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问题 选择题

如图所示，a和b是地球的两个卫星，则下列说法中正确的是（　　）

A．卫星A的线速度小于卫星B的线速度

B．卫星A的角速度速度小于卫星B的角速度

C．卫星A的周期小于卫星B的周期

D．卫星A的向心加速度小于卫星B的向心加速度

答案

根据万有引力提供向心力得出：

GMm
r2
=m
v2
r
=ma=mω2r=m
4π2r
T2

A、v=

GM
r
，卫星A的轨道半径小于B的轨道半径，所以卫星A的线速度大于卫星B的线速度，故A错误

B、ω=

GM
r3
，卫星A的轨道半径小于B的轨道半径，所以卫星A的角速度速度大于卫星B的角速度，故B错误

C、T=2π

r3
GM
，卫星A的轨道半径小于B的轨道半径，所以卫星A的周期小于卫星B的周期，故C正确

D、a=

GM
r2
，卫星A的轨道半径小于B的轨道半径，所以卫星A的向心加速度大于卫星B的向心加速度，故D错误

故选C．

单项选择题

How to protect children Web fans from unsuitable material on-line while encouraging them to use the Internet has long been discussed in the US.
For some parents, the Internet can seem like a jungle, filled with danger for their children. But jungles contain wonders as well as dangers and with good guides, some education, and a few precautions, the wilds of the Internet can be safely navigated. "Kids have to be on-line. If we tell our kids they can’t have access to the Internet, we’re cutting them off from their future, " said an expert.
Most kids have started to use search engines. Many of them are great for finding tons of interesting Internet sites, and they can also locate places where you might not want your kids to go. There are search engines designed just for kids. A certain software contains only sites that have been selected as safe. The most popular way to limit access would be to use what is known as a "content screener". But this can’t be wholly reliable, and the best thing parents can do is to talk to their kids and let them know what is OK or not OK to see or do on the Internet. Another way is that mum or dad is nearby when the child is surfing the Internet.
A few other tips
·Don’t put the PC in a child’s room but keep it in an area where mum or dad can keep an eye on things. That also makes the Internet more of a family activity.
·Ask your child what he or she has been doing and about any friends they make on-line.
·Tell your child not to give on-line strangers personal information, especially like address and phone number.
·And tell your children never to talk to anyone they meet on-line over the phone, send them anything, accept anything from them or agree to meet with them unless you go along.

According to the passage, we can infer that ______.

A. soft wares fit for children want programming
B. a child who is on-line is in danger
C. Internet is a jungle full of danger
D. Internet contains a lot of harmful sites
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判断题

含N有机污染物在水体中的转化过程实际上是从一种污染方式向另一种污染方式转化。
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