如图所示，一矩形线圈在匀强磁场中绕OO′轴匀速转动，磁场方向与转轴垂直，磁场的磁感应强度为B．线圈匝数为n，电阻为r，长为l₁，宽为l₂，转动角速度为ω．线圈两端外接阻值为R的电阻和一个理想交流电流表．求：

如图所示，两根足够长的光滑平行金属导轨MN、PQ间距为l=0.5m，其电阻不计，两导轨及其构成的平面均与水平面成30°角．完全相同的两金属棒ab、cd分别垂直导轨放置，每棒两端都与导轨始终有良好接触，已知两棒质量均为m=0.02kg，电阻均为R=0.1Ω，整个装置处在垂直于导轨平面向上的匀强磁场中，磁感应强度B=0.2T，棒ab在平行于导轨向上的力F作用下，沿导轨向上匀速运动，而棒cd恰好能够保持静止．g取10m/s²，求：

阅读理解

    A handshake is one of the most common ways to greet others, but US President Donald Trump's unusual method has been put under the microscope lately.

    Much like an arm wrestler in a match, the recently-elected leader has a habit of yanking (猛拉) people's hands toward himself during handshakes. And while a typical handshake is only brief, the one between him and Japanese Prime Minister Shinzo Abe on Feb 10 lasted a full 19 seconds.

    Simply having a strong hand is not the explanation here. According to Darren Stanton, a body language expert from the UK, while handshakes are usually an exchange of kindness, Trump uses his as a way to show power and control, regardless of how uncomfortable it makes people. “It is as if to say, 'Hey, I'm in charge, don't mess with me,' ” Stanton told The Independent.

    Apart from Trump's “yank-shake”, there are other ways people display power with their hands. At business talks or political meetings, for example, some people may rotate (旋转) their wrists during handshakes so that their hand ends up on top instead of underneath. Some may squeeze so hard that it leaves the other person's hand in pain.

    According to Stanton, by pulling people into his personal space, Trump is also testing whether they are willing to cooperate with him. “For example, if someone was resistant to being yanked towards him and stood their ground, he would know that he has work to do with them before he got what he wanted,” Stanton told Express.

    This is probably why on Feb 13 when Canadian Prime Minister Justin Trudeau visited the White House, all eyes were on how he was going to handle Trump's handshake. Fortunately, Trudeau managed to avoid the embarrassment by grabbing Trump's shoulder to stop himself being pulled in. Afterward, some Twitter users wrote that this proved Trudeau's strong leadership, with one even calling the moment “one of Canada's greatest victories”.

    Vice magazine summed up the exchange between the two leaders as: “... no regular handshake. This was the first shot in a bloodless war.”

阅读理解

    A new study has discovered that meditation（冥想）and oxygen sport together reduce depression. The Rutgers University study found that this mind and body combination, done twice a week for only two months, reduced the symptoms for a group of students by 40 percent.

    “We are excited by the findings because we saw such a meaningful improvement in both clinically depressed and non-depressed students,” said lead author Dr. Brandon Alderman. “It is the first time that both of these two behavioral ways have been looked at together for dealing with depression.”

    Researchers believe the two activities have an interactive effect in combating depression. Alderman and Dr. Tracey Shors discovered that a combination of mental and physical training (MAP) enabled students with major depressive disorder not to let problems or negative thoughts defeat them.

    Rutgers researchers say those who participated in the study began with 30 minutes of focused attention meditation followed by 30 minutes of oxygen sport. They were told that if their thoughts drifted to the past or the future they should refocus on their breathing, enabling those with depression to accept moment-to-moment changes in attention.

    Shors, who studies the production of new brain cells in the hippocampus—part of the brain involved in memory and learning—says scientists have shown in animal models that oxygen sport exercise keeps a large number of certain cells alive.

    The idea for the human intervention（干预）came from her laboratory studies, she says, with the main goal of helping individuals acquire new skills so that they can learn to recover from stressful life events.

    By learning to focus their attention and exercise, people who are fighting depression can acquire new learning skills that can help them process information and reduce the overwhelming recollection of memories from the past, Shors says.

    “We know these treatments can be practiced over a lifetime and that they will be effective in improving mental health.” said Alderman. “The good news is that this intervention can be practiced by anyone at any time and at no cost.”

一质量为M_B=6kg的木板B静止于光滑水平面上，物块A质量M_A=6kg，停在B的左端，一质量为m=1kg的小球用长为L=0.8m的轻绳悬挂在固定点O上，将轻绳拉直至水平位置后，由静止释放小球，小球在最低点与A发生碰撞后反弹，A在B上滑动，恰好未从B右端滑出，在此过程中，木板获得的最大速度为0.5m/s，不计空气阻力，物块与小球可视为质点，g取10m/s²，求：