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 ... "
ARISTOTLE , 384-322 BCE