问题 多选题

如图所示,A为一放在竖直轻弹簧上的小球,在竖直向下恒力F的作用下,在弹簧弹性限度内,弹簧被压缩到B点,现突然撤去力F,小球将向上弹起直至速度为零,则小球在上升过程中(不计空气阻力)(  )

A.小球,弹簧和地球组成的系统(包括弹性势能在内)机械能守恒

B.小球速度减为零时,还未脱离弹簧

C.小球机械能逐渐增大

D.小球动能先增大后减小

答案

A、小球上升过程中,只有重力和弹簧的弹力做功,小球、弹簧和地球组成的系统(包括弹性势能在内)机械能守恒.故A正确;

B、小球上升过程为离开弹簧之前做简谐运动,由于推力F不一定等于重力,根据简谐运动的对称性可知,故弹簧恢复原长时,小球速度减为零时,可能脱离弹簧,也可能没有脱离弹簧.故B错误;

C、由于除重力外,其余力做的功等于机械能的变化量;小球上升过程中,除重力外,弹簧的弹力对其做正功,故机械能增加,当小球离开弹簧后,做上抛运动,故机械能守恒,所以小球的机械能先增大后不变,故C错误;

D、小球上升过程受到重力和弹簧的弹力,当弹力大于重力时,小球向上加速,当弹力小于重力后,小球开始减速,还有可能离开弹簧做上抛运动,即小球动能先增大后减小.故D正确;

故选AD

单项选择题

Researchers at the University of Arizona, led by Teresa Cummins, conducted an exploratory study on the online learning modules, designed to supplement hands-on classes taught by local experts and supported by an overview text, to help users increase their understanding of key concepts in the Arizona Master Watershed Steward program. They also sought to determine whether program participants would use the non-compulsory modules.

Their evaluation revealed that module users increased their understanding of key watershed concepts; participants in the evaluation demonstrated a 30 percent increase in content knowledge following module use. Additionally, 70 percent of participants retained this knowledge through a two-month follow-up test.

A follow-up survey showed that approximately half of the participants returned to the online modules on their own; several of these users returned several times and spent multiple hours per session. Comments from the follow-up survey suggested that the users accessed the site either as they had the time or as they needed the information.

Online usage statistics indicated participants continued to visit the site for many months following the modules’ release and advertisement. Though many visits were very brief (a single pageview; only a couple of seconds), visitors with many returns to the site and/or long visits appeared to be working through the modules.

One participant stated, "My brain can only hold so much information; the modules keep information on-hand." Other participants commented that the modules were "more interesting and interactive" than the text material, and a "very effective tool" and "a great resource to the Master Watershed Steward community."

The Arizona Master Watershed Steward program, sponsored by University of Arizona, prepares adults to serve as volunteers in the conservation of water resources and the protection, restoration, and monitoring of their watersheds. The modules were intended to reinforce concepts covered in classes and further engage participants in the learning process.

One key finding was that the modules were not clearly preferred by users over in-person instruction. One participant stated, "For me, they are just another avenue for learning, a supplement or additional reference. I prefer in person and real hands-on learning." Other participants similarly expressed their desire for hands-on and face-to-face interaction. Several participants noted that the usefulness of in-person lectures—was a function of the presenter.

Overall, the exploratory evaluation indicated that the modules were a welcome supplement to the course and were effective in reinforcing key concepts. Participants retained knowledge for several weeks, although since subjects were self-selected, they may have been self-motivated to pay attention and master the online module materials.

"I am constantly searching for new ways to educate our program’s diverse audiences and reinforce watershed science concepts, " says Candice Rupprecht, state coordinator for the Master Watershed Steward program. "I am excited to know that online learning modules can enhance our program by offering additional independent learning opportunities for our volunteers.

From the second paragraph, we can infer that()

A. learners might make no progress without the online modules

B. learners are conscientious in memorizing the content knowledge

C. learners gained improvement in their study by using the online modules

D. 70 percent of program participants took the follow-up test

单项选择题 A1型题