I'm new to Unity.
I have Rigidbody2D and I want to add constant velocity to my player.
I want to know whats are differences or advantages/disadvantages or bast practice in applying velocity to Rigidbody2D in Start or Update (or FixedUpate)
When I apply velocity on start everything works fine, so why we add velocity every frame ( Update method ) ?
I want to add constant velocity to my player.
The Start function is eliminated in this case since it's called once only when the script is GameObject and script enabled. If you need to apply force over and over again then the Start function is not something you should use.
The Update function is used to do anything every frame. For example, manually moving object every frame.
The FixedUpdate function is used to do something to Rigidbody and this includes adding force to it every fixed frame. This is what you need to use since you're doing something to a Rigidbody object.
The other one you didn't mention is the LateUpdate function. This is used when you want to make a GameObject follow a camera since it's calld after all Update functions have been called.
Unity also has a ConstantForce utility to simplifies adding constant force to Rigidbody. With it, you can add force once to ConstantForce in the Start function and it will handle the rest until you change the force.
ConstantForce2D cForce = targetObj.GetComponent<ConstantForce2D>();
cForce.force = new Vector2(0, 100);