题干

如图所示,bacd为导体做成的框架,其平面与水平面成θ角,质量为m的导体棒PQabcd接触良好,回路的电阻为R,整个装置放于垂直框架平面的变化磁场中,磁感应强度B的变化情况如图乙所示,PQ能够始终保持静止,则0~t2时间内,PQ受到的安培力F和摩擦力Ff随时间变化的图象可能不正确的是(取平行斜面向上为正方向)(  )

A:

B:

C:

D:

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B

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    I was at my parent's dinner table. Before me was a worn journal of thin and discolored pages. It was my grandfather's journal and now belonged to my father. My grandfather had passed away in the months leading up to my birth. I never got to visit the places he had frequently and people who had been a part of his life's journey.

    I was now about to enter his world, through the words he had left behind. Within minutes, I was attracted by the power of the written words. In the magical script before me, I was transported to another age when food was an everyday art, planned, prepared and enjoyed in the company of others, and a time when people had the heart to pause,their own lives to embrace (拥抱)each other's struggles. All this was conveyed to me in the beauty of the words that flowed together to connect with the writer's mind and understand the world they lived in.

    That kind of writing seems to be lost on us today. We have gotten used to writing in bite-sized pieces for a public looking for entertainment, and hungry for information. No wonder, there are nearly 200 million bloggers on the Internet and a new blog is created somewhere in the world every half a second. Instead of adding to our collective wisdom, most of these writings reflect the superficiality (肤浅)and impatience of our day and age.

    This not only robs us of the skill of writing impressive essays, it also prevents us from exploring what is truly important. Writing humbles us in a way that is vital for our character growth, by reminding us about the limits of the self and our appropriate place in the vast flow of life. Writing frees us by helping us explore the unknown so that we truly open up to the magic of the world around us. I saw all of this in the writing s of my grandfather. And I've seen it again and again in the writings of the greatest thinkers of humanity. Their writings reflect deep thought on issues of human importance.