Rigidbody2D loses speed on collisions

Question

I'm trying to create a simple Breakout-style game. In my game, the ball strikes a paddle that moves with the player's mouse. Depending on the position of the strike, the ball changes its direction. This allows the player to aim the ball.

The problem I'm having is that each time I set the position, the ball loses velocity. I've tweaked every Physics2D setting I could find, but I'm not seeing any changes. Here's what my Ball class looks like:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Ball : MonoBehaviour {

  public float initialVelocity;
  public float collisionAngleRange;
  public Paddle paddle;

  private Rigidbody2D body;

  public void Start() {
    body = GetComponent<Rigidbody2D>();
    body.velocity = VectorUtilities.AngleToVector2(135) * initialVelocity;
  }

  // When colliding with the paddle, use the position of the strike to adjust the ball's position.
  public void OnCollisionEnter2D(Collision2D collision) {

    // Ignore the object if it's not the paddle.
    if (!collision.gameObject.CompareTag("Paddle")) { return; }

    // Get the collision coordinates
    Bounds bounds = collision.gameObject.GetComponent<BoxCollider2D>().bounds;
    float left = bounds.min.x;
    float right = bounds.max.x;

    Vector2 contact = collision.contacts[0].point;

    // Interpolate the contact point and determine the angle.
    float interpolation = (contact.x - left) / (right - left);
    float angle = 90 - collisionAngleRange / 2 + collisionAngleRange * (1 - interpolation);

    // Change the direction of the ball without affecting its speed.
    body.velocity = VectorUtilities.AngleToVector2(angle) * body.velocity.magnitude;

    // Log what's happening with the velocity
    Debug.Log(body.velocity.magnitude);
  }
}

The VectorUtilities class looks like this:

using UnityEngine;

// A set of utility methods and extensions for Vector2 and Vector3.
public static class VectorUtilities {

  public static Vector2 AngleToVector2(float degrees) {
    return new Vector2(Mathf.Cos(degrees * Mathf.Deg2Rad), Mathf.Sin(degrees * Mathf.Deg2Rad));
  }
}

Here are my Ball game object's Rigidbody2D and CircleCollider2D settings:

The paddle's settings:

The material settings:

Physics2D settings:

I recognize I could solve this problem by multiplying the ball's normal vector by my desired speed, but it's more important to me to understand why this is happening and to fix the underlying problem. Why does the ball lose speed on every collision, and how can I prevent that from happening?

Answer 1

I finally figured out what was going on here. Because the mass of the Ball and the mass of the Paddle are the same, the Ball was transferring some of its force into the Paddle when it collided with it. However, since the Paddle had its y position and its rotation frozen, I couldn't actually see the effect of the Ball striking it. Turning off those properties allowed me to see the problem.

The solution was to set the body type of the Paddle to Kinematic, which allows it to register collisions without being affected by them.