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Outlook
As we discussed in the introductory chapter of this book, friction played
an important role in ancient and medieval physics, somewhat similar to
the force of inertia. These two forces—inertia and friction—cannot exist
without each other. To put it simply, if friction existed without inertia, noth-
ing could move, whereas if inertia existed without friction, nothing could
stop. However, Aristotle’s Physics lacked a correct understanding of both
friction and inertia. Aristotle believed that a body keeps moving only when
a force is applied and that velocity is proportional to the applied force. He
also believed that straightforward and ...