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Section 5.1.1

Topic Sentences

State the central idea of a paragraph in a topic sentence. The other sentences should elaborateon the topic sentence with relevant information and evidence. Topic sentences are a key element in developing unified and coherent paragraphs.

Although the topic sentence may appear anywhere in the paragraph, it is most often the firstsentence. A topic statement may also contain a transitionalelement from the previous paragraph.

In the first paragraph of the next example, the topic sentence is the first sentence, stating thesubject. However, a paragraph can also lead up to the topic sentence, as in the secondparagraph. When a topic sentence concludes a paragraph, it receives extra emphasis and canalso serve as a summary or conclusion.

Time became a key word in the language of physics during the seventeenthcentury. Isaac Newton wove the passage of time directly into his equations, as inforce = mass × acceleration. Today, it is difficult for any physicist toexamine the universe without thinking of time in mich the same way as the illustrious Briton didmore than 300 years ago. Most of the laws of physics continue to be written in the style ofNewton; they are designed to show how things change from one moment to the next. Each eventunder study, such as the path of a ball thrown into the air or the thermodynamics of a meltingice cube, is broken down into a series of freeze-frames that, run like a movie, show how natureworks.

Newton had placed a clock upon the mantel of the universe. This Newtonian timepiece tickedand ticked, chiming like some cosmic Big Ben, in step with all the celestial inhabitants, no matterwhat their speed or position. That meant that a clock situated at the edge of the universe orzipping about the cosmos at high velocities would register the same passage of time, identicalminutes and identical seconds, as an earthbound clock. More important, the Newtonian clockwas never affected by the events going on around it. Time was aloof and absolute, alike for allas galaxies collided, solar systems formed, and moons orbited planets. Time led anindependent existence, separate from nature itself.

--Marcia Bartusiak,"When the Universe Began, What Time Was It?" TechnologyReview

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