Question How to use Multiply Velocity over Life in VFX graph? (slow-down motion effect)

Hello and thanks for your questions, I will try my best to answer.
First thing first, let's talk about Contexts:

Contexts have different purposes:

The Initialize Context lets you set the “Initial” values and properties of your particles. In other words: What are the properties of my particles when they're spawn? Those properties, How long do they live, Position, Velocity are called “Particles Attributes”. So usually, in the Initialize Context, we set the Initial values of those particles attributes.

So, while it make sense to set the Velocity attribute in the Init Context, the "Over-life" isn't really useful as in the Init Context, particles are just born and have an Age of zero which mean that you will always sample the Overlife curve at position 0.

The Update Context:

The Update Context is where the particle's behaviors and properties are going to be updated.
If you select, this Context, in the Inspector, you'll find different checkboxes. By default, even without any Blocks/nodes inside it, the Update Context does some implicit calculation for you (Velocity Integration, Aging, Kill Particles if age>= Lifetime).

Update Position: → Velocity Integration, This means that each frame, the Update context takes the Velocity Attributes values of your particles and uses it to calculate the new Position of your Particles.



Update Position: → Each Frame, the Update Context is also increasing the values of the Age attribute.
So this Context is usually where you're going to set or modify attributes' values over time and/or Particles's Lifetime.

While It makes sense to use the Multiply Velocity over lifetime in the Update, you need to understand that the Update is called each Frame. So each frame the current Velocity is going to be multiplied, so it can change exponential.
A stupid examples with an initial Speed of 1.0 being multiply by a Constant 0.9 in the Update:

Frame 01 = 1 × 0.9 = 0.9 | F2 = 0.9 × .9 = 0.81| F3 = .81 × .9 = 0.72| F4 = 0.72 × .9 = 0.64| F5 = 0.64 × 0.9 = 0.57

F6 = 0.57 ×0.9 = 0.51 | F7 = 0.51 × 0.9 = 0.45 | F8 = 0.45 × 0.9 = 0.40 etc…

Output Context:
This Context Render the particles based on their properties (Attributes). So here, we typically set attributes that drives what the particles will look like: Their visual's properties : Size, Alpha, Color etc…

So while it make sense to use a Size overLife in this Context, doing a "Velocity overlife” in this context will not yield any results as the Velocity Integration is done before in the Update Context.

I'm guessing that now, you must be saying: This is really nice and all, but how do you slow down the Particles's velocity over their Lifetime.
Well as always with VFX Graph they're multiples ways of doing of thing. Here are some of those solutions:

A:
As you did, use a Multiply Over Lifetime block. But as mention earlier, be sure to change the curve so that you don't multiply by low values as it's going to be executed each-frame. The pleasant thing is that you can manipulate the tangent of your curve.


B:
You can use a Multiply Velocity in Custom mode in the Update context, and use the DeltaTime as sample time.
This is simple to set up and can give some pleasant control while keeping the Curve in a 0-1 range. But it's not on the Particle's Lifetime.

C:
Another very simple solution is to use a Linear Drag Force Block. This will apply a force on particles that will slow them down. While it very simple to use, it can be hard to master and isn't the adequate solution if you want to control precisely when your particles slow-down on their lifetime.

D:
The Last solution that I want to provide is little but more involved and use Custom Attributes.


I'm finishing this post here as I see that @Qriva already made a great answer.
Hope this helps a bit and I wish you a great day.

 

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