问题 问答题

如图所示,在倾角θ=37°的绝缘面所在空间存在着竖直向上的匀强电场,场强E=4.0×103N/C,在斜面底端有一与斜面垂直的绝缘弹性挡板.质量m=0.20kg的带电滑块从斜面顶端由静止开始滑下,滑到斜面底端与挡板相碰后以碰前的速率返回.已知斜面的高度h=0.24m,滑块与斜面间的动摩擦因数μ=0.30,滑块带电荷q=-5.0×10-4C.取重力加速度g=10m/s2,sin37°=0.60,cos37°=0.80.求:

(1)滑块从斜面最高点滑到斜面底端时的速度大小.

(2)滑块被挡板弹回能够沿斜面上升的最大高度.

(3)滑块从开始运动到停下来的整个过程中产生的热量Q.(计算结果保留2位有效数字)

答案

(1)滑块沿斜面下滑时受到的滑动摩擦力为:f=μ(mg+qE)cos37°

设到达斜面底端时速度大小为v1,重力和电场力做正功,摩擦力做负功,支持力不做功,

根据动能定理:(mg+qE)h-f

h
cos37°
=
1
2
mv12

解得:v1=2.4m/s

(2)滑块第一次与挡板碰撞后返回的高度最大,设此高度为h1,因为滑块偏向上运动所以重力电场力摩擦力都做负功,根据动能定理:

-(mg+qE)h1-f

h1
sin37°
=-
1
2
mv12

解得:h1=0.10m

(3)因为重力在斜面方向的分力大于滑块受到的摩擦力,所以滑块最终静止在斜面底端,因此重力势能和电势能减少和等于克服摩擦力做的功,即等于产生的热量:

Q=(mg+qE)h=0.96J

答:(1)滑块从斜面最高点滑到斜面底端时的速度大小为2.4m/s.

(2)沿斜面上升的最大高度为0.10m.

(3)从开始运动到停下来的整个过程中产生的热量Q为0.96J.

单项选择题
阅读理解

第三部分:阅读理解 (共20小题;每小题2分, 满分40分)

阅读下列短文,从每题所给的四个选项(A、B、C和D)中,选出最佳选项。

When you look at a map of the Caribbean, you will see a bunch of little islands surrounded by. Blue waters, which include places that aren't always household names like Montserrat and Anguilla.

While some of the names may not be familiar, the fact is that the Caribbean is a popular vacation region. Those little clots on the map often turn out to be island paradises. There are more than 7,000 islands, islets, and reefs in the area, many of which are lightly populated and stunningly beautiful.

The biggest challenge you might face in planning your trip is choosing where in the Caribbean to go. All of the nations feature excellent weather. Most feature beautiful beaches. You can enjoy the waters in most of the Caribbean nations and as is often the case with island nations, you will be welcomed warmly by the locals. Here are some resources to help you.

Unless you can walk on water or you cruise the Caribbean, the best way to visit is to fly. Some destinations have more frequent and cheaper flights than others. You should do some comparison shopping on flights to the Caribbean. And if you are planning on flying there during the Christmas holiday weeks it's important to book as early as possible because prices will just keep going up as they get closer.

The Caribbean offers a wide variety of accommodation options. Check out the Caribbean hotels that are available to get a better idea of the various options. If you are on a tight budget then, the Caribbean could be challenging, as there are only a few cheap hotels and hostels that might help you save some money.

Many people see the Caribbean from a cruise ship. There is no shortage of cruises that will take you through the region. Cruise and Vacation Travel Packages is the place to go to find the cruise or vacation package that is right for you.

56. The Caribbean is popular with tourists because ________.

A. the area has a small population             B. the area has a long history.

C. there are many beautiful islands there             D. tire area is very developed

57. The underlined word "they" in the fourth paragraph refers to _________.

A. prices of air tickets                           B. tourists to the Caribbean

C. flights to the Caribbean                  D. the Christmas holiday weeks

58. Which of the following statements is NOT true according to the passage?

A. The weather in all of the Caribbean nations is fine.

B. Most of the Caribbean hotels are cheap.

C. The local people of the Caribbean are very friendly.

D. It is hard to choose a destination in the Caribbean.