下列说法正确的是（ ）
A．加速度增大，速度一定增大
B．有加速度的物体，其速度一定增加
C．物体的加速度不断减小，速度一定越来越小
D．速度很大的物体，加速度可能很小
A、加速度是描述速度变化快慢的物理量，加速度大表示速度变化快，
而速度增大还是减小要看加速度方向与速度方向的关系，故A错误
B、有加速度的物体，如果加速度方向与速度方向相反，那么速度就减小，故B错误
C、物体是加速还是减速是看加速度的方向与速度的方向相同还是相反，相同就加速，反之就减速，与加速度增大还是减小无关，故C错误
D、物体的速度大，但不一定变化，变化也不一定快，加速度不一定大．故D正确．
故选D．
| 阅读理解。 | |
1. To be a good teacher,you need some of the gifts of a good actor:you must be able to hold the attention and interest of your audience; you must be a clear speaker,with a good, strong,pleasing voice which is fully under your control；and you must be able to act what you are teaching in order to make its meaning clear. 2. Watch a good teacher,and you will see that he does not sit unmoved before his class:he stands the whole time he is teaching; he walks about,using his arms,his hands and fingers to help him in his explanations,and his face to express feeling.Listen to him,and you will hear the loudness,the quality and the musical note if his voice always changes according to what he is talking about. 3. The fact that a good teacher has some of the gifts of a good actor doesn't mean that he will indeed be able to act well on the stage,for there are very important differences between the teacher's work and the actor's.The actor has to speak words which he has learnt by heart; he has to repeat exactly the same words each time he plays a certain part, and even his movements and the ways in which he uses his voice are usually fixed beforehand. What he has to do is to make all these carefully learnt words and actions seem natural on the stage. 4. A good teacher works in quite a different way.His audience takes an active part in his play:they ask questions,they obey orders,and if they don't understand something,they say so.The teacher therefore has to suit his act to the needs of his audience.He cannot learn his part by heart,but must invent it as he goes along. 5. As a good teacher presently,you must take your audience as your friends,take care of them,help them and give them enough freedom and space.I have known many teachers who are fine actors in class but are unable to take part in a stage play because their brains can't keep discipline:they cannot keep strictly to what another has written. |