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The phenomenon of gravitation is different from the contact forces encountered in earlier chapters in that it is capable of acting over a distance to produce forces without direct contact between the affected objects. This behaviour, called 'action at a distance', was a matter of considerable philosophical debate in Newton's time. In physics, this issue is addressed by the introduction of the model of a force field.

The term 'force field' has many other uses in modern English language (recall Star Trek, for example) but its use in physics is different and highly specific, namely

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field of force
n. pl. fields of force
A region of space throughout which the force produced by an agent or several
agents, such as an electric charge, is operative. Also called force field.
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The American Heritage Dictionary of the English Language, Fourth Edition copyright 2000
by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.


Wherever one encounters a force field, the concept of a field strength at a point is usually introduced. This is a (vector) physical quantity of a much more abstract nature than other quantities previously encountered and it may take you some time to become familiar enough with it to use the concept confidently. Nevertheless, it turns out to be a very useful tool and repays the time and effort required, not least because very similar concepts arise again in the study of electric fields (Chapter 16) and magnetic fields (Chapter 18).

Note that the physical quantity 'field strength' is often defined as 'field' in textbooks.  This usage is not recommended by IUPAP and other standards authorities and is not used in Understanding Physics, where the term field is confined to that in the definition above.