问题 问答题

如图所示,一根原长为L0的轻质弹簧,下端固定在水平桌面上,上端固定一个质量为m的物体A,A静止时弹簧的压缩量为△L1,在A上再放一个质量为m的物体B,待A、B静止后,再在B上施加一个竖直向下的力,力使弹簧又缩短了△L2,这时弹簧的弹性势能为EP.现突然撤去力F.求:

(1)撤除力F以后B脱离A向上飞出的瞬间弹簧长度为多少?要有分析过程.

(2)B脱离A时B的速度大小为多少?

答案

(1)分离瞬间加速度相同,相互作用力为零,而此时B物体只受重力,加速度为重力加速度,故A物体加速度也为重力加速度,弹簧长度为原长L0

(2)从撤除力到A、B分离,系统机械能守恒,则有:

EP=2mg(2△L1+△L2)+

1
2
•2mv2

解得:v=

EP
m
-2g(2△L1+△L2)

答:(1)撤除力F以后B脱离A向上飞出的瞬间弹簧长度为原长L0

(2)B脱离A时B的速度大小为

EP
m
-2g(2△L1+△L2)

单项选择题

(A)

Since the 1970s, scientists have been searching for ways to link the brain with computers. Brain-computer interface (BCI) technology could help people with disabilities send commands to machines.

Recently, two researchers, Jose Millan and Michele Tavella from the Federal Polytechnic school in Lausanne, Switzerland, demonstrated (展示) a small robotic wheelchair directed by a person’ s thoughts.

In the laboratory, Tavella operated the wheelchair just by thinking about moving his left or right band. He could even talk as he watched the vehicle and guided it with his thoughts.

"Our brain has billions of nerve ceils. These send signals through the spinal cord (脊髓) to the muscles to give us the ability to move. But spinal cord injuries or other conditions can prevent these weak electrical signals from reaching the muscles," Tavella says. "Our system allows disabled people to communicate with external world and also to control devices."

The researchers designed a special cap for the user. This head cover picks up the signals from the scalp(头皮) and sends them to a computer. The computer interprets the signals and commands the motorized wheelchair. The wheelchair also has two cameras that identify objects in its path. They help the computer react to commands from the brain.

Prof. Millan, the team leader, says scientists keep improving the computer software that interprets brain signals and turns them into simple commands. "The practical possibilities that BCI technology offers to disabled people can be grouped in two categories: communication, and controlling devices. One example is this wheelchair."

He says his team has set two goals. One is testing with real patients, so as to prove that this is a technology they can benefit from. And the other is to guarantee that they can use the technology over long periods of time.

Which of the following shows the path of the signals described in Paragraph 5 ()

A. scalp→computer→cap→wheelchair

B. computer→cap→scalp→wheelchair

C. scalp→cap→computer→wheelchair

D. cap→computer→scalp→wheelchair

单项选择题