问题 问答题

如图所示,竖直放置的半径为R的光滑圆环上,穿过一个绝缘小球,小球质量为m,带电量为q,整个装置置于水平向左的匀强电场中.今将小球从与环心O在同一水平线上的A点由静止释放,它刚能顺时针方向运动到环的最高点D,而速度为零,则电场强度大小为多大?小球到达最低点B时对环的压力为多大?若欲使此小球从与环心O在同一水平线上的A点开始绕环做完整的圆周运动,则小球的初动能至少为多大?

答案

小球在运动过程中受到重力、电场力与环的弹力作用,环的弹力对小球不做功;

(1)从A到D的过程中,

由动能定理得:-mgr+qEr=0-0,

解得:E=

mg
q

(2)从A到B的过程中,

由动能定理得:mgr+qEr=

1
2
mv2-0,

解得:v=2

gr

小球在B点做圆周运动,环的支持力与重力的合力提供向心力,

由牛顿第二定律得:F-mg=m

v2
r

解得:F=5mg,

由牛顿第三定律得:小球在B点时对环的压力大小F′=F=5mg;

(3)从A点到最高点(D与A中点)过程中,

由动能定理可得,W+WG=0-Ek0

-mg

2
2
R+mg(R-
2
2
R)=0-EK0

解得:Eko=(

2
-1)mgR

答:(1)电场强度大小为

mg
q

(2)小球在B点时对环的压力大小为5mg.

(3)若欲使此小球从与环心O在同一水平线上的A点开始绕环做完整的圆周运动,则小球的初动能至少为(

2
-1)mgR.

阅读理解

Dallas Children’s Theater Academy

ShowBiz Summer 2008

Rosewood Center for Family Arts

    5938 Skillman * Dallas, TX 75231 * (214) 978-0110

Important Information

* All classes are taught by DCT Theater Professionals!

* All Grade Levels indicated are for Fall 2008.

* Tuition Deposit required for registration.

* Tuition balance due on the first day of class. No tuition refunds (退款).

* DCT reserves the right to cancel any class; refunds made for cancellation.

* Please accompany student to first day of class.

* Registration begins one-half hour before class time.

* Questions? Call Nancy Schaeffer at (214) 978-0110 or e-mail nancy@dct.org

How To Register

MAIL: * Fill out the form in this brochure.

* Be sure to list the class and group that you want & the dates.

* The registration form may be copied.

FAX:  Complete the registration form and fax it to DCT at (214) 978-0118.

PHONE: Call (214) 978-0110.

WEB: Visit www.dct.org, click on Academy Classes

Drama Days! — $175

Entering Grades: Group A: K   Group B: 1st & 2nd   Group C: 3rd –– 5th

Mon. –– Fri.  9: 30 a .m. –– 1 p.m.   June 2 –– June 6

* Day One –– come up with a character, work with your class to plan the play.

* Day Two –– create the situations and scenes for your one-of-a-kind show!

* Day Three –– rehearse (排演) your part.

* Day Four –– pick a costume, get ready!

* Day Five –– SHOWTIME –– invite family and friends to your Friday Showcase!

* Bring a sack lunch and drink each day!

Laugh Out Loud –– $175

Entering Grades: 5+  Mon. –– Fri.  1:30 p.m. –– 5:00 p.m.  June 23 –– June 27  Show Friday, June 27

* Do you like comedy? Try your hand at the world of comedy in the class just for you!

* Learn a comedy sketch from the old masters!

* Tell a joke! Learn why old jokes can be the best!

* Work with your class to put on a one-of-a-kind Comedy Show for family and friends!

Acting –– Just Acting –– $225

Group A: Entering Grades K; Group B: Entering Grades 1st –– 2nd; Group C: Entering Grades 3rd –– 4th; Mon. –– Fri.  9: 30 a .m. –– 2:30 p.m.; Session I: July 28 –– Aug. 1; Session II: Aug. 4 –– Aug. 8; Session III: Aug. 11 –– Aug. 15

* Are you ready to challenge your acting skills and try something new?!

* Start with the characters: Who are you? Be the Star of your Dreams.

* Become the character you’ve always wanted to be.

* Work with your class to create the plot.

* Get ready to present your show for family and friends on the final Friday!

Summer Scenes –– $225

Entering Grades 5th and 6th; Mon. –– Fri. 9: 30 a .m. –– 2:30 p.m.; Session I: July 28 –– Aug.1; Session II: Aug. 4 –– Aug. 8; Session III: Aug. 11 –– Aug. 15

* Do you want to be an actor?

* Improve your skills with instruction from professional actors & directors.

* Rehearse and perform scenes from your favorite plays.

* Develop characters and polish performances!

14. What is the purpose of the text?

A. To attract readers to attend Summer Scenes.

B. To persuade readers to attend ShowBiz Summer 2008.

C. To earn money from the introduction.

D. To teach readers how to register for ShowBiz Summer 2008.

15. If you want to attend Showbiz Summer 2008, you should be aware that ______.

A. tuition must be paid at least before June 2

B. there’ll be no refunds in any case.

C. registration begins half an hour after class time

D. students had better be accompanied on the first day

16. It can be inferred from the text that all the four one-week specials ______.

A. are intended for children aged below 5

B. are all held at   Rosewood  Center   for Family Arts

C. will take place in June or July and last five days

D. need students to bring a sack lunch and drink each day

17. Which of the following classes suits those who want to be actors?

A. Drama Days!                                   B. Laugh Out Loud.

C. Acting – Just Acting.                      D. Summer Scenes.

18. Where is the text most probably from?

A. Part of a brochure.                           B. An entertainment magazine.

C. A newspaper.                                   D. A guide book.

单项选择题