问题 选择题

重为G的汽车以速度v通过半径为R的圆弧形路面,在圆弧的底部受到的支持力大小为N,则(  )

A.N>G

B.N<G

C.速度v越大,则N越小

D.汽车的速度如果增大为2v,则在圆弧底部受到的支持力大小将变为4N

答案

A、B汽车以速度v通过圆弧形路面的底部时,由汽车的重力和路面的支持力的合力提供向心力.向心加速度竖直向上,由牛顿第二定律分析可知,支持力N大于重力G,即N>G.故A正确,B错误.

    C、根据牛顿第二定律得,N-G=m

v2
R
,得到N=G+m
v2
R
,可见,速度v越大,N越大.故C错误.

    D、由N=G+m

v2
R
得知,N与速度v的平方不是成正比,所以速度如果增大为2v,支持力大小不是变为4N.故D错误.

故选A

阅读理解

阅读理解

     根据短文内容,从短文后的选项中选出能填入空白处的最佳选项,选项中有两项为多

余选项。

     The person behind you constantly kicks the back of your seat. Your talkative seatmate doesn't

understand your need for sleep. And the aircraft's bathroom is a total mess. These situations can

make even a short flight unbearable . Hopefully you don't cause these unpleasant experiences for

others.   1  .

     Always recline(向后倾斜) your seat slowly. There's nothing worse than suddenly being hit in the

knees by the seat in front of you. In addition, don't keep your seat reclined for the entire flight. Always

keep it upright during mealtimes. And remember to put it in the upright position before going to the

restroom (or anytime you leave your seat. )

       2  Wait until the meal is done and all the food trays(盘子)have been collected . It's hard for

passengers to stand up to let you pass when they still have their food trays. And when using the

bathroom ,always clean up after yourself -the next user will be grateful !

     Keep your body -and your possessions -to yourself as much as possible so as not to crowd your

in-flight seatmate(s) . Share the armrest , especially on a long flight . Also, be careful not to kick or

push on the seat in front of you.

     While some people enjoy chatting with other passengers during a flight, not everyone does. Some

people may want to nap ,read or work.    3  

     If you are traveling with someone and want to chat, keep your voices low,   4    People can still

hear through your headphones if the volume is too high.

    When exiting the plane, if others are having trouble with their carry-on luggage . help them if you

can.  If you can't help ,wait patiently, and don't push past people to get off the airplane. On your flight ,

remember the golden rule:   5   .

A. Don't leave your carry-on in the aircraft.

B. If the conversation seems one-sided , you should notice it.

C. Treat others the way you want to be treated!

D. Avoid going to the bathroom during mealtimes.

E. Instead ,you can set an example by following these common airplane courtesies(礼貌)

F. it is not polite to talk loudly in the aircraft.

G. If using electronic gadgets(小装置),keep the volume down.

单项选择题