问题 问答题

如图11-3-8所示,一质量为M的无底木箱,放在水平地面上,一轻质弹簧一端悬于木箱的上边,另一端挂着用细线连接在一起的两物体A和B,mA=mB=m,剪断A、B间的细线后,A做简谐运动,则当A振动到最高点时,木箱对地面的压力是多少?

图11-3-8

答案

Mg

本题考查简谐运动的特点及物体受力情况的分析.剪断细线前A的受力情况:

重力:mg,向下;细线拉力:F=mg,向下;弹簧对A的弹力:F="2" mg,向上.此时弹簧的伸长量为Δx==.

剪断细线后,A做简谐运动,其平衡位置在弹簧的伸长量为Δx=处,最低点即刚剪断细线时的位置,离平衡位置的距离为,由简谐运动的特点知最高点离平衡位置的距离也为,所以最高点的位置恰好在弹簧的原长处,此时弹簧对木箱作用力为零,所以此时木箱对地面的压力为Mg.

单项选择题
阅读理解

How many coins have you got in your pocket right now? Three? Two? or One?

With a phonecard you can make up to 200 calls without any change at all.

1. What do you do with it?

Go to a telephone box marked “Phonecard”. Put in your card to start, make your call and when you have finished, a screen tells you how much is left on your card.

It costs no extra for the cards, and the calls cost 10p per unit, the same as any other pay-phone call.

You can buy them in units of 10, 20, 40, 100 or 200.

2. Now appearing in a shop near you

Near each phonecard place you will find a shop where you can buy one. They’re at bus, train and city tube stations.

At many universities, hospitals and clubs, restaurants and gas stations on the highway and shopping centers.

At airports and seaports.

3. No more broken payphones

Most broken payphones are like that because they’ve been damaged. There are no coins in a cardphone to excite thieves’ interest in it. So you’re not probably to find a broken one.

Get a phonecard yourself and try it out. Or get a bigger wallet.

小题1:The passage is most probably          .

A.a warning

B.a notice

C.an advertisement

D.an announcement小题2:There are three sections in the passage. Which one do you think is about why phonecards are good?

A.Section 1

B.Section 2.

C.Section 3.

D.none.小题3:Which statement of the following is right by inference(推断)?

A.Using a phonecard will cost you less money than payphone call.

B.Phonecards are easier to carry.

C.If a thief steals your phonecard, he can’t use it any more.

D.When you finish your call, take out your card first and then you will see how many calls you can still make.