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在水平桌面上，用弹簧测力计沿水平方向拉着物体，当弹簧测力计示数为6N时，该物体做匀速直线运动，此时物体所受的合力为______N，这时当弹簧测力计的示数突然减小为4N时，该物体受到的摩擦力大小为______N，合力为______N，最终物体静止在地面上受到2N的拉力，所受的合力为______N．

答案

（1）第一空：物体做匀速直线运动，拉力与摩擦力二力平衡，拉力大小为6N，方向水平向前，摩擦力大小也为6N，方向水平向后，合力大小为二力之差，所以是0N；

（2）第二空：拉力大小不影响动摩擦力的大小，影响动摩擦力大小的因素没变，所以拉力由6N变为4N时，动摩擦力的大小不变，仍为6N；

（3）第三空：当拉力为4N时，摩擦力仍为6N，合力大小为：F合=f-F拉=6N-4N=2N，方向与摩擦力方向相同；

（4）第四空：物体静止时，所受水平拉力与摩擦力二力平衡，大小相等，拉力为2N，方向水平向前，摩擦力也为2N，方向水平向后，合力大小为二力之差，所以也是0N．

故答案为：0；6；2；0．

阅读理解
阅读理解。

     A desert is a very dry land where there is hardly any rain. In the day, it can be so hot that you
could fry an egg on the ground. But during the night it is sometimes so could that water would turn
into ice.
    Many desserts are covered in sand. But some have mountains, rocks, or lakes that have dried up
in the heat. The Great American Dessert has cliffs (悬崖) that have been made into amazing and
beautiful shapes by the wind. The Sahara Desert in Africa is mostly sand. The Australian Desert has
red sand.
     Sometimes it does not rain for a whole year in the desert. But there is water deep under the ground
and some plants have roots that go very deep to find it. Other plants have roots that spread out (延伸)
a long way so they can suck up the rain as soon as it falls.

     One desert plant is the cactus, which has long, spreading roots. The cactus stores water inside
its hard skin. This skin protects it from heat and cold.

     Camels are a bit like cactus plants! They store water too-inside their bodies. They have special fur
which protects them from the sun during the day and keeps them warm at night. 
     Some other animals live in deserts, too. They have to be very good at seeing and hearing because
most of them only come out at night. This is why many desert animals have extra big eyes or ears.

     A desert fox has bigger ears than a normal for. A gerbil has huge eye, so that it can see at night.
It stays cool in the day by staying underground, like most desert animals.

     Some people live in the desert but they always make their homes near an oasis. An oasis is where
water comes from an underground river. Near an oasis, people can grow lots of plants and keep
animals. 
     Life is hard in the desert, for people, animals and plants. But desert people like to live there.
1. A desert is very dry because _________.
A. it is sandy
B. there is so little rain
C. the wind is strong
D. there is no water deep under the ground
2. What does the underlined phrase "suck up" in paragraph 3 mean _________?
A. 围堵
B. 讨好
C. 释放
D. 吸收
3. Some desert animals have extra big eyes or ears because _________.
A. the sunlight is too strong in the day
B. they need to see and hear well at night
C. it is very cold at nigh
D. they store water in them.
4. Most desert animals stay cool in the day by _________.
A. storing water in side their bodies
B. drinking water as much as they can
C. staying underground
D. growing lots of plants
5. Which of the following is true?
A. Desert people like to live in the desert though life is hard.
B. People in the desert usually fry eggs on the ground.
C. Camels have special fur to store water.
D. All the deserts are covered in sand.
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解答题
已知中心在原点的双曲线的一条渐近线方程是y=-
3
2
x，焦距为2
13
，求此双曲线的标准方程．
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