问题 选择题

如图所示,在某空间同时存在着相互正交的匀强电场E匀强磁场B电场方向竖直向下,有质量分别为m1,m2的a,b两带负电的微粒,a电量为q1,恰能静止于场中空间的c点,b电量为q2,在过C点的竖直平面内做半径为r匀速圆周运动,在c点a、b相碰并粘在一起后做匀速圆周运动,则(  )

A.a、b粘在一起后在竖直平面内以速率

B(q1+q2)
m1+m2
r做匀速圆周运动

B.a、b粘在一起后仍在竖直平面内做半径为r匀速圆周运动

C.a、b粘在一起后在竖直平面内做半径大于r匀速圆周运动

D.a、b粘在一起后在竖直平面内做半径为

q2
q1+q2r的匀速圆周运动

答案

粒子b受到的洛伦兹力提供向心力,有

q2vB=m2

v2
r

解得

v=

q2Br
m2

两个电荷碰撞过程,系统总动量守恒,有

m2v=(m1+m2)v′

解得

v′=

m2
m1+m2
v=
q2Br
m1+m2

整体做匀速圆周运动,有

r′=

(m1+m2)v′
(q1+q2)B
=
q2
q1+q2
r

故选D.

阅读理解

(E)

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BOWLING

Ten-pin bowling is old-school fun for all ages.Besides the 100 bowling lanes for which it is named,Gongti 100 has ping-pong rooms,kiddy rides, a games arcade(拱廊)and cheap beer. 8 Gongti Xilu,010-65522688

SKATING

Get in some practice for the outdoor skating season at one of the capital’s ice-rinks.Le Cool on the ground floor of China World Mall(6505-5776)offers 90 min sessions for 30 yuan during the week.and 50 yuan on weekends,including skate hire. On a budget? Go retro at New Trena Rollerskatillg World,Baishiqiao Lu Haidian(6218-4225).Girls pay 5 yuan on weekdays and 7 yuan on weekends,while guys pay 8 yuan on weekdays and 15 yuan on weekends.

MARTIAL ARTS

Want to release that pent-up aggression and get fit in the process? Black Tiger Gym has classes in many Thai kickboxing and Brazilian jiujitsu, taught by world champions. Suite 906,Bldg 9,Jianwai SOHO,Chaoyang District(131 2042 9724)

HOT YOGA

This is t11e workout guaranteed to make you sweat, no matter how cold the weater is. Bikram hot yoga is a series of 26 postures practiced in a heated room.Fine-Yoga teachers Hot yoga and other styles at 2 studios.SOHO New Town(85896414)and Pinochle Plaza(139 1120 9563).

MUSEUM

CuIture vultures should drop by Beijing Capital Museum(010—63370491),where a major

exhibition of ancient Chinese Painting show is currently on show.9 am to 4 pm and 20 yuan for the ticket.Take Metro line 1 and get off at Muxidi station.

72.What does the underlined word in the first paragraph probably mean?

A.hitting something with the foot.    B.relaxation.

C.feeling of pleasure.D.giving up.

73.How many outdoor activities are mentioned in the passage?

A.5.    B.4.    C.2.     D.1.

74.In_______you don't have to exercise much.

A.BOWLING    B.SKATING  C.MARTIAL ARTS     D.MUSEUM

75.Where does this passage probably appear?

A.website.    B.textbook.    C.science magazine.D.travel guide.

填空题