How do you know that a object's velocity is 0.00 or below meaning that is not moving at all?
You're confusing speed and velocity. Velocity is a vector, it has direction and magnitude. The speed is that magnitude. Speed is strictly positive, while velocity can be negative.
But when I get the length of the velocity vector, it can never get to negative and I think that the lowest is 1?
The length is the magnitude, aka the speed, which can never be negative (of course it can be 0, that just means theres no movement).
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