题目

一物体在以xOy为直角坐标系的平面上运动，其运动规律为x=-2t2-4t，y=3t2+6t(式中的物理量单位均为国际单位)，关于物体的运动，下列说法不正确的是  ( ) A.物体在x轴方向上做匀加速直线运动 B.物体在y轴方向上做匀加速直线运动 C.物体运动的轨迹是一条直线 D.物体运动的轨迹是一条曲线 答案：D 【解题指导】位移对时间的一次导数是速度，速度对时间的一次导数是加速度，求解出x、y两个方向的分运动的速度和加速度后进行合成，得到合速度与合加速度，若两者共线，物体做直线运动。 【解析】位移对时间的一次导数是速度，x=-2t2-4t，y=3t2+6t，故：vx=-4t-4，vy=6t+6；故初速度：v0x=-4 m/s，v0y=6 m/阅读表达Confidence is very important in daily life. It can help you to develop a healthy attitude （态度）.A study shows that the people who are more confident are much happier. They can have more chance to make themselves successful. But how to be more confident? Here are some suggestions:·Speak loud When you are not confident, you can't do well what you want to do. You speak in a voice so low that other people can hardly hear you. Try to speak loud enough so that people can hear you clearly. The high voice can help you become more confident.·Play sports Physical exercise makes you tired but completely relaxed. A strong body helps you be full of confidence.·Encourage yourself Write down a list of things you did during the day to see how many things you have done well. Did you finish your homework? Did you tell a joke that made everybody laugh? Give yourself praise for the good things you've done.·Get rid of （消除） fear Fear comes along with failure （失败）. But it's easy to overcome （克服） if you know that failure is part of your life. Don't hide your head just because you said something stupid last time. Try to start again and believe you can do better.·Pick up a hobby If you like singing, sing as much as you can. In some ways, a hobby can make you outstanding. And it will make you happy and confident.（一）根据短文内容简要回答问题。1.How many suggestions can help you to be more confident?______________________________________________________________2.How can you encourage yourself?______________________________________________________________（二） 将短文中划线的句子译成汉3.___________________________________________________________4.___________________________________________________________（三）请给短文拟一个适当的标题。5.___________________________________________________________