问题 问答题

航空母舰上的飞机跑道长度有限.飞机回舰时,机尾有一个钩爪,能钩住舰上的一根弹性钢索,利用弹性钢索的弹力使飞机很快减速.若飞机的质量为M=4.0×103kg,同舰时的速度为v=160m/s,在t=2.0s内速度减为零,弹性钢索对飞机的平均拉力F=______N(飞机与甲板间的摩擦忽略不计).

答案

飞机回舰时做匀减速运动,

由匀变速直线运动的速度公式:v=v0+at,

可得,加速度:a=

v-v0
t
=
0-160
2
=-80m/s2

负号表示加速度方向与速度方向相反,

对飞机,由牛顿第二定律得:

F=Ma=4.0×103×80=3.2×105N;

故答案为:3.2×105

完形填空
Cars are an important part of life in the United States. Without a car, most people feel that they are___36___.And even if a person is poor, he doesn’t feel___37___poor when he has a car.
Henry Ford was the man who first started ___38___cars in large numbers. He probably didn’t know how much the car was going to ___39___American culture.The car made the United States a nation on ___ 40___. And it ___41___to make the United States what it is today.
There are three main reasons that the car has become so___42___in the United States. ___43___,the country is a huge one and Americans like to __44__around in it .The car__45__the most comfortable and cheapest form of transportation. With a car, people can go any place___46___spending a lot of money.The second reason is that the United States never really developed an efficient(有效地) and inexpensive form of public___47___. Long distance trains have never been as ___48___in the United States as they are in other parts of the world.Nowadays there is a good system of air-service provided by ___49__.But it is too expensive to be used ___50__. The third reason is the most ___51__one.The American spirit of ___52___is what really made cars popular. Americans don’t like to wait for a bus or a train ___53___a plane. They don’t like to have to ___54___an exact schedule. A car gives them the freedom to schedule their own time.And this is the freedom that Americans want___55___to have.
小题1:
A.healthyB. poorC.prettyD.rich
小题2:
A.actuallyB.trulyC.reallyD.exactly
小题3:
A.makingB.doingC.designingD.owing
小题4:
A.affordB.affectC.effortD.effect
小题5:
A.footB.horse backsC.wheelsD.chairs
小题6:
A.helpedB.triedC.hopedD.got
小题7:
A.well-knownB.popularC.lovelyD.famous
小题8:
A.In allB.After allC.Above allD.First of all
小题9:
A.walkB.transportC.moveD.go
小题10:
A.providesB.offersC.takesD.proves
小题11:
A.withB.byC.forD.without
小题12:
A.journeyB.transportationC.travelD.trip
小题13:
A.unusualB.usualC.commonD.ordinary
小题14:
A.bikesB.shipsC.planesD.rockets
小题15:
A.completelyB.continuously C.frequently D.fully
小题16:
A.importantB.funnyC.commonD.simple
小题17:
A.dependenceB.independenceC.selfishnessD.self-confidence
小题18:
A.norB.butC.andD.or even
小题19:
A.takeB.receiveC.acceptD.follow
小题20:
A.mostB.worstC.bestD.least
单项选择题