... It was clearer before ... :DDD
Le 13/02/2013 23:17, Matt Lind a écrit :
I think its other people who are blowing this out of proportion.
After sending my initial email on the subject, I encountered this one
from Olivier which precedes mine:
>Thank's César ! Now it's clear :)
>
>Le 13/02/2013 11:13, César Sáez a écrit :
>
> They are completely different concepts, in short:
>
> Force = mass * acceleration
>
> Acceleration = Velocity / time
>
> Velocity = distance-traveled / time
>
Seeing how I didn't do much differently than Caesar as far as
explanation other than to give an example, I don't think I've done
anything to split hairs or mislead people. I gave a very introductory
explanation as is normally given on the 1^st or 2^nd day of a physics
course. If you want to dive deeper and differentiate between
instantaneous velocities and whatever everybody else is throwing
about, then you are the ones splitting the hairs, not me.
Matt
*From:*softimage-boun...@listproc.autodesk.com
[mailto:softimage-boun...@listproc.autodesk.com] *On Behalf Of *Ed Manning
*Sent:* Wednesday, February 13, 2013 2:05 PM
*To:* softimage@listproc.autodesk.com
*Subject:* Re: Difference between a force and a velocity ?
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
<mailto: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>
[mailto: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
<mailto: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
<mailto: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>
[mailto: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
<mailto: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
<mailto: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>
[mailto: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
<mailto: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...
