问题 选择题

质量相等的两木块A、B用一轻弹簧栓接,静置于水平地面上,如图a所示.现用一竖直向上的力F拉动木块A,使木块A向上做匀加速直线运动,弹簧始终处于弹性限度内,如图b所示.则从木块A开始运动到木块B将要离开地面的过程中(  )

A.力F大小先增加后减小

B.弹簧的弹性势能先增加后减小

C.木块A的动能和重力势能之和先增大后减小

D.两木块A、B和轻弹簧组成的系统的机械能一直增大

答案

A、弹簧的弹力变化规律是先减小到零后反向增加,由于木块A向上做匀加速直线运动,故开始阶段有:

F+F-mg=ma,后来则有F-F-mg=ma,故F一直增加,故A错误

B、弹簧的弹性势能应该先减小后增加,故B错误

C、木块匀加速向上运动,动能和重力势能均增加,故其机械能增加,故C错误

D、外力F对A、B和轻弹簧组成的系统做正功,由功能关系得,系统的机械能一直增大,故D正确

故选D

阅读理解

Here is a poster on a middle school web site. Read the web-site poster. Then do Numbers 1 to 4.

Going, Going, Gone!

The Haynes Middle School Parent—Teacher Organization

invites you to attend our latest fund(基金)-raiser,

The Fourth Annual(每年一次的) Haynes School Auction(拍卖)!

Saturday, May 10

6:00 pm — 11:00 pm 

in the school hall

6:00 pm — 6:30 pm: All items for auction are previewed.

6:30 pm — 7:00 pm: Silent auction begins.

7:00 pm — 7:30 pm: Highest bidders(出价人) from silent auction are determined.

7:30 pm — 11:00 pm: Main auction begins.

Items up for bid in the silent auction range in value from $ 5.00 to $ 30.00.

Items up for bid in the main auction include the following:

●Airline tickets to a place of your choice

●Weekend getaways at first-class hotels

●Season tickets to the Chicago Bears football game

●$ 50 gift tickets to local gift shops, restaurants, and salons

●Theater tickets to The Merchant of Venice

Don’t miss the boat! Book your tickets today.

Last year, tickets sold out in five days!

Tickets are sold on a first-come, first-served basis.

$ 15.00 per person

All the money from the auction will be given to the Haynes School computer lab.

小题1: Which of the following is not mentioned as being up for bid at the auction?

A.A movie pass to the local cinema.

B.A weekend stay at a hotel.

C.A ticket to a restaurant.

D.Airline tickets.小题2:The more items that are given or bought for auction, _______.

A.the less money that will be charged for the tickets to the auction

B.the more money that can be raised to support the computer lab

C.the faster the ticket will be sold to the auction

D.the higher the value that will be placed on the items in the silent auction小题3:Which of the following is most likely to happen at the year’s auction?

A.The airline tickets will receive higher bids than any other item.

B.All items up for bid will be sold for at least twice their value.

C.More money will be raised this year than in any other year before.

D.Tickets for the event will be sold out in less than one week.小题4:The phrase “first-come, first-served” tells you that _______.

A.people can buy the tickets on the first day only

B.the person who is first to arrive will receive a ticket at no charge

C.food and drinks will be served at the auction

D.tickets are sold in the order of who arrives first to buy them

单项选择题