问题 选择题

物体同时受到同一平面内的三个力作用,下列几组力中,它们的合力不可能为零的是(  )

A.5N 7N 8N

B.2N 3N 5N

C.1N 5N 10N

D.1N 10N 10N

答案

三个力合力为0时,则任意两个力的合力与第三个力大小相等,方向相反,由此可知,任意一个力在另外两个力的合力范围.

A、5N和7N的合力范围为:2N-12N,8N在合力范围里,故三个力的合力可能为0;故A错误

B、2N和3N的合力范围为:1N-5N,5N在合力范围里,故三个力的合力可能为0;故B错误

C、1N和5N的合力范围为:4N-6N,10N不在合力范围里,故三个力的合力不可能为0;故C正确

D、1N和10N的合力范围为:9N-11N,10N在合力的范围里,故三个力的合力可能为0.故D错误

故选:C.

阅读理解

While only about 200 astronauts have had the excitement of looking down at Earth and commanding systems on the International Space Station, actually thousands of everyday middle­school students have experienced by Middle School Students, or EarthKAM, a camera system.

EarthKAM is an educational activity and outreach(扩展)investigation that also results in remote Earth sensing and observation. Using the Internet, the students control a special digital camera fixed aboard the space station, enabling them to photograph the Earth's coastlines, mountain ranges and other geographic items of interest from the unique advantage point of space.

While EarthKAM offers a powerful way for students to investigate Earth from the unique perspective of space, it is also inspiring the next generation of flight controllers for space programs—involving university students to control and operate the camera system and related activities from the ground.

In the interview, Brion Au, one of the investigation developers at NASA's Johnson Space Centre, said,“EarthKAM is a payload(有效载荷)by students, for students. They are in charge. This system provides a viewpoint that the astronauts have...it's just awe­inspiring!”So far, students have captured more than 40,000 photos of the Earth from the space station as it orbited the Earth once every 90 minutes, traveling at 17,500 miles an hour. The team at EarthKAM posts these photographs online for the public and participating classrooms around the world to view.

Au explains that this education investigation is inspiring students to explore the world by examining Earth, while promoting social studies, art, geography, science, technology, and math, among other important lessons involving research and teamwork. EarthKAM was started by Dr. Sally Ride, originally flying on the shuttles. The camera is located in the window Observational Research Facility, also known as the WORF, one of many the station's research facilities.

小题1:How have thousands of students experienced similar excitement as astronauts?

A.In the space station.

B.In the spaceship.

C.Using a camera system.

D.Teaming up with astronauts.小题2:What equipment should the students need for the research?

A.Computer.

B.Recorder.

C.Telephone.

D.Radios.小题3:What doesn't belong to the next generation of flight controllers for space programs?

A.University students' controlling the camera system.

B.University students' operating the camera system.

C.University students' related activities from the ground.

D.University students' investigating Earth from space.小题4:Who are responsible for EarthKAM?

A.The astronauts.

B.The students.

C.The investigation developers.

D.The teachers.小题5:What's the main idea of the last paragraph?

A.DR.Sally Ride explains the details of EarthKAM

B.Why the team at EarthKAM posts these photographs online.

C.EarthKAM provides a viewpoint that the astronauts have.

D.Brion Au explains the details of EarthKAM.

填空题