There's nothing wrong with the running track example. You are overthinking the problem. It's an introductory level illustration to clarify the difference between speed (scalar) and velocity (vector), and is used in physics text books believe it or not. If you do a google search on "physics definition of velocity", the top three results all explain velocity as the change in position divided by the change in time. (e.g. v = x1 - x0 / t1 - t0). If the start position and end position are the same, then the velocity is indeed zero over any time interval because it has no magnitude, and proven mathematically. If you have a problem with that, take it up with the physicists and mathematicians.
From: softimage-boun...@listproc.autodesk.com [mailto:softimage-boun...@listproc.autodesk.com] On Behalf Of Andy Jones Sent: Wednesday, February 13, 2013 6:08 PM To: softimage@listproc.autodesk.com Subject: Re: Difference between a force and a velocity ? The example of running around on a track is wrong unless you're on a stationary planet. Way to confuse Olivier, guys :( Here's my stab: Definitions aside, velocity describes how something is moving at an instant in time. Force, however, is more closely related to how an object will accelerate over time (in that the acceleration of the object is directly proportional to the forces acting upon it). So if a force is acting on an object, that will cause it to start moving differently as time passes, like what happens when you drop something. When you drop a ball, gravity (a force) causes its downward velocity to increase more and more as time passes. A noteworthy exception to this simple explanation of forces and velocity are forces that are actually dependent on velocity. In practice, one of the most common such forces is the force of friction with a surface, such as a table top (or even the surface of air against the object, which is a component of the "drag" force affecting an object traveling through a medium). For example, as an object travels faster and faster through the air, the force of friction with the air will increase. This is why when you first drop a ball it speeds up on its way down, but once it's falling fast enough, it will stop speeding up and continue at a constant speed (and constant velocity, if you're a stickler for terminology). Once the ball is falling at a fixed speed, you know that the force of gravity and the force of drag/friction are equal and opposite, such that they cancel out. This is of course ignoring secondary effects, such as the heat generated by the people below arguing over email forums about how physics works. In this case, as the arguing increases, the amount of hot air released increases, and the air surrounding the ball becomes thinner, reducing the drag force on the ball, and causing it to fall even faster, thereby increasing the speed at which the ball hits its intended target, hopefully reminding that target that it's silly to get worked up over Newtonian physics, since it's a simplified approximation of reality anyway. - Andy On Wed, Feb 13, 2013 at 3:54 PM, Raffaele Fragapane <raffsxsil...@googlemail.com<mailto:raffsxsil...@googlemail.com>> wrote: I don't have an account, please feel free to do so in my place. I googled it though, didn't do all the post myself ;) On Thu, Feb 14, 2013 at 10:24 AM, Steven Caron <car...@gmail.com<mailto:car...@gmail.com>> wrote: please add... http://cgmemes.blogspot.com/ On Wed, Feb 13, 2013 at 3:01 PM, Raffaele Fragapane <raffsxsil...@googlemail.com<mailto:raffsxsil...@googlemail.com>> wrote: -- Our users will know fear and cower before our software! Ship it! Ship it and let them flee like the dogs they are!
