问题 多选题

如图所示,斜面上除了AB段粗糙外,其余部分均是光滑的,小物体与AB段的动摩擦因数处处相等.今使该物体从斜面的顶端由静止开始下滑,经过A点时的速度与经过C点时的速度相等,已知AB=BC,则下列说法正确的是(  )

A.物体在AB段与BC段的加速度大小相等

B.物体在AB段与BC段的运动时间相等

C.重力在这两段中所做的功相等

D.物体在AB段与BC段的动量变化相等

答案

A、AB段粗糙,物体受重力、支持力和摩擦力,BC段仅受重力和支持力,合力大于AB段的合力,根据牛顿第二定律,知BC段的加速度大于AB段的加速度.故A错误.

B、经过A点时的速度与经过C点时的速度相等,根据

.
v
=
v0+v
2
,知AB段和BC段的平均速度相等,两段位移相等,所以运动时间相等.故B正确.

C、重力做功与首末位置的高度差有关,在两段运动过程中,高度差相同,所以重力做功相等.故C正确.

D、A到B、B到C速度变化量的大小相等,根据△P=m△v,知动量变化的大小相等,但方向不同.故D错误.

故选BC.

阅读理解

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.Instead,you can set an example by following these common airplane courtesies

Always recline your seat slowly.There’s nothing worse than suddenly being slammed 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 position before going to the restroom(or anytime you leave your seat).

Avoid going to the bathroom during mealtime.Wait until the meal is done and all the food trays 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 your-self-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,and don’t allow your children to do so either.

While some people enjoy chatting with other passengers during a flight,not everyone does.Some people may want to nap,read or work.If the conversation seems one—sided,take the hint.

If you are traveling with someone and want to chat,keep your voices low.If using electronic gadgets,keep the volume down.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 next flight,remember that it all boils down to the golden rule.Treat others the way you want to be treated !

72.Which of the following manners we should follow while flying?

A.Always keep your seat upright during meal time.

B.Keep your belongings to yourself as much as possible.

C.Use your headphones and keep the volume down.

D.All the above.

73.Where do we most probably read this passage?

A.a text book                                               B.a newspaper

C.a magazine                                                 D.a science book

74.Which of the following word has the closest meaning with the word courtesies?

A.golden rules            B.manners                  C.experiences             D.passengers

75.What is the best title of this passage?

A.Manners in the sky.

B.Bad manners in the sky.

C.Behave yourself in the sky.

D.Treat others the way you want to be treated.

单项选择题