Moving models with a matrix

Question

Currently I have a playerWorld matrix which holds the position of my player, just a simple sphere. When I press a key to move the model, it does some crazy thing and flies off the screen really fast, and not even in the direction I want it to go.

    if (kbs.IsKeyDown(Keys.W))
        {
            playerWorld += Matrix.CreateTranslation(playerWorld.Forward * elapsed);
        }

Why is this happening? elapsed is a float variable:

    float elapsed = (float)gameTime.ElapsedGameTime.TotalSeconds;

Here is the draw code, maybe it's being drawn wrong?

    public void Render(ThirdPersonCam cam)
    {
        foreach (ModelMesh mesh in playerModel.Meshes)
        {
            foreach (BasicEffect effect in mesh.Effects)
            {
                effect.View = cam.view;
                effect.Projection = cam.proj;
                effect.World = playerTransforms[mesh.ParentBone.Index] * playerWorld;
            }
            mesh.Draw();
        }
    }

Answer 1

When adding (combining) the movement or rotation (or scale) of two matrices together the arithmetic to apply on them is multiplication rather than addition.

change your += to *= and your original code will work as expected.

if (kbs.IsKeyDown(Keys.W))
    {
        playerWorld *= Matrix.CreateTranslation(playerWorld.Forward * elapsed);
    }

one matrix multiplied by another matrix results in what you would conceptualize as adding the SRT of one matrix to the SRT of the second. (SRT is Scale/Rotation/Translation)

Answer 2

Use a vector to store translation and build the matrix from it.

   if (kbs.IsKeyDown(Keys.W))
      {
          playerPos += playerForward * speed * elapsed; 
      }      

   playerWorld = Matrix.CreateScale(playerScale) 
               * Matrix.CreateFromQuaternion(playerRotation)
               * Matrix.CreateTranslation(playerPos);