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
3-hydroxycorticosteroid dehydrogenase
actual time of departure (atd)
acute injury
air cooled jacket
alternating current chronopotentiometry
antambulacral
as is clause
bamlite
BancoCompartamos
barrel vaults
benefactive roles
beta-aminobutyric acid
boiled dumplings
bonds outstanding
bring sb. back to reality
calciotborite
carbon-dioxide process
carragheenan
chemical glass
conspirators
Cotrim
Couesnon
coupling cone
creakle
cryptocolpate
cyclazodone
distorsio perdistorta
dunkleosteids
eighty-column card
error-generating process
export credit insurance
fainly
for the better
galvanized iron screw
Gay Games
genus lathyruss
geriatric education
gimlets
gnathocerus maxillosus
grid-filament capacitance
higher order wave
horn gate
hydroxyacetic-formic acid
I AM movement
infirmed
inspiratory muscles spasm
internetwork protocol
Jeziorak, Jezioro
loose constraint
luteus
machine weight
Mahoua
masculine pelvis
mastery of the electromagnetic
memcapacitors
microgyria
mitzvahed
mobile offshore drilling unit
near-adult
noise deadener
nomadization
non-livings
OIM
on the phone
out-flows
out-spoken
planings
plowgirls
pot resistance
prophasic
quarry run
refer to acceptor
reference materials
registrary
resiliency
retinaculum Morgagni
rubricator
saturation prospecting
scatter load
shakataks
show layer
soda circulating pump
spark-ignited
St. Elmo's fire
stagnation temperature
staircase tower
standard nomenclature
suppression chamber structure
swan-song
temperature increment quantity
the eyes of night
tip surface of wormwheel
tracees
trirexes
Unterföhring
Urodiaspidae
venessa
verbed
W-dolostone
water issues
work district for telemechanical system