Suppose I am pushing a block across a horizontal table.  Once I stop pushing on the block, it quickly comes to a stop.  This situation occurs often enough in the real world – stop pushing or pulling on an object and it comes to rest – that it leads us to think that the most fundamental to divide objects in terms of their motion is into objects that are at rest and objects that are moving.  To keep an object moving, you need a force acting on it, pushing or pulling the object in the direction it moves.

The problem is that, no matter how much this thinking resonates with your real world experiences, it isn’t correct.  In fact, this is probably the most common misconception students have about forces. Culminating with Newton, what natural philosophers (scientists of their day) realized is that the critical distinction is not between objects that are moving and objects that aren’t, but between objects whose motion is changing and objects whose motion isn’t.  Forces are not required to keep things moving. Instead, forces are what cause an object’s motion to change.  As Newton’s makes clear in his Laws of Motion, the difference between objects undergoing uniform motion (motion that doesn’t change) and non-uniform motion (motion that does change) is the presence of a net, external force acting on the object.

So why does our everyday experience lead us to the wrong conclusions about forces and their role in motion?  Because the vast majority of objects that we watch move do so in the presence of retarding forces like friction.  In the case of the block being pushed across a horizontal table, it moves at a constant speed while you push because there is no net force acting on it.  All the forces acting on the block cancel out with each other and the block undergoes uniform motion, sliding across the table at a constant speed.  Once you stop pushing, however, no force is left to cancel out the frictional component from the interaction between the block and the table and the block slows to a stop.

The genius of Newton, and others like Galileo before him, was to step out of the real world and think about how things would work in the absence of forces like friction.  What he created was an incredibly powerful framework for determining how forces act to change an object’s motion.  Learning how to use the framework provided by Newton’s second law is one of the core focuses of this course.