问题 选择题

如图,木块放在木板AB上,B端固定,现使木板从水平面绕B端逆时针缓慢转动,木块开始相对木板静止,后沿木板下滑,在此过程中(  )

A.木块所受的静摩擦力逐渐增大

B.木块所受的滑动摩擦力逐渐增大

C.木块所受的静摩擦力方向沿斜面向下

D.木块对木板的压力逐渐增大

答案

A、木块开始相对木板静止,对物体进行受力分析:

其中f为静摩擦力.

将重力分解,由于木板从水平面绕B端逆时针缓慢转动,所以物体可以认为是处于平衡状态.

根据平衡条件得:

沿斜面方向:静摩擦力f=mgsinα,方向沿斜面向上.

由于木板从水平面绕B端逆时针缓慢转动,所以α逐渐增大,所以木块所受的静摩擦力逐渐增大.故A正确.

B、后沿木板下滑,对物体进行受力分析:

其中f为滑动摩擦力.

根据垂直斜面方向平衡条得:N=mgcosα

根据滑动摩擦力公式得:f=μN=μmgcosα

由于α逐渐增大,所以木块所受的滑动摩擦力逐渐减小,故B错误.

C、根据A选项分析,故C错误.

D、根据A、B选项分析知道斜面对木块的支持力N=N=mgcosα,由于α逐渐增大,斜面对木块的支持力N在减小.

根据牛顿第三定律得木块对木板的压力逐渐减小,故D错误.

故选A.

完形填空

Life comes in a package, which includes love and hate, happiness and sorrow, failure and success, hope and despair. Life is a learning process. Experiences in life teach us new lessons and make us a better person. With each passing day we learn to handle various situations.

• Love and hate

Love plays a key role in our life. Love makes us feel wanted. In the early stage of our life, our parents shower us with unconditional love and care. They teach us about what is right and wrong, good and bad. 小题1: Only after we get married and have kids can we understand and become sensitive to others, feelings. On the contrary, hate can not solve any problems, but make things worse. Thus, we’d better hate less and love more.

•   Happiness and sorrow

Material happiness is short-lived, but happiness achieved by bringing a smile to others gives a certain level of fulfillment. 小题2:____ No mind is happy without peace. We realize the true worth of happiness when we are in sorrow. Sorrow is basically due to death of a loved one, failure or despair. But these things are temporary and will pass away anyhow.

•   Failure and success

Failure is the path to success. It helps us to touch the sky, teaches us to survive and shows us a specific way. Success brings in money, fame, pride and self-respect. 小题3:____ The only way to show our gratitude to God for giving success to us is by being modest, polite and respectful to the less fortunate ones.

•   Hope and despair

Hope is what keeps life going. Hope makes us dream and builds patience and confidence in us. Life teaches us not to despair even in the darkest hour because after every night there is a day. Nothing remains the same.

小题4:____

Life teaches us not to regret over yesterday, for it has passed and is beyond our control. Tomorrow is unknown , for it can either be bright or dull. 小题5:___

A. Every defeat is just a small part of victory.

B. Peace of mind is the main link to happiness.

C. But we always tend to take this for granted.

D. Happiness is always what we want to achieve.E.  It's important to keep our head on our shoulder.

F.  The alternative is to work hard today to enjoy a better tomorrow.

G.  We have only one choice — keep moving on with hope in life.

问答题 简答题