问题 问答题

如图所示,一光滑的半径为R的半圆形轨道底部固定竖直放置在水平面上,一个质量为m的小球以某一速度冲上轨道,然后小球从轨道口B处飞出,最后落在水平面上,已知小球落地点C距B处的距离为3R.求:小球对轨道口B处的压力为多大?

答案

设小球经过B点时速度为v0,则:

小球平抛的水平位移为:

x=

BC2-(2R)2
=
(3R)2-(2R)2
=
5
R,

小球离开B后做平抛运动,

在水平方向:v=vt,

在竖直方向上:2R=

1
2
gt2

解得:v=

5gR
2

在B点小球做圆周运动,由牛顿第二定律得:

F+mg=m

v2
R
,解得F=
1
4
mg,

由牛顿第三定律,小球对轨道的压力F′=F=

1
4
mg.

答:小球对轨道口B处的压力为

1
4
mg.

单项选择题

D

Love, success, happiness, family and freedom—how important are these values to you Here is one interview which explores the fundamental questions in life.

Question: Could you introduce yourself first

Answer: My name is Misbah, 27 years old. I was born in a war-torn area. Right now I’m a web designer.

Q: What are your great memories

A: My parents used to take us to hunt birds, climb trees, and play in the fields. For me it was like a holiday because we were going to have fun all day long. Those are my great memories.

Q: Does your childhood mean a lot to you

A: Yes. As life was very" hard, I used to work to help bring money in for the family. I spent my childhood working, with responsibilities beyond my age. However, it taught me to deal with problems all alone. I learnt to be independent.

Q: What changes would you like to make in your life

A : If I could change something in my life, I’ d change it so that my childhood could have taken place in another area. I would have loved to live with my family in freedom. Who cares whether we have much money, or whether we have a beautiful house It doesn’t matter as long as I can live with my family and we are safe.

Q: How do you get along with your parents

A : My parents supported me until I came of age. I want to give back what I’ ve got. That’ s our way. But I am working in another city. My only contact with my parents now is through the phone, but I hate using it. It filters(过滤) out your emotion and leaves your voice only. My deepest feelings should be passed through sight, hearing and touch.

How would Misbah prefer to communicate with his parents()

A. By chatting on the Internet.

B. By calling them sometimes.

C. By paying weekly visits.

D. By writing them letters.

单项选择题