Hi,
I am making a little physics "engine" for my NXT Robot.
So far I have a lot of things covered, like gravity, velocity, bouncing, etc.
Except the problem is, is that when 2 objects collide, a lot of times they seem to get stuck together, so it takes a large velocity to break that bond. Also, if the player collides with another object from the top (Player on top, object on bottom), it seems to stop moving (It doesnt even bounce, and it should), but its Y velocity is still very low, even though it should be at 0.
BTW, This program was made possible by BuilderBoy (Thanks ).

Here's my code:
//Made by NXT with major physics help from BuilderBoy

#define LEFT_OF_SCREEN_X 0
#define RIGHT_OF_SCREEN_X 99
#define TOP_OF_SCREEN_Y 63
#define BOTTOM_OF_SCREEN_Y 0
#define SCREEN_WIDTH 100
#define SCREEN_HEIGHT 64

#define TOTAL_OBJECTS 5

#define BOUNCE (0.5)
#define GRAVITY -0.2
#define FRICTION 0.3


//define the object
struct object
{
 float X;
 float Y;
 int width;
 int height;
 float mass;
 float velX;
 float velY;
 //Displaying stuff
 byte DRAW_OPT;
};

object box[TOTAL_OBJECTS];

bool Collide(object ob1, object ob2)
{
   if(((trunc(ob1.X) + ob1.width + 1) >= ob2.X) && (trunc(ob1.X) <= (trunc(ob2.X) + ob2.width + 1)) &&
      ((trunc(ob1.Y) + ob1.height + 1) >= trunc(ob2.Y)) && (trunc(ob1.Y) <= (trunc(ob2.Y) + ob2.height + 1)))
   {
      return(true);
   }

   return(false);
}

float muldiv(float _input, float _interval)
{
 _input = floor(_input * _interval);
 return(_input / _interval);
}


task main()
{
 bool collided;
 //Setting variables for boxes
 for(int c=0; c < TOTAL_OBJECTS; c++)
 {
  box[c].X = (40) + Random(5);
  box[c].Y = Random(40);
  box[c].width = Random(6) + 2;
  box[c].height = Random(5) + 2;
  box[c].velX = Random(3) - 1;
  box[c].velY = Random(2) - 1;
  box[c].mass = Random(4) + 1;
 }
 while(1)
 {
  collided = 0;
  for(int c=0; c < TOTAL_OBJECTS; c++)
  {
   for(int d=0; d < TOTAL_OBJECTS; d++)
   {
    if(d != c)
    {
     if(Collide(box[c],box[d]))
     {
      box[c].X = (40) + Random(5);
      box[c].Y = Random(40);
      collided = 1;
     }
    }
   }
  }
  if(collided == 0)
   break;
 }
 float backupBoxX;
 float backupBoxY;
 float backupBoxX2;
 float backupBoxY2;
 float backupVelX;
 float backupVelY;
 float backupVelX2;
 float backupVelY2;
 box[0].DRAW_OPT = DRAW_OPT_FILL_SHAPE | DRAW_OPT_LOGICAL_XOR;
 box[0].mass = 3;
 while(true)
 {
  SetDisplayFlags(DISPLAY_REFRESH_DISABLED);
  ClearScreen();
  if(ButtonPressed(BTNCENTER,0))
  {
   box[0].velY += 0.6;
  }
  else if(ButtonPressed(BTNEXIT,0))
  {
   box[0].velY -= 0.6;
  }
  if(ButtonPressed(BTNRIGHT,0))
  {
   box[0].velX += 0.6;
  }
  if(ButtonPressed(BTNLEFT,0))
  {
   box[0].velX -= 0.6;
  }
  //Processing for each box and displaying
  for(int c=0; c < TOTAL_OBJECTS; c++)
  {
   //Backs up X and Y Coordinates for collision
   backupBoxX = box[c].X;
   backupBoxY = box[c].Y;
   //Calculating X and Y Coordinates
   box[c].velY += GRAVITY;
   backupVelY2 = box[c].velY;
   backupVelX2 = box[c].velX;
   box[c].X += box[c].velX;
   box[c].Y += box[c].velY;
   //Collisions with walls of screen
   if(box[c].Y <= BOTTOM_OF_SCREEN_Y) //Collide with bottom of screen
   {
    box[c].Y = backupBoxY;
    if(abs(box[c].velX) < FRICTION)
    {
     box[c].velX = 0;
    }
    else if(box[c].velX < 0)
    {
     box[c].velX += FRICTION;
    }
    else
    {
     box[c].velX -= FRICTION;
    }
    box[c].velY = -box[c].velY*BOUNCE;
   }
   if(box[c].Y + box[c].height >= TOP_OF_SCREEN_Y) //Collide with bottom of screen
   {
    box[c].Y = backupBoxY;
    box[c].velY = -box[c].velY*BOUNCE;
   }
   if(box[c].X <= LEFT_OF_SCREEN_X) //Collide with left of screen
   {
    box[c].X = backupBoxX;
    box[c].velX = -box[c].velX*BOUNCE;
   }
   if(box[c].X + box[c].width >= RIGHT_OF_SCREEN_X) //Collide with right of screen
   {
    box[c].X = backupBoxX;
    box[c].velX = -box[c].velX*BOUNCE;
   }
   //Collisions with other objects
   for(int d=0; d < TOTAL_OBJECTS; d++)
   {
    if(d != c)
    {
     if(Collide(box[c],box[d]))
     {
      box[c].X = backupBoxX;
      backupVelY = box[c].velY;
      backupVelX = box[c].velX;
      if(Collide(box[c],box[d]))
      {
       //Collision on Y axis
       box[c].Y = backupBoxY;
       box[c].velX = backupVelX2;
       box[c].velY = backupVelY2;
       //V1 = (C*M2*(V2-V1)+M1*V1+M2*V2)/(M1+M2)
       box[c].velY = (BOUNCE*box[d].mass*(box[d].velY - backupVelY) + box[c].mass*backupVelY + box[d].mass*box[d].velY)/(box[c].mass+box[d].mass);
       box[d].velY = (BOUNCE*box[c].mass*(backupVelY - box[d].velY) + box[d].mass*box[d].velY + box[c].mass*backupVelY)/(box[d].mass+box[c].mass);
      }
      else
      {
       //Collision on X axis
       box[c].velX = backupVelX2;
       box[c].velY = backupVelY2;
       box[c].Y = backupBoxY;
       box[c].velX = (BOUNCE*box[d].mass*(box[d].velX - backupVelX) + box[c].mass*backupVelX + box[d].mass*box[d].velX)/(box[c].mass+box[d].mass);
       box[d].velX = (BOUNCE*box[c].mass*(backupVelX - box[d].velX) + box[d].mass*box[d].velX + box[c].mass*backupVelX)/(box[d].mass+box[c].mass);
      }
     }
    }
   }
   RectOut(trunc(box[c].X),trunc(box[c].Y),box[c].width,box[c].height,box[c].DRAW_OPT);
  }
  SetDisplayFlags(DISPLAY_REFRESH | DISPLAY_ON);
  Wait(17); //Waits 17ms to refresh the screen
 }
}

And also, in the video the player is the black box.
And here's a video:

I am already doing that. If it collided on the Y axis, it only goes on that axis, see:
if(Collide(box[c],box[d]))
      {
       //Collision on Y axis
       box[c].Y = backupBoxY;
       box[c].velX = backupVelX2;
       box[c].velY = backupVelY2;
       //V1 = (C*M2*(V2-V1)+M1*V1+M2*V2)/(M1+M2)
       box[c].velY = (BOUNCE*box[d].mass*(box[d].velY - backupVelY) + box[c].mass*backupVelY + box[d].mass*box[d].velY)/(box[c].mass+box[d].mass);
       box[d].velY = (BOUNCE*box[c].mass*(backupVelY - box[d].velY) + box[d].mass*box[d].velY + box[c].mass*backupVelY)/(box[d].mass+box[c].mass);
      }


I'm confused. So am I supposed to only move the Y axis back on 1 axis? Or on none of them??

Hmm.....
It works, but....
For some weird reason sometimes objects go through eachother!!