问题 填空题

如图是小明自制的一个电磁跷跷板.小狗、小猫站在一个可绕光滑转轴的轻质杆上.它的工作过程是:当处于如图位置时,两触点断开,电磁铁失去磁性,在小狗重力作用下,小狗端下降,反复上述过程.

 

(1)如果小猫、小狗站立点离转轴的距离分别是7cm和5cm,小猫的质量为1Kg,则该跷跷板要能正常工作,小狗的重力应略大于  ______ N;(取g=9.8N/kg)

(2)如果该跷跷板电路连接正确、两触点接触良好,工作一段时间后,跷跷板不能正常工作,可以改进的方法是  ______________________________ (只要写出一点即可)

答案

13.72、将小狗模型适当向左移动(或提高电源的电压,或适当增加小猫模型的质量等)

题目分析:(1)已知:小猫的力臂为,小狗的力臂为,小猫的重力为

要使跷跷板能正常工作,则应使小狗的重力与力臂的乘积大于或等于小猫的重力与力臂的乘积,即

则解得:

即小狗模型的重力应略大于13.72N;

(2)跷跷板不能正常工作说明左侧小猫模型及磁力和力臂的乘积小于小狗的力及力臂的乘积,无法将左端压下,故可以增大左侧力及力臂的乘积或减小右侧力及力臂的乘积;方法有:将小狗模型向左移动,减小右侧力臂;增大电压提高线圈中的电流,从而增大磁力;增加小猫模型的质量,从而增大压力;以上任选一个回答即可.

点评:本题为力电综合类题目,考查学生利用杠杆的平衡条件进行分析计算的能力及运用所学知识分析问题的能力;在审题时要注意挖掘有用信息.

选择题
阅读理解

阅读理解。

     Passport Control

     If you are arriving at London Heathrow Airport and are not transferring (转换) another flight outside

Britain or Northern Ireland, you must pass through Passport Control and Customs (海关) immediately

after leaving your plane. If you are not British or a citizen of the European Community, you must fill out

a special form before your passport is examined. This form is called a landing card and should be given

to you during the flight to London. After landing, follow the ARRIVALS signs. Make sure you are in the

right channel when you reach Passport Control. There is one channel for holders of European Community

passports, and a second channel marked

     "Other Passports".

      Baggage Reclaim (行李领取) Area

     After passing through Passport Control, follow the signs to the Baggage Reclaim Area. If you have

luggage which was carried in the aircraft along with other cargo, it will be delivered to you in the Baggage

Reclaim Area. Look for the sign with your flight number. If you have only hand luggage, go directly through

the Baggage Reclaim Area to Customs.

     Customs

     All passengers must pass through Customs after Passport Control. There is a choice of two channels,

Green and Red. If you have nothing to declare, go through the Green Channel. If you are not sure about

your Duty Free allowances (免税额), or if you have something to declare, go through the Red Channel.

Information about Duty Free allowances can be found on special notice-boards in the Baggage Reclaim

Area. You should study this information carefully whether you are going through the Red Channel or not.

Please note that if you go through the Green Channel, you may be stopped and asked to open you luggage

for inspection.

  

1. When a Chinese traveler is arriving at Heathrow Airport for a trip in London, he must ______.

A. fill out a landing card first

B. pass through Customs before Passport Control

C. walk through the Red Channel

D. immediately reclaim his baggage after landing

2. Those who are not sure about their Duty Free allowances _______.

A. can check the information in the Baggage Reclaim Area

B. need to go through the Green Channel

C. will be asked to open the luggage for inspection

D. can do as those who have nothing to declare

3. Which of the following travelers DO NOT have to go through Passport Control and Customs?

A. People traveling straight to London.

B. Holders of European Community passports.

C. Passengers transferring to a city in Ireland.

D. Passengers only with some hand luggage.