问题 多选题

如图所示,汽车在拱形桥上由A匀速率运动到B,以下说法正确的是(  )

A.牵引力与克服摩擦力做的功相等

B.牵引力和重力做的功大于克服摩擦力做的功

C.合外力对汽车不做功

D.重力做功的功率会变化

答案

A、汽车运动过程中,牵引力做正功设为WF,摩擦力做负功其大小设为Wf,重力做负功其大小设为WG,支持力不做功,

根据动能定理得:

WF-Wf-WG=0,故A错误;

B、牵引力、重力和摩擦力三个力总功为0,牵引力和重力做的总功等于克服摩擦力做的功,故B错误;

C、根据动能定理得:汽车由A匀速率运动到B的过程中动能变化为0,所以合外力对汽车不做功,故C正确;

D、重力的大小方向不变,但是汽车的速度方向时刻变化,因此根据P=Fv=mgv,可知速度在重力方向上的分量越来越小,所以重力的功率是变化的,故D正确;

故选CD.

单项选择题
阅读理解

Why does the rate of heart attacks increase during the World Cup football finals? How can we help an overweight patient to lose weight? They’re just some of the topics covered in a new book by University of Sussex academics, which is helping student doctors to consider the importance of psychology in medical practice.

Psychology for Medicine is the first textbook of its kind, providing medical trainees and new doctors with all the relevant psychological knowledge they need. Previously, students had to refer to many different books for the relevant psychology.

The book, by Sussex psychologists Dr. Sussex Ayers and Dr. Richard de Visser, has been well received by fellow academics and medical doctors and was finished with the help of the students and staff of the Brighton and Sussex Medical School. The students proofread(校对) the chapters, provided ideas for the content and even modeled for the photographs. The book provides a solid grounding in psychological study relevant to medicine, along with practical tips and advice for practice. One student, Simon Hall, drew cartoons for the book.

The study psychology is important for doctors for a number of reasons. Psychological and physical symptoms are highly related. Up to a third of patients will have psychological disorders, while physical causes are usually only found in around 15 per cent of people’s symptoms. In treatment, a lot of the effect drugs can be due to patients believing they will recover rather than the drug itself.

Dr. Ayers says, “The important thing about this book is that it’s applied science --- it shows why psychology is important to medicine and how we should use it. The book’s presentation is really important. It’s not just lots of theory, but full of tips and advice so that students can take what they learn in the classroom and apply it to real-life situations.”

小题1:The book Psychology for Medicine is mainly written for ________.

A.scientists

B.overweight people

C.patients

D.student doctors小题2:What is the third paragraph mainly about?

A.The topics of the book.

B.The comments on the book.

C.The contributors to the book.

D.The characteristics of the book.小题3:According to the text, the book Psychology for Medicine _________.

A.is a bit difficult to understand.

B.is mainly about how to lose weight.

C.included tips on training football players.

D.focuses on the importance of psychology in treatment小题4:We can infer that the text is a(n) ______.

A.research report

B.introduction to a book

C.advertisement for a book

D.introduction to a psychologist