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Motion , Inertia , and Friction

One of the great scientific breakthroughs of the 17th century was to understand how and why things move and stop moving . The key to the problem is that objects have inertia , which means that they slow down and stop only when something , typically friction , forces them to do so .

For almost 2,000 years people subscribed to Aristotle's assertion that , unless things are continually propelled onwards by some force , they soon slow down naturally to rest . It seemed only common sense . Yet there were problems with this view . How , for example , does an arrow continue in its flight long after it has left the bow ? In the Middle Ages the Islamic scholar Avicenna and the French priest Jean Buridan spoke of moving objects having " impetus " . At first sight , this idea seems close to the modern idea of momentum - the natural tendency of an object in motion to keep on moving - but for Avicenna and Buridan , impetus was an active internal motive force pushing things onwards against the natural tendency to slow down .

For a long time , even thinkers such as Aristotle were misled about the nature of motion by common - sense experience .


Aristotle argued that things only keep moving because they are continually pushed or pulled by an external force and slow down as soon as the force is removed , just as a cart slows down when the ox stops pulling .

" ... A body in motion

can maintain this

motion only if it

remains in contact

with a mover ... "


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