问题 问答题

如图所示,倾角为30°的光滑斜面的下端与水平地面平滑连接(可认为物体在连接处速率不变).一个质量为m的小物体(可视为质点),从距地面h=3.2m高处由静止沿斜面下滑.物体与水平地面间的动摩擦因数为0.4,重力加速度g=10m/s2,求:(1)物体沿斜面下滑的加速度大小;

(2)物体由静止沿斜面下滑,到达斜面底端的速度大小;

(3)物体在水平地面滑行的时间.

答案

(1)物体由静止沿斜面下滑,加速度为a1,由牛顿运动定律有  mgsinθ=ma1

解得  a1=5m/s2                       

(2)物体由静止沿斜面下滑,经时间t1到达斜面底端速度为v1

 

h
sinθ
=
1
2
a
t21

解得,t1=1.6s

v1=a1t1=8m/s              

(3)物体滑到地面作匀减速运动,加速度大小为a2

根据μmg=ma2,a2=4m/s2

经时间t2减速到零,则t2=

v 1
a2
=2s

答:

(1)物体沿斜面下滑的加速度大小是5m/s2

(2)物体由静止沿斜面下滑,到达斜面底端的速度大小是8m/s;

(3)物体在水平地面滑行的时间是2s.

阅读理解

阅读短文,完成下列各题。

      Snow fell on the mountain. It snowed and snowed. The snow did not melt (融化). It became deep and

heavy. The snow on the bottom pressed (挤压) together, it became ice.

      The ice was very wide and thick. It began to move down the mountain. It was like a river of ice. It was a

glacier (冰河).

      Sometimes the glacier moved only a few inches (英寸) each day. As it moved, it took rocks and dirt with it.

It changed the land. In some places, it left hills. In some places, when the glacier melted, it made rivers and

lakes.

      A million years ago, there were many big glaciers. Glaciers covered many parts of the world. The glaciers

changed the land.

      Glaciers are still at work today. A glacier in the north of Canada is cutting a new path (路) down the side of

a mountain. This glacier will change the land, too.

1. The snow that fell on the mountain               .[ ]

A. became snowman

B. melted

C. became ice

D. turned to rain

2. The word in the story that means a river of ice is ________.

3. The story says, "The snow did not melt. It became deep and heavy". The word "it" means              .

4. Which of the following does this story lead you to believe? [ ]

A. There are not as many glaciers as there used to be

B. Glaciers do not change the land as they move over it.

C. Glaciers are found only in warm places.

5. How fast did the glacier move? (Which sentence is exactly like the one in your book?)[ ]

A. Sometimes the glacier moved only once a year.

B. Sometimes the glacier moved over ten miles each day.

C. Sometimes the glacier moved only a few inches each day.

6. The main idea of the whole story is that _______. [ ]

A. snow is heavy

B. the high land never changes

C. glaciers changed the land

名词解释