| The American Heritage® Book of English Usage. |
A Practical and Authoritative Guide to Contemporary English. 1996.
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Page 152
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| kinds of cement and concrete used today are similar to those used by the ancient Romans. After the Roman Empire fell in the fifth century, people lost the art of making cement and it was not rediscovered until the mid-1700s. Cement, from Latin caementum, “rough-cut stone,” is made from limestone and clay that is crushed, heated, and ground into a powder. It is mixed with water and materials such as sand, gravel, and broken stone to make concrete. Cement and water form a paste that binds the other materials together as the concrete hardens, a process reflected in the etymology of the term itself. Concrete is a combination of the Latin prefix com-, “together,” and crescere, “to grow.” Reinforced concrete is made by pouring concrete around steel bars. Without this added strength, the building of modern skyscrapers would be impossible. Prestressed concrete is made by pouring concrete around steel cables stretched by jacks. When the jacks are released, the cables compress the concrete and strengthen it. | 1 |
centrifugal / centripetal / inertia
| | The word centrifugal is used to describe any feature that appears to “flee the center of,” while centripetal is used to describe any feature that appears to “seek the center of.” These terms have been used to describe the growth of flower petals and the transmission of nerve impulses. However, the most common use of centrifugal and centripetal is to describe the forces present in circular motion. | 2 |
| A centrifugal force is a force on an object that tends to move it away from a center of rotation and always results from the inertia of the object. Inertia is the property of an object proportional to mass that opposes acceleration. A centripetal force is a force on an object that tends to move it toward a center of rotation and can be a result of gravitation, electricity, or any other naturally occurring force. For example, if you hold a bucket of water by the handle and spin in a circle, you can feel the bucket pull your arms away from the center of your rotation. This is the centrifugal force and is a result primarily of the inertia of the bucket and the water. The effort that it requires to hold onto the bucket and keep spinning is the source of the centripetal force. You can “feel” the centrifugal force in this example, and you can observe (due to the circular motion) that a centripetal force is present. When Apollo 8 orbited the Moon in 1968, the centripetal force was provided by the gravitational attraction between the spacecraft and the Moon. The centrifugal force, which opposed gravity in this example, was provided by the inertia of the spacecraft and the astronauts. Because both gravity and inertia are proportional to the mass of an object, the astronauts could not “feel” either the centripetal or centrifugal force. This cancellation of effects that are proportional to mass is the source of weightlessness when in outer space. | 3 |
| More at
mass / weight and
revolve / rotate. | 4 |
Centigrade / Celsius / Kelvin
| | The history of naming temperature intervals can be considered a history of removing the ambiguity introduced by the word degree. In 1742, Anders Celsius, a Swedish astronomer, proposed dividing up the temperature interval between the boiling point of water and the | 5 |
| The American Heritage® Book of English Usage. Copyright © 1996 by Houghton Mifflin Company. All rights reserved. |
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