Abstract
This text-book follows ordinary lines. The author does well to direct the attention of the beginner at the outset to the fact that weight and mass are by no means the same thing. Newton's laws are given almost unchanged in words, the second being altered to “rate of change of momentum is proportional to the impressed force, &c.” The word rate is, in strictness, ambiguous, since it does not necessarily imply time-rate; and the explanation (p. 33) that “rate of change of momentum” means the change of momentum in the unit of time is not quite accurate, since the unit of time may be an hour or a week. The poundal figures a great deal; but, happily, as a rule, the values of forces are given in gravitation measure in the answers. The antiquated and inaccurate terms power and weight are used in the discussion of machines, although power has been very properly defined as time-rate of doing work. The old method of defining the instantaneous value of a variable angular velocity as “the number of unit angles which would be described in the unit of time, if during that unit the angular velocity remained the same as at the instant under consideration” is adhered to; but this definition defines nothing. The author is commendably clear in his warning to the student that “centrifugal force” is not a force acting on a revolving body. In the discussion of projectiles, the eye is not pleased by the sight of “u sin at–1/2gt2” for ut sin α–1/2gt2; and it is just possible that a beginner may (by the inscrutable ingenuity for error which students sometimes exhibit) misunderstand the expression altogether.
Elementary Dynamics.
By W. M. Baker. Second edition, revised. Pp. x + 318. (London: George Bell and Sons, 1905.) Price 4s. 6d.
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Elementary Dynamics . Nature 73, 245 (1906). https://doi.org/10.1038/073245a0
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DOI: https://doi.org/10.1038/073245a0