题干

如图所示,已知一带负电的滑块从粗糙斜面的顶端静止下滑,滑至底端时的速率为。若加上一个垂直纸面向外的匀强磁场,,同样让它从粗糙斜面的顶端静止下滑,则它滑至底端时的速率与相比(    )

A:变大

B:变小

C:不变

D:条件不足,无法判断

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答案(点此获取答案解析)

B

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      Held on the second Wednesday of March each year, No Smoking Day helps people across the UK who want to stop smoking. The activity encourages and helps smokers to give up, and publicizes(宣传) the many sources of help offered to people. Each year over a million smokers take part in the day, while about 40,000 actually give up smoking.
      The much-publicized effects of smoking include increased risk (风险) of heart disease, lung cancer and asthma. However, smoking also causes dull skin and wrinkles. The reducedamount of oxygen getting to the skin accelerates (加速)aging, and makes the skin look sallow (病黄色的). And while many smokers believe that cigarettes help them to cope with stress (对付压力), scientific research has shown that worry levels actually go down after giving up smoking.
    Set a date and time to stop, and keep to it .Talk to your doctor about whether any kind of medicines might suit you. Find out if there is a stop-smoking health center near you .Try to get the help of your friends and family. Keep away from situation in which you used to reach for cigarette. If you need to, use nicotine patches (尼古丁戒烟贴片) or gum.
    NHS Stop-smoking Centers are all over the UK, providing free services. They have specially trained employees who can advise you on the best way to stop smoking. There are also a number of free national helpline in England, Scotland, Wales, and Northern Ireland, where expert counselors can discuss with you how to give up smoking step by step.