buttons to increase and decrease variables at runtime

Bellow is my code which shows the movement of the ball. Its bouncing with the parameters of gravity.

I want to add a button that should increase its movement and another button to decrease its movement.

==> to summarize, place two buttons: one should increase the value of gravity and another should decrease the value of gravity



import java.applet.Applet;
import java.applet.*;
import java.lang.*;
import java.awt.*;

//Creating a Main class Moving Ball
public class BouncingBall extends Applet implements Runnable {
 

   //Variable Initialization  
   int Direction_On_X = 0;  
   int Direction_On_Y = 0;
   int x_position     = 0;    
   int y_position     = 0;
   int oldx_position  = 0;  
   int oldy_position  = 0;
   int Squashed      = 0;       
   //For movemnt Calculation
   double gravity         = 9.83;
   double dist            = 0.0;
   double velocity        = 0.0;  
   double BouncingFactor  = 0.4;  
   double Temporaray;
   Double Doublenum;
   String bname;
   //Initializing a thread
   Thread thread_Instance;
 
   
//Method to squashed the ball
public void SquashedTheBall()
{
  Squashed = 1;
  repaint();  
}

//Method to beginthe animation , calling by start method
 public void Begin()
  {
     if (thread_Instance == null)
     {
      //
      thread_Instance = new Thread(this);
      thread_Instance.start();
     }
  }


//It will satrt the animation
  public void start()  
  {
      Begin();      
  }
   
  public void run()  
  {
     Thread.currentThread().setPriority(Thread.NORM_PRIORITY);
     while (true)
     {  
      motion();  
      try {
      Thread.sleep(50);  
     }
    catch (Exception sleepProblem)
     {}
 }
}

//Painting the ball , normaly and Squashed
public void paint(Graphics g)  
 {      

     g.setColor(getBackground());  
     g.fillOval(oldx_position,oldy_position,40,40);
     g.setColor(Color.red);       
     if(Squashed == 1 )
     {
      g.fillOval(x_position, y_position, 33, 25);
      Squashed = 0;
     }
     else
     {
      g.fillOval(x_position, y_position, 40, 40);
     }
 }

 //It will display the ball motion
private void motion()  
   {      
     //Variables updation
     oldx_position = x_position;      
     oldy_position = y_position;      
 
        if (Direction_On_Y == 0)
       {      
        //calculate the distance
        dist = velocity + (1/2)*gravity;    
        //Calculate the velocity      
        velocity = velocity + gravity;      
        //Calculate Y posiiton
         y_position+=(int)dist/10;
        //If ball reached bottom
        if (y_position > 300)
         {
           SquashedTheBall();
           Direction_On_Y = 1;                         
          //Adjust for going below 390...  
         dist = y_position - 300;      
         velocity = Math.sqrt((velocity*velocity) -  2*gravity*dist);
           Temporaray = velocity * (BouncingFactor/50);  //Calculate the new velocity because of collision.    
           velocity = velocity - Temporaray;      
         y_position = 300;
        }
     }
      //If Ball is bouncing up, do calculations and adjust y_position accordingly      
      
      else      
      {
        
      if(velocity <= 0)
       {
            Direction_On_Y = 0;
         }
      else
       {
             dist = velocity + (1/2)*(gravity);
             velocity = velocity - gravity;           
             y_position -= dist/20;
         }       
            
               
      if(y_position <= 0)      
      {
      Direction_On_Y = 0;
      y_position = 0;
      }

  }            
       //If ball is moving on right hand
       if (Direction_On_X == 0)
          {  
            //Move the ball by the factor of 2
            x_position+=2;
         
      if (x_position > 800)
         {
           // If it is opposite
           Direction_On_X = 1;
             x_position = 390;
         }
       }
      else
          {
          x_position-=2;

         if (x_position <= 0)
         {  
            Direction_On_X = 0;
              x_position = 0;
         }
        }
       repaint();  
     }
 
   
 
 }