Vector3f velocity(Vector3f storeResult) {
        Vector3f result = (storeResult == null) ? new Vector3f() : storeResult;
        if (isKinematic()) {
            result.set(kpVelocity);
        } else {
            assert !rigidBody.isKinematic();
            rigidBody.getLinearVelocity(result);
        }

        return result;
    }
Copy the body's linear velocity, or an estimate thereof.

@param storeResult (modified if not null)
@return a new velocity vector (psu/sec in physics-space coordinates)
jme3-jbullet/src/main/java/com/jme3/bullet/animation/PhysicsLink.java