首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   291篇
  免费   2篇
  国内免费   2篇
教育与普及   3篇
理论与方法论   4篇
现状及发展   40篇
研究方法   53篇
综合类   160篇
自然研究   35篇
  2020年   1篇
  2016年   1篇
  2015年   1篇
  2014年   1篇
  2013年   9篇
  2012年   14篇
  2011年   58篇
  2010年   9篇
  2009年   1篇
  2008年   17篇
  2007年   28篇
  2006年   19篇
  2005年   27篇
  2004年   25篇
  2003年   15篇
  2002年   26篇
  2001年   4篇
  2000年   2篇
  1999年   2篇
  1998年   2篇
  1996年   2篇
  1994年   1篇
  1992年   2篇
  1990年   3篇
  1988年   1篇
  1985年   2篇
  1983年   1篇
  1978年   1篇
  1975年   2篇
  1974年   1篇
  1971年   2篇
  1970年   5篇
  1969年   4篇
  1968年   3篇
  1967年   1篇
  1966年   1篇
  1965年   1篇
排序方式: 共有295条查询结果,搜索用时 15 毫秒
191.
The first proposition of the Principia records two fundamental properties of an orbital motion: the Fixed Plane Property (that the orbit lies in a fixed plane) and the Area Property (that the radius sweeps out equal areas in equal times). Taking at the start the traditional view, that by an orbital motion Newton means a centripetal motion – this is a motion ``continually deflected from the tangent toward a fixed center' – we describe two serious flaws in the Principia's argument for Proposition 1, an argument based on a polygonal impulse approximation. First, the persuasiveness of the argument depends crucially on the validity of the Impulse Assumption: that every centripetal motion can be represented as a limit of polygonal impulse motions. Yet Newton tacitly takes the Impulse Assumption for granted. The resulting gap in the argument for Proposition 1 is serious, for only a nontrivial analysis, involving the careful estimation of accumulating local errors, verifies the Impulse Assumption. Second, Newton's polygonal approximation scheme has an inherent and ultimately fatal disability: it does not establish nor can it be adapted to establish the Fixed Plane Property. Taking then a different view of what Newton means by an orbital motion – namely that an orbital motion is by definition a limit of polygonal impulse motions – we show in this case that polygonal approximation can be used to establish both the fixed plane and area properties without too much trouble, but that Newton's own argument still has flaws. Moreover, a crucial question, haunted by error accumulation and planarity problems, now arises: How plentiful are these differently defined orbital motions? Returning to the traditional view, that Newton's orbital motions are by definition centripetal motions, we go on to give three proofs of the Area Property which Newton ``could have given' – two using polygonal approximation and a third using curvature – as well as a proof of the Fixed Plane Property which he ``almost could have given.' (Received August 14, 2002) Published online March 26, 2003 Communicated by G. Smith  相似文献   
192.
In this study, we test the security of a crucial plank in the Principia’s mathematical foundation, namely Newton’s path leading to his solution of the famous Inverse Kepler Problem: a body attracted toward an immovable center by a centripetal force inversely proportional to the square of the distance from the center must move on a conic having a focus in that center. This path begins with his definitions of centripetal and motive force, moves through the second law of motion, then traverses Propositions I, II, and VI, before coming to an end with Propositions XI, XII, XIII and this trio’s first corollary. To test the security of this path, we answer the following questions. How far is Newton’s path from being truly rigorous? What would it take to clarify his ambiguous definitions and laws, supply missing details, and close logical gaps? In short, what would it take to make Newton’s route to the Inverse Kepler Problem completely convincing? The answer is very surprising: it takes far less than one might have expected, given that Newton carved this path in 1687.  相似文献   
193.
194.
195.
Communicated by C. Wilson  相似文献   
196.
Zusammenfassung Nachweis, dass der DNS-Gehalt der Dottergranula in Hühnerembryonalzellen in der frühesten Entwicklungsperiode (0–18 h) temporär ansteigt.

The scanning electron microscope was made available by Analytica, Sollentuna and the scanning micrograph was taken by Mr.G. Alsterborg.

This work has been supported by the Swedish Natural Science Research Council (H.E.) and Kungliga Fysiografiska Sällskapet, Lund.  相似文献   
197.
As the Earth warms, many species are likely to disappear, often because of changing disease dynamics. Here we show that a recent mass extinction associated with pathogen outbreaks is tied to global warming. Seventeen years ago, in the mountains of Costa Rica, the Monteverde harlequin frog (Atelopus sp.) vanished along with the golden toad (Bufo periglenes). An estimated 67% of the 110 or so species of Atelopus, which are endemic to the American tropics, have met the same fate, and a pathogenic chytrid fungus (Batrachochytrium dendrobatidis) is implicated. Analysing the timing of losses in relation to changes in sea surface and air temperatures, we conclude with 'very high confidence' (> 99%, following the Intergovernmental Panel on Climate Change, IPCC) that large-scale warming is a key factor in the disappearances. We propose that temperatures at many highland localities are shifting towards the growth optimum of Batrachochytrium, thus encouraging outbreaks. With climate change promoting infectious disease and eroding biodiversity, the urgency of reducing greenhouse-gas concentrations is now undeniable.  相似文献   
198.
199.
200.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号