Attention strengths from

To nurture strengths from attention in their only child, parents can make sure to diversify the kinds of attention they give, each of the great variety signifying a different cause for valuing the child s worth. A few of the more common kinds include ... 

Let us fix attention on a particular H20 molecule A in the interior of water (if we wish to identify this molecule we can suppose that it contains a nucleus of the oxygen isotope 01S) and let us consider the water molecules which happen to be nearest neighbors of this molecule at the moment. These molecules have been in contact with A for different lengths of time. Since all the molecules in the liquid wander about, there was a time when none of these molecules was in contact with A. Further, if we could now begin to watch these molecules, we should find that, after the lapse of different periods of time, they become separated from A and each is replaced by another molecule. Similar remarks can be made about the molecules which come into contact with any chosen molecule. We can now raise the question—-What is the rate of turnover of this process The rate depends on the degree of local order and disorder, which in turn depends on the strength and character of the forces between adjacent molecules. 

For these reasons, attention has been paid to energy criteria as a measure of toughness. The following points need to be noted. Materials do not reach their theoretical strength (that is of their primary chemical bonds), because of the presence of minute flaws. Stress is concentrated at these flaws and so is enhanced. In effect this amounts to a weakening of the material. Under load, cracks propagate from these flaws and lead to failure. 

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