问题 计算题

如图所示,倾角为θ=30°的斜面固定在地面上,物体A与斜面间的动摩擦因数为μ=,轻弹簧下端固定在斜面底端,弹簧处于原长时上端位于B点,开始时物体A到B的距离为L=1 m,现给A一个沿斜面向下的初速度v0=2 m/s,使物体A开始沿斜面向下运动,物体A将弹簧压缩到最短后又恰好被弹回到B点,取g=10 m/s2(不计空气阻力),求:

(1)物体A第一次运动到B点时的速度大小;

(2)弹簧的最大压缩量。

答案

(1)3m/s;(2)0.9m

题目分析:(1)物体A由开始运动直至B点的过程,由动能定理得 mgLsinθ-μmgLcosθ=-

解得,代入数值解得:vB=3m/s

(2)设弹簧最大压缩量为x.在物体A刚好接触弹簧直至恰好返回到B点的过程中,由动能定理得

-2μmg x cosθ=0-,解得:x=0.9m

默写题
阅读理解

A simple piece of clothesline hangs between some environmentally friendly Americans and their neighbors.

On one side stand those who see clothes dryers (干衣机) as a waste of energy and a major polluter of the environment.As a result, they are turning to clotheslines as part of the “what-I-can-do environmentalism.”

On the other side are people who are against drying clothes outside, arguing that clotheslines are unpleasant to look at.They have persuaded Homeowners Associations (HOAs) access the U.S.to ban outdoor clotheslines, because clothesline drying also tends to lower home value in the neighborhood.

This has led to a Right-to-Dry Movement that is calling for laws to be passed to protect people’s right to use clotheslines.So far, only three states have laws to protect clothesline.Right-to-Dry supporters argue that there should be more.

Matt Reck, 37, is the kind of eco-conscious(有生态意识的)person who feeds his trees with bathwater and reuses water drops from his air conditioners to water plants.His family also uses a clothesline.But on July 9, 2007, the HOA in Wake Forest, North Carolina, told him that a dissatisfied neighbor had telephoned them about his clothesline.The Recks paid no attention to the warning and still dried their clothes on a line in the yard.“Many people say they are environmentally friendly but they don’t take matters into their own hands,” says Reck.The local HOA has decided not to take any action, unless more neighbors come to them.

North Carolina lawmakers are saying that banning clotheslines is not the right thing to do.But HOAs and housing businesses believe that clothesline drying reminds people of poor neighborhoods.They worry that if buyers think their future neighbors can’t even afford dryers, housing prices will fall.

Environmentalists say such worries are not necessary, and in view of global warming, that idea needs to change.As they say, “The clothesline is beautiful.Hanging clothes outside should be encouraged.We all have to do at least something to slow down the process of global warming.”

小题1:One of the reasons why supporters of clothes dryers are trying to ban clothesline drying is that ______.

A.clothes dryers are more efficient

B.clothesline drying reduces home value

C.clothes dryers are energy-saving

D.clothesline drying is not allowed in most U.S.states小题2:Who are in favor of clothesline drying?

A.Housing businesses.

B.Environmentalists.

C.Homeowners Associations.

D.Reck’s dissatisfied neighbors.小题3:What is mainly discussed in the text?

A.Clothesline drying: a way to save energy and money.

B.Clothesline drying: a lost art rediscovered.

C.Opposite opinions on clothesline drying.

D.Different varieties of clotheslines.