如图所示，x轴上M点和N点的坐标分别是(0.5 m，0)、(2 m，0)，虚线MM′与y轴平行。在y轴和MM′之间加垂直纸面方向的匀强磁场，在N点固定一个正的点电荷 $Q=+1\times {10}^{-4}C$ 。现有一个质量 $m=3\times {10}^{-7}kg$ 、电荷量的大小 $q=1\times {10}^{-4}C$ 的粒子，从y轴上某点沿x轴的正方向以初速度 $v_0=1\times {10}^{4}m/s$ 进入磁场，在第四象限里一直做匀速圆周运动，最后从x轴上的Q点（没画出）射入第一象限，不计带电粒子的重力，粒子在磁场中运动时就不受在N点的固定点电荷影响。已知静电力常量k=9×10⁹ N·m²/C²。求：

    It was a cold night in Washington, D. C., and I was heading back to the hotel when a man approached me. He asked if l would give him some money so he could get something to eat. I'd read the signs "Don't give money to beggars." So I shook my head and kept walking.

    I wasn't prepared for a reply, but he said, "I really am homeless and I really am hungry! You can come with me and watch me eat!" But I kept on walking.

    The incident bothered me for the rest of the week. I had money in my pocket and it wouldn't have killed me to hand over a buck or two even if he had been lying. Flying back to Anchorage, I couldn't help thinking of him. I tried to rationalize (找借口)my failure to help by thinking government agencies, churches and charities were there to feed him. Besides, you're not supposed to give money to beggars.

    Somewhere over Seattle, I started to write my weekly garden column for The Anchorage Daily News. Out of the blue, I came up with an idea. Bean's Cafe, the soup kitchen in Anchorage, feeds hundreds of hungry Alaskans every day. Why not try to get all my readers to plant one row in their gardens dedicated to Bean's? Dedicate a row and take it down to Bean's. Clean and simple.

    The idea began to take off. Readers would fax or call me when they got something in their garden. Those who only grew flowers donated them. Food for the spirit.

    In 1995, the Garden Writers Association of America held their annual convention in Anchorage and after learning of Anchorage's program, Plant a Row for Bean's became Plant a Row for the Hungry. The original idea was to have every member of the Garden Writers Association of America write or talk about planting a row for the hungry sometime during the month of April.

    As more and more people started working with the Plant a Row idea, new changes appeared unexpectedly. Many companies gave free seed to customers and displayed the logo, which also appeared in national gardening publications. Row markers with the Plant a Row logo were delivered to gardeners to set apart their "Row for the Hungry."

    Garden editor Joan Jackson, supported by The San Jose Mercury News and California's nearly year-round growing season, raised more than 30,000 pounds of fruits and vegetables her first year, and showed GWAA how the program could really work. Texas fruit farms donated food to their local food bank after being inspired by Plant a Row. Today the program continues to thrive and grow.

    I am surprised that millions of Americans are threatened by hunger. If every gardener in America--and we're seventy million strong--plants one row for the hungry, we can make quite a decrease in the number of neighbors who don't have enough to eat. Maybe then I will stop feeling guilty about abandoning a hungry man I could have helped.