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    It's the holiday season, the time when we connect with family and friends. Social media sites like Twitter and Facebook are full of festive pictures, featuring parents' catching and sharing those special family moments, their child's wish list, and maybe even a cute video of their child dancing to “Jingle Bell Rock” while wearing a diaper (尿片) and Santa hat. Swelling with pride, parents can't wait to get approval with a “thumbs up” or better yet a personalized message on their treasured post.

    Adults should be able to post what they want online. However, when exposing family moments online, are they sharing too much information? Do parents have the right to share those cute now, but embarrassing later moments about kids? Have children willingly given their agreement to sharing their cute but funny video online?

    A recent study found that 75% of parents turn to social media for parenting-related information and social support. There is even a term used to describe the overuse of sharing too much information about kids on social media: “sharenting”. Research also finds that “sharenting” isn't going anywhere anytime soon. What's troublesome is that a typical parent has about 150 Facebook friends and only a third of them are actual friends. So that brings up good questions – Who are we really sharing our information with and why? Who knows when and where that photo could resurface in the future?

    While there's no reliable information on how young children feel about things posted online, we do have information about how teens feel. According to a report by the Family Online Safety Institute, 76% of teens are concerned about their privacy. Many teens constantly search for new apps that allow anonymity. When names are required, they use screen names that don't reveal real information. If our teens are doing a better job of protecting themselves online, shouldn't parents take the lead and do the same? Plus, with more and more college admission representatives and potential employers surfing the internet for potential candidates, we'd hate for one of our posts to change an important decision. Think about it … online reputations are now becoming inseparable with real life ones.

    Of course, we can secure our privacy settings, only allowing our friends to view pictures, posts and videos, but that doesn't stop others from uploading our pictures. Adults need to be cautious of sharing information online, especially information about children.

    So, this holiday season, enjoy family time and share those special memories with family and friends. Before clicking the app to upload photos or videos, stop and think twice.

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太空中的绿色动力﹣﹣太阳帆

    ①太阳光传送光和热,照到人身上,人会感到暖洋洋的,但从来也没有人感觉到太阳

    光有压力。实际上,太阳光是有压力的,因为光具有两重性,既是电磁波,又是粒子﹣﹣光子。光线实际上是光子流,当光子流受到物体阻挡时,光子就撞到该物体上,就像空气分子撞到物体上一样,它的动能就转化成对物体的压力。

    ②不过,太阳光产生的压力﹣﹣光压是非常非常小的。不仅人感受不到,就连普通的仪器也测不出来。在地球附近,太阳光照射到一个平整、光亮、能完全反射光的表面时,产生的压力最大,大约是9×10﹣6牛/平方米,也就是说100万平方米平整光亮的面积上才受到9牛的压力,只相当于一个2分硬币的重量。在地面上,由于重力、大气压力、空气阻力、摩擦力等力的存在,微乎其微的太阳光压力被淹没在这些宏观力的汪洋大海之中。

    ③“山中无老虎,猴子称大王。”到了太空中,重力、大气压力、空气阻力、摩擦力等几乎完全消失,太阳光压才有了出头之日。一些具有创新思维的人开始想到利用太阳光压来推动航天器在太空飞行。早在上一世纪初,俄罗斯宇航理论先驱齐奥尔科夫斯基就提出过这一大胆的设想。以后,又有不少科学家进行过研究。然而,只有当科学技术发展到今天的水平,在有强大的火箭把航天器送入太空的条件下,利用太阳光作为航天推进力才有了实现的可能。

    ④太阳光压的大小是与接受太阳照射的面积成正比的。受照面积越大,产生的压力越大。为了获得一定的压力,必须有足够大的受照面积,从而引出了太阳帆的概念。

    ⑤太阳帆是一种面积很大,表面平整、光滑、无斑点和皱纹的薄膜,一般由聚酯或聚酰亚胺等高分子材料制成,表面镀铝或银,使其具有全反射的特性。

    ⑥一块面积为105×105平方米的太阳帆,在太阳光正射下可获得大约100毫牛的力,用它推动100千克的物体,可产生1毫米/平方秒的加速度。这个加速度极其微小,只有地面重力加速度的一万分之一。

    ⑦俗话说:“涓涓细流汇成大海,块块碎土堆成高山。”速度等于加速度与时间的乘积,尽管加速度非常小,只要时间足够长,终能达到一定的速度。即使航天器的加速度只有1毫米/平方秒,那么,一天以后,速度达到86.4米/秒;一个月后达到2592米/秒;130天后,就可超过第二宇宙速度,达到11.23千米/秒;一年后可达到31.54千米/秒,足以飞出太阳系,由此可见加速度不在大,时间长则灵。