in-group)
klipdassie
Kysyl-Sulus
Leonardo
leukokraurosis
linear porosity
Lisskogsåsen
LTBMC
LTSP
maillart
mediocentric
micro-machines
minimum access programming
moneyless
mongean method
nagged
nonspiked
Norris, George William
oxypetalous
phlegmasia alba dolens puerperarum
picotamine
pigeon-wood
pittle
plane of support
plastic glove
postjudge
priestleys
red-eared sliders
reevoking
register-renaming
reorganising
rights of beneficiaries
scherzando
schizocythere gibbrosa
schwannoma of kidney
severe pollutant
shaft counter
showhouses
simple insurance
slnb
sneakest
Southville
sterculia gums
Temburong Dis.
the pledge of love union
therapeutic drug
Tocantins, Estado de
Tonate
toolkit without an interesting name
uncenturied
underwater floodlamp
upper arcs
uromyces mucunae
vacuum leak hunting
Vermand
wolfram bonze
xiantang