Unity Scripting API

Need help updating. I am following the Angry Bird Tutorial for my college class, and unfortunately in the tutorial online the guy doesn't provide the code that I can cross check with mine. I am getting multiple errors. I've attempted changing rigidbody2d to rb how unity 5 now classifies it but it gave me even more errors.

 using UnityEngine;
 using System.Collections;
 
 public class NewBehaviourScript : MonoBehaviour {
     public float maxStretch = 3.0f;
     public LineRenderer catapultLineFront;
     public LineRenderer catapultLineBack;
 
 
     private SpringJoint2D spring;
     private Transform catapult;
     private Ray rayToMouse;
     private Ray leftCatapultToProjectile;
     private float maxStretchSqr;
     private float circleRadius;
     private bool clickedOn;
     private Vector2 prevVelocity;
     public Rigidbody2D rb;
 
     void Awake () {
         rb = GetComponent<Rigidbody2D> ();
         spring = GetComponent <SpringJoint2D> ();
     }
 
     // Use this for initialization
     void Start () {
         LineRendererSetup ();
         rayToMouse = new Ray (catapult.position, Vector3.zero);
         leftCatapultToProjectile = new Ray (catapultLineFront.transform.position, Vector3.zero);
         maxStretchSqr = maxStretch * maxStretch;
         CircleCollider2D circle = Collider2D as CircleCollider2D;
         circleRadius = circle.radius;
     }
     
     // Update is called once per frame
     void Update () {
         if (clickedOn)
             Dragging ();
 
         if (spring != null) 
             if (!GetComponent<Rigidbody2D>().isKinematic && prevVelocity.sqrMagnitude > Rigidbody2D.velocity.sqrMagnitude){
             Destroy(spring);
             rb.velocity = prevVelocity;
         }
         if (!clickedOn) {
             prevVelocity = rb.velocity;
         
             LineRendererUpdate ();
         }
         else {
             catapultLineFront.enabled = false;
             catapultLineBack.enabled = false;
         }
     }
 
     void LineRendererSetup() {
         catapultLineFront.SetPosition (0, catapultLineFront.transform.position);
         catapultLineBack.SetPosition (0, catapultLineBack.transform.position);
 
         catapultLineFront.sortingLayerName = "foreground";
         catapultLineBack.sortingLayerName = "foreground";
 
         catapultLineFront.sortingOrder = 3;
         catapultLineBack.sortingOrder = 1;
     }
     void OnMouseDown (){
         spring.enabled = false;
         clickedOn = true;
     }
     void OnMouseUp (){
         spring.enabled = true;
         rb.isKinematic = true;
         clickedOn = false;
     }
     void Dragging () {
         Vector3 mouseWorldPoint = Camera.main.ScreenToViewportPoint (Input.mousePosition);
         Vector2 catapultToMouse = mouseWorldPoint - catapult.position;
         if (catapultToMouse > maxStretchSqr){
             rayToMouse.direction = catapultToMouse;
 
         mouseWorldPoint.z = 0f;
         transform.position = mouseWorldPoint;
 }
     
 }
     void LineRendererUpdate () {
         Vector2 catapultToProjectile = transform.position - catapultLineFront.transform.position;
         leftCatapultToProjectile.direction = catapultToProjectile;
         Vector3 holdPoint = leftCatapultToProjectile.GetPoint (catapultToProjectile.magnitude + circleRadius);
         catapultLineFront.SetPosition (1, holdPoint);
         catapultLineBack.SetPosition (1, holdPoint);
     }
 
 
 }

I am getting: Assets/Scripts/ProjectileDragging.cs(31,43): error CS0119: Expression denotes a type', where a variable', value' or method group' was expected An object reference is required to access non-static member UnityEngine.Rigidbody2D.velocity' error CS0019: Operator >' cannot be applied to operands of type UnityEngine.Vector2' and float'