# Why we have velocity value in voppop?

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Hi.

I would like to know about velocity in Houdini.

Please see the attached file and let me know.

I don't understand why I get those value.

Have a great day.

Best,

Jamie

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Hi

I'm a Houdini beginner so apologies if I am wrong... It looks to me though like you're setting the particle's velocity to be the same as its position so on frame 1, y position is 0.1 and velocity is 0.1 so by frame 2 it has moved to 0.104167 in y and its velocity is set to the same value. This setup will cause the particle to accelerate upwards.

Hope that helps

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Hi Tim,

Thanks for your help, but for more talking to.

The frame 1, x position is 0.1 and velocity is 0.1.

There is no matter. Absolutely right.

When the frame 2, it moves 0.104167. Why add 0.04167?

Is there a formula?

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frame rate is 24 frames per second so pop multiplies the velocity by 1/24 to get the correct part per frame

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velocity is in units per second so the formula is

P+v/fps

so as your P is 0.1

then v is 0.1 (per second)

so v per frame is 0.1/fps = in your case 0.1/24 ~= 0.004167

so P next frame will be 0.104167 etc

the formula is actually P + v*timestep

as usually you are not computing data only from frame to frame but a few times inbetween using oversampling

and the duration of this computing step in seconds is called timestep

so if you are computing it only once per frame it will be just 1/fps, therefore P + v/fps

but with oversampling its rather 1/fps/samples

EDIT: too late

Edited by anim
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Here is the base formula P += v * TimeInc;

v += acceleration * TimeInc;

Time inc = 1/\$FPS

Edited by Filipp Elizarov
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@Martin : Thank you for you support.

@Tomas : Thank you so much for your kindly teaching.

@Flipp : Really thank you so much.

All thanks.

Really I was confusing about fps.

I have calculated that P/(frame/fps).

So I had gotten wrong the value.

I am happy to understanding that. Good tutors and nice solve.

Have a great weekend.

Best,

Jamie

Edited by jungjaehwa
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One more question.

Where can I find the formula in Houdini help documentation?

I can not find it out.

And then next step, I have studied that output P connect input acceleration in voppop.

I want to understand that there is a difference between velocity and acceleration.

Help me.

Edited by jungjaehwa
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at the beginning of each timestep acceleration is 0 and gets accumulated as a result of different forces (or direct modification) and at the end of the timestep it is added to velocity for the next timestep

something like v = v + acc*timestep (or as Fillip mentioned timestep is called TimeInc in vops)

so if initial particle v is 0 and you have constant acceleration of 1 then in one second v reaches value 1 in 2 seconds 2 etc

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Hi Tomas,

Thank you so much for your help and sharing the time.

For understanding, I am repeatedly reading the theory velocity and acceleration.

Actually I forget the theory.

When I get it, I want to talk again.

Have a great day.

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