如图所示，两块平行金属板M、N竖直放置，两板间的电势差U=1.5×l03V，现将一质量m=1×10-2 kg、电荷量q=4×10-5 C的带电小球从两板上方的A点以V0=4 m/s的初速度水平抛出，且小球恰好从靠近M板上端处进入两板间，沿直线运动碰到N板上的B点，已知A点距两板上端的高度h=0.2m，不计空气阻力，取g=10 m/s2。求：
（1）M、N两板间的距离d；
（2）小球到达B点时的动能Ek。
解：（1）小球进入电场区域前做平抛运动，竖直方向上：vy2=2gh，vy=2 m/s
进入电场后做直线运动，满足：
解得：d=0.3m
（2）从A到B的过程中，应用动能定理有
解得：Ek=0.175 J
根据短文内容，从短文后的选项中选出能填入空白处的最佳选项。 Smiling is a great way to make yourself stand out while helping your body to function better. Smile to improve your health, your stress level, and your attractiveness; 1 We are drawn to people who smile．There is an attraction factor．We want to know a smiling person and figure out what is so good because anxieties can push people away, but a smile draws them in． Smiling changes our mood． Next time you are feeling down, try putting on a smile． 2 Smiling can trick the body into helping you change your mood． Smiling is appealing． When someone is smiling they lighten up the room, change the moods of others, and make things happier．A smiling person brings happiness with them．Smile a lot and you will draw people to you． Smiling relieves stress． Stress can really show up in our faces．Smiling helps to prevent us from looking tired, worn down, and overwhelmed． 3 The stress should be reduced and you’ll be better able to take action． Smiling lowers your blood pressure． When you smile, there is a measurable reduction in your blood pressure．Give it a try if you have a blood pressure monitor at home．Sit for a few minutes and take a reading．Then smile for a minute and take another reading while still smiling．____4____ Smiling lifts the face and makes you look younger． The muscles we use to smile lift the face, making a person appear younger．Don’t go for a face lift, just try smiling your way through the day—you’ll look younger and feel better． 5 Try this test: Smile．Now try to think of something negative without losing the smile．It’s hard．When we smile, our body is sending the rest of us a message that "Life is Good! "
|