题目

如图所示，水平绝缘轨道AB与处于竖直平面内的半圆形绝缘光滑轨道BC平滑连接，半圆形轨道的半径R=0.40m．轨道所在空间存在水平向右的匀强电场，电场强度E=1.0×104 N/C．现有一电荷量q=+1.0×10﹣4C，质量m=0.10kg的带电体（可视为质点），在水平轨道上的P点由静止释放，带电体运动到圆形轨道最低点B时的速度vB=5.0m/s．已知带电体与水平轨道间的动摩擦因数μ=0.50，重力加速度g=10m/s2 ． 求： （1） 带电体运动到圆形轨道的最低点B时，圆形轨道对带电体The home of the future won’t be completely different and we will be living in houses and flats just as we do today. But people will want to shape their homes to match their dreams. No two homes will be the same. People will be able to buy “house kits” containing a basic house structure, with movable walls, doors and windows. They will put together the different parts to create the home they want.        Many jobs that we do today will disappear, others will still exist but will change and new jobs will be created. Skilled workers such as builders, gardeners and electricians won’t disappear because machines can’t replace them. Teachers will still exist because students need human contact. But they will be using modern technology in class more and students will be working more from home. The medical technology revolution and space travel will create new jobs which we can only imagine today.        Space holidays will develop in the future, but these holidays won’t be for everyone because they won’t be cheap. Short space trips will develop first, then space hotels will orbit the earth where it will be possible to have a longer vacation. By the end of the next century, there will be holiday centres on the moon with leisure facilities for families.        Paper won’t exist in the future. Instead, there will be e-paper which people will be able to use over and over again. This will develop in order to save natural resources. E-newspapers and e-magazines will replace traditional newspapers and magazines and we will download information and news articles from the Internet every day onto our reuseable paper.        The laws of physics tell us that the earth is going to disappear some time in the future. This isn’t going to happen tomorrow but scientists predict that it will happen in five billion years when our sun explodes（爆炸）.We will have to explore the universe and find another home. At some point in the distant future, either we stay on the earth and die with it, or we leave and move to another planet. There won’t be any other choice. 60．Homes of the future will ________.        A．be completely different from those of today.        B．be very similar to our homes        C．all be different form one another                         D．be movable as you want 61．Space trips and staying in space hotels will _________.        A．become a very common way to spend a holiday        B．be the cheapest holiday option for families        C．attract a lot of people        D．still only be for very rich people 62．E-paper will replace traditional paper because _________.        A．we will use it again  B．it won’t waste natural resource        C．it will be cheaper to produce   D．it will be convenient to carry 63．We will have move to another planet ________.        A．if we want to save the human race  B．when the sun explodes        C．when the earth disappears       D．when the earth is too crowded to fill people on