C'mon, Matt -- You're sounding a little like Chris Cox here... ;-)
You may be technically correct, but if we are going to split hairs, then we could also differentiate between net or average velocity, which you describe, and instantaneous velocity. (If I could figure out how to print proper equations and math symbols in email, then I'd probably go ahead and make a fool of myself trying to demonstrate my memory of 35-year old pre-calculus courses.) Anyway, change in position over change in time isn't really what I remember calling velocity in physics or math class -- velocity is the *limit* of change in position over change in time, as the amount of change in time nears zero. Or we could just say it's the derivative of position with respect to time, but then we need more definitions, which might mean more disagreements... Suffice to say that you can of course have any number of velocity vectors sum to zero over time. What might be more helpful to the OP would be using your example to introduce the concept of temporal sampling. Velocity isn't necessarily position now minus position a frame ago, even in an animation application -- it can be computed over any time interval, including subframe steps. And of course your example also illustrates temporal aliasing -- it's easy to miss important changes when you only look once every frame. When we compute motion vectors, we usually try to temporally subsample for exactly this reason. On Wed, Feb 13, 2013 at 4:32 PM, Matt Lind <ml...@carbinestudios.com> wrote: > If you have a problem with the definitions, talk to mathematicians and > physicists.**** > > ** ** > > I only put out the information for clarity as confusing speed with > velocity is very common and there are times when it does indeed matter.*** > * > > ** ** > > ** ** > > Matt**** > > ** ** > > ** ** > > ** ** > > *From:* softimage-boun...@listproc.autodesk.com [mailto: > softimage-boun...@listproc.autodesk.com] *On Behalf Of *Steven Caron > *Sent:* Wednesday, February 13, 2013 12:47 PM > > *To:* softimage@listproc.autodesk.com > *Subject:* Re: Difference between a force and a velocity ?**** > > ** ** > > i know velocity is a delta and i have used it many a time as part of an > equation which is variable.**** > > ** ** > > while your distinction is accurate i was concerned it wasn't helping > olivier with that example which illustrates no displacement. i just dont > think it was an intuitive example of velocity. in your example it would > mean you sat in the bleachers to watch the race but left right before it > starts and returned the moment it ends. so you say to your friend, "man > these guys haven't moved an inch!" and she looks at you weird...**** > > ** ** > > ** ** > > On Wed, Feb 13, 2013 at 12:03 PM, Matt Lind <ml...@carbinestudios.com> > wrote:**** > > Yes it’s important because there are many equations which rely on velocity > as a variable, not speed.**** > > **** > > If you’re computing a speed, you need to accumulate the distance travelled > over time. If you’re computing a velocity, you’re working with deltas.*** > * > > **** > > In more explicit terms, if you’re computing motion vectors, you’re > comparing the current frame to the previous frame. If you’re computing > speed, you’re comparing the current frame to the first frame. How you code > for those scenarios is vastly different.**** > > **** > > **** > > Matt**** > > **** > > **** > > **** > > *From:* softimage-boun...@listproc.autodesk.com [mailto: > softimage-boun...@listproc.autodesk.com] *On Behalf Of *Steven Caron > *Sent:* Wednesday, February 13, 2013 11:58 AM > *To:* softimage@listproc.autodesk.com > *Subject:* Re: Difference between a force and a velocity ?**** > > **** > > is that distinctions helpful here?**** > > **** > > i mean we aren't just going to render the last frame of our particle going > around the track, we are going to render 1440 frames (24fps*60secs) and at > each frame the instantaneous velocity is going to have some direction and > magnitude.**** > > **** > > On Wed, Feb 13, 2013 at 11:13 AM, Matt Lind <ml...@carbinestudios.com> > wrote:**** > > Velocity = net displacement / time. > > Force = Mass * Acceleration > > > NOTE: Speed and velocity are very different. > > Speed is distance traveled over time (scalar) > Velocity is net displacement over time (vector) > > Example: Running around a track in a stadium. > > If you make a complete lap in 60 seconds, then your speed is 6.66 meters > per second, while your velocity is 0 meters per second because you haven't > been displaced from your starting position. > > > > Matt**** > > > > -----Original Message----- > From: softimage-boun...@listproc.autodesk.com [mailto: > softimage-boun...@listproc.autodesk.com] On Behalf Of olivier jeannel > Sent: Wednesday, February 13, 2013 1:50 AM > To: softimage@listproc.autodesk.com > Subject: Difference between a force and a velocity ? > > Hi list, > Asking this question with no shame ^^; > This might be basic, but what is the difference between a force and a > velocity ? > > Sorry for being half brained...**** > > **** > > ** ** >
