Making my XNA sprite jump properly

Question

I have been having great trouble getting my sprite to jump. So far I have a section of code which with a single tap of "W" will send the sprite in a constant velocity upwards. I need to be able to make my sprite come back down to the ground a certain time or height after begining the jump. There is also a constant pull of velocity 2 on the sprite to simulate some kind of gravity.

// Walking = true when there is collision detected between the sprite and ground

if (Walking == true)
    if (keystate.IsKeyDown(Keys.W))
        {
            Jumping = true;
        }

if (Jumping == true)
    {
        spritePosition.Y -= 10;
    }

Any ideas and help would be appreciated but I'd prefer a modified version of my code posted if at all possible.

Answer 1

    public class Player
    {
      private Vector2 Velocity, Position;
      private bool is_jumping; 
      private const float LandPosition = 500f; 

      public Player(Vector2 Position)
      {
         this.Position = new Vector2(Position.X, LandPosition); 
         Velocity = Vector2.Zero;
         is_jumping = false; 
      }
      public void UpdatePlayer(Gametime gametime, KeyboardState keystate, KeyboardState previousKeyBoardState)
      {
       if (!is_jumping)
         if (keystate.isKeyDown(Keys.Space) && previousKeyBoardState.isKeyUp(Keys.Space))
          do_jump(10f); 
       else
       {
        Velocity.Y++; 
        if (Position.Y >= LandPosition)
           is_jumping = false; 
       } 
       Position += Velocity; 

     }

     private void do_jump(float speed)
     {
            is_jumping = true; 
            Velocity = new Vector2(Velocity.X, -speed); 
     }
   }

fun little mix of psuedocode and some real code, just add the variables that I didn't include at the top.

Also check out Stack Overflow Physics ;) Good luck with your game.

edit: that should be complete, let me know how things go.