Suppose I am pushing a block across a horizontal table.\u00a0 Once I stop pushing on the block, it quickly comes to a stop.\u00a0 This situation occurs often enough in the real world \u2013 stop pushing or pulling on an object and it comes to rest \u2013 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.\u00a0 To keep an object moving, you need a force acting on it, pushing or pulling the object in the direction it moves.<\/p>\n
The problem is that, no matter how much this thinking resonates with your real world experiences, it isn\u2019t correct.\u00a0 In fact, this is probably the most common misconception students have about forces.\u00a0Culminating 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\u2019t, but between objects whose motion is changing and objects whose motion isn\u2019t.\u00a0 Forces are not required to keep things moving.\u00a0Instead, forces are what cause an object\u2019s motion to change.\u00a0 As Newton\u2019s makes clear in his Laws of Motion, the difference between objects undergoing uniform motion (motion that doesn\u2019t change) and non-uniform motion (motion that does change) is the presence of a net, external force acting on the object.<\/p>\n
So why does our everyday experience lead us to the wrong conclusions about forces and their role in motion?\u00a0 Because the vast majority of objects that we watch move do so in the presence of retarding forces like friction.\u00a0 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.\u00a0 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.\u00a0 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.<\/p>\n
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.\u00a0 What he created was an incredibly powerful framework for determining how forces act to change an object\u2019s motion.\u00a0 Learning how to use the framework provided by Newton\u2019s second law is one of the core focuses of this course.<\/p>\n\n\t\t\t Candela Citations<\/h3>\n\t\t\t\t\t