For an overview on creating, visualizing, and simulating mover constraints, including LinearVelocity, see Mover Constraints. Also see Roblox Units to understand how Roblox units compare to metric units.
A LinearVelocity constraint applies force on an assembly to maintain a constant velocity. It can be set to apply force along a Vector3, line, or 2D plane.
The LinearVelocity constraint applies a force that attempts to maintain a constant linear velocity. If you want to control the amount of force applied, use a VectorForce constraint. Alternatively, if you only need initial linear velocity, set the AssemblyLinearVelocity property directly on the assembly.
Relativity
Application of velocity can be controlled through the constraint's RelativeTo property. If set to World, force will be applied in world coordinates, independent of the parent or attachment orientations. If set to Attachment0 or Attachment1, force will be applied relative to Attachment0 or Attachment1 respectively.