After a hard campaign, Barack Obama won the US president election on November 4. He will become the 44th President of the United States---the first African-American in the country's history.
On the election eve, Obama's grandmother died---only a few days before seeing her grandson's victory. Tears ran down the face of Obama as he spoke about his grandmother. “She taught me about hard work.She was the one who put off buying a new car or a new dress for herself so that I could have a better life. She poured everything she had into me.”
In his first speech as the president, Obama declared to tens of thousands of cheering supporters, “…The road ahead will be long. Our climb will be steep. We may not get there in one year, or even one term, but America---I have never been more hopeful than I am tonight that we will get there. I promise you: We as a people will get there…” The crowd broke out into the same chant(咏唱，吟诵), “Yes, we can!”Those three short words have become the defining phrase of Obama's campaign.
Forty-five years ago, Martin Luther King had a dream of an America where men and women would be judged not by the colour of their skin but by the content of their character. Today the dream has come true.
时刻导线框的上边恰好与磁场的下边界重合(图中位置I)，导线框的速度为
经历一段时间后，当导线框的下边恰好与磁场的上边界重合时(图中位置II)，导线框的速度刚好为零。此后，导线框下落，经过一段时间回到初始位置I。则