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Accurate Clock Pendulums$
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Robert J. Matthys

Print publication date: 2004

Print ISBN-13: 9780198529712

Published to Oxford Scholarship Online: January 2010

DOI: 10.1093/acprof:oso/9780198529712.001.0001

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Bob shape

Bob shape

Chapter:
(p.87) Chapter 13 Bob shape
Source:
Accurate Clock Pendulums
Author(s):

Robert James Matthys

Publisher:
Oxford University Press
DOI:10.1093/acprof:oso/9780198529712.003.0013

The sphere and the vertically oriented cylinder are more repeatable and predictable bob shapes than the more efficient, lower-drag prolate spheroid and football shapes. As a bob shape, prolate spheroids have low air drag for their volume, and give a high Q pendulum. Only football-shaped bobs, that is, bobs with pointed ends (120 degrees included angles) in the direction of travel give a higher Q. A prolate spheroid shape can be obtained by rotating an ellipse 360 degrees around its long axis. The improved repeatability and predictability of spheres and vertically oriented cylinders can be attributed to their not having the rotational alignment uncertainty that is present in the prolate spheroid and football bob shapes.

Keywords:   air drag, bob shape, sphere, vertically oriented cylinder, prolate spheroid, football shape, pendulum, rotational alignment

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