问题 问答题

在水平地面上有一质量为2kg的物体,物体在水平拉力F的作用下由静止开始运动,10s后拉力大小减为F/3,该物体的运动速度随时间t的变化规律如图所示.求:

(1)物体受到的拉力F的大小.

(2)物体与地面之间的动摩擦因素.(g取10m/s2)

答案

(1)        (2)

设物体所受滑动摩擦力为,物体在O~10s的运动过程中,

由牛顿第二运动定律得

    ①                              

根据v—t图象及运动学公式得

    ②                                   

物体在10s~14s的运动过程中,由牛顿第二运动定律得

    ③                             

根据v一t图象及运动学公式得

    ④                                  

由①、②、③、④式求得

拉力    ⑤                              

又       ⑥                            

动摩擦因数    ⑦

阅读理解

阅读理解。

    When we think of Hollywood, we think of movies and famous movie stars. They are part of Hollywood's

history. Today people make movies in other places too. Not all famous movie stars live in Hollywood. But

Hollywood is still a very special city in Los Angeles, California.

    You can easily see where Hollywood is in Los Angeles. There is a big sign on the hills. It says

"HOLLYWOOD". The white letters are fifty feet tall. You can see the sign from far away. The Hollywood

sign is a famous landmark in Los Angeles. Many postcards show this famous Hollywood landmark.

    In the hills of Hollywood, there is also the Hollywood Bowl. This is an open-air theatre. It is one of the

largest open-air theatres in the world. It has seventeen thousand seats and a very special stage. The stage

was designed by the great American architect (建筑师) Frank Lloyd Wright. You can listen to all kinds of

concerts at the Hollywood Bowl.

1. Today Hollywood is ______.[ ]

A. a special city in Los Angeles

B. a place where all the movie stars live

C. a place where they make all the movies

D. the biggest city in California

2. You can see where Hollywood is in Los Angeles because ______.[ ]

A. it is a hill

B. postcards show you

C. there is a big sign on the hills

D. local people will tell you

3. The Hollywood Bowl is ______. [ ]

A. a place where you see movies

B. one of the largest open-air theatres in the world

C. a hill in Hollywood

D. a big sign in Los Angeles

4. Which is not mentioned about the Hollywood Bowl? [ ]

A. Seats.

B. Size.

C. Stage.

D. Designer.

5. What special things can we see in Hollywood? [ ]

A. The Hollywood hills.

B. The Hollywood landmark.

C. The Hollywood Bowl.

D. Both B and C.

名词解释