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Posted 25 Dec 2007

Gravity and Collision Simulation in C#

, 31 Dec 2007
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This article demonstrates how to simulate the motion of an object within a Gravity and Collision enabled environment.


Simulating the motion of an object within a real time environment with Gravity and Collision effects may not be a straightforward task in ordinary programming languages; doing such tasks require a good understanding of using timers and sometimes thread management, and this is why there are separate simulation tools for this and other similar tasks.

In this article, I will demonstrate to you how, using a timer and basic motion and collision equations, we could model the motion of three balls in a gravity enabled environment. You will see how these balls are going to collide each other and reflect from a wall, and even more, you can control their motion by updating some motion variables.

The motion of these balls is controlled and operated under the gravity and collision systems, using Newton's basic motion equations and collision equations. The positions of the three balls are updated every 20ms using a timer which will also take a snapshot of that motion.


Before you see the code, I believe we should review the basic Gravity and Collision equations first.

Theoretical Analysis:

Gravity and motion equations:

The position equation:

X = Xi + Vx * tx
  • Xi: Is the initial position of the object.
  • Vx: Is a constant speed if we ignore the friction and air resistance, which I do include in this simulation. I consider the air as a constant resistance that must be taken care of periodically, so I divide the motion into separate periods and calculate the new position for each period.
  • tx: Is the time.

Motion third equation:

Y = Y0 + Vy * ty – 0.5 * g * t^2
  • g: The gravity acceleration.

The velocity equations:

Vy = Vy0 – g*t
  • Vy: The final Y speed.
  • Vy0: The initial Y speed.
  • g: The gravity acceleration.
Vx = 0.99*Vx0
  • Vx0: The old X speed.
  • Vx: The new X speed after including air resistance.
  • 0.99: Is a constant ratio representing air resistance.

Collision and preserved momentum:

Collision: an action between two or more bodies, each one affecting the others by a great power in a very short time, the bodies might not even touch!!!

+ Notes:

  • The two bodies may be moving in opposite directions.
  • The two bodies may be moving in the same direction.
  • One is moving while the other is still.
  • The power at the time of collision between two bodies can be represented as shown in the graph below:
  • Collision power

  • If two or more bodies collide, then the sum of their momentum before the collision is equal to the sum of their momentum after the collision.
  • Theory:

    V1×m1 + V2×m2 = V1`×m1 + V2`×m2
    Assuming same mass:
    V1 + V2 = V1` + V2`


The code below will clarify these equations in their context.

Using the code

First of all, we should define the motion variables for each ball in our simulation.

///////////////////////////////////////// ball /////////////////////////////////////
// xspeed: The X axis speed of the ball –                                         //
//           it will be calculated based on the mouse movement speed.             //
// yspeed: The Y axis speed of the ball –                                         //
//           it will be calculated based on the mouse movement speed.             //
// newyspeed: The updated Y acis speed of the ball                                //
//              after applying Newton and collision equations.                    //
// startingypos: The initial Y position of the ball –                             //
//               when stop dragging the ball.                                     //
// newxpos: The updated X position of the ball                                    //
// newypos: The updated Y position of the ball                                    //
// oldxpos: The previous X position of the ball                                   //
// oldypos: The previous Y position of the ball                                   //
// newx: The new X position of the mouse after dragging                           //
// oldx: The old X position of the mouse after dragging                           //
// newy: The new Y position of the mouse after dragging                           //
// oldy: The old Y position of the mouse after dragging                           //    
// acc: The acceleration = 10                                                     //
// t: The time                                                                    //
// xmouse: The X axis of the mouse pointer position                               //    
// ymouse: The Y axis of the mouse pointer position                               //
// dragging: Boolian variable to check whether the ball is being dragged or not.  //
// trace: Boolian variable to check if the trace option is on or off.             //
// collisiony: Boolian variable to check if the ball hits the ground or not.      //

// ball 1 variables
double xspeed,yspeed,newyspeed,startingypos;
double newxpos,newypos,oldxpos,oldypos;
double newx,oldx,newy,oldy;
double acc,t;
const int ground = 500;
int xmouse,ymouse;
bool dragging=true,trace,collisiony;

int choice = 1;
int numberofballs = 1;

Ballinstance b1 = new Ballinstance();

Next, we will track the ball motion and check for a collision every 20 ms in our timer, and accordingly we will update the balls' positions.

private void timer_Elapsed(object sender, System.Timers.ElapsedEventArgs e)
{ xspeed,ref yspeed,
            ref newyspeed,ref startingypos,
            ref newxpos,ref newypos,ref oldxpos,ref oldypos,
            ref newx,ref oldx,ref newy,ref oldy,
            ref acc,ref t,
            ref xmouse,ref ymouse,
            ref dragging,ref trace,ref collisiony);
    ball.Left = (int)newxpos;
    ball.Top = (int)(ground - newypos);


Below is the Ballinstance class, and the play function where most of the work is done. As you will see, this function will be visited every 20 ms, the timer period, and then will check for the calling ball status, which can be as follows:

  • Dragging state:
  • If the ball calling the play function was in the drag mode, then the ball position will be updated according to the mouse pointer position, and the ball's initial speed will be calculated by measuring the change of the ball position between two successive calls to the play function; within 20 ms.

  • Motion state:
  • If the ball calling the play function wasn't in the drag mode, then the ball position will be updated according to Newton's and projectile motion equations and the Collision preserved momentum equation.

public class Ballinstance
    int xpos,ypos;
    const int ground = 500;

    public void play(ref double xspeed,
                     ref double yspeed,
                     ref double newyspeed,
                     ref double startingypos,
                     ref double newxpos,
                     ref double newypos,
                     ref double oldxpos,
                     ref double oldypos,
                     ref double newx,
                     ref double oldx,
                     ref double newy,
                     ref double oldy,
                     ref double acc,
                     ref double t,
                     ref int xmouse,
                     ref int ymouse,
                     ref bool dragging,
                     ref bool trace,
                     ref bool collisiony)

        xpos = (int)newxpos;
        ypos = (int)newypos;
        // this code will be visited 50 times per second while dragging
        if (dragging) 
            // Grip the center of the ball when dragging
            xpos = xmouse;
            ypos = ymouse;

            // While dragging the starting y-axis position of the ball is ball.Top
            startingypos = ground - ypos;

            // Calculate the x and y speed based
            // on the mouse movement within 20 msec
            // speed = distance/time  ->  time = 20 millisecond
            // the speed is the change in the displacement
            // with respect to the time which
            // is already running (the code is within
            // the timer), so we don't have to divide 
            // by the time
            newx = xpos;
            newy = ground - ypos;
            xspeed = (newx-oldx)/1;
            yspeed = (newy-oldy)/1;
            oldx = newx;
            oldy = newy;

            // The time -while dragging- will not start yet

            // This code will be visited 50 times per second while not dragging
            // The ball position is where it's last dragged
            oldxpos = xpos;
            // X-axis motion
            if(xpos < 580 && 0 < xpos)
                newxpos = oldxpos + xspeed;
                // Here the ball will hits the wall
                // Ball xspeed will decrease every time it hits the wall
                // Minus sign: to change the ball direction
                // when it collides with the walls
                // wall resestance, the ball will
                // lose some energy when hitting the wall
                xspeed *= -0.9;        
                newxpos = oldxpos + xspeed;
            // Y-axis motion
            if(0 < newypos || collisiony)
                // Newton first motion equation
                newyspeed = yspeed - (acc*t);
                // Newton third motion equation
                newypos = startingypos + ((yspeed*t)- 0.5*acc*(t*t));
                // no collision happend
                collisiony = false;
                // Here the ball will hits the ground
                // Initialize the ball variables again
                startingypos = -1;    
                // Here set startingypos=-1 not 0, because
                // if 0 newypos will be 0 every time the ball 
                // hits the ground so no bouncing
                // will happens to the ball, look to the 
                // eguation of newypos below when t = 0
                t = 0;
                // Ball yspeed will decrease every time it hits the ground
                // 0.75 is the elasticity coefficient
                // the initial speed(yspeed)
                // is 0.75 of the final speed(newyspeed)
                yspeed = newyspeed * -0.75;
                newypos = startingypos + ((yspeed*t)- 0.5*acc*(t*t));
                collisiony = true;
            // Always
            // Ball xspeed will always decrease, even if it didn't hit the wall
            xspeed *= 0.99;    // air resistance

            #region explination of xspeed condition
            // This to stop the ball when it heading
            // to the left, you can notice that removeing
            // this condition will make the ball never
            // stop while its heading to the left until it will
            // hit the left wall, to know why,
            // run the simulation under the debuging mode and watch
            // the value of newxpos
            // newxpos = oldxpos + xspeed
            // when 0 < xspeed < 1 (the ball heading right),
            // ball.left = (int)newxpos, the casting 
            // forces the ball left position value
            // to be the same as its previous value
            // because oldxpos and newxpos are equals,
            // and hence the ball will stop.
            // but when -1 < xspeed < 0 (the ball heading left),
            // ball.left = (int)newxpos, the casting
            // here will not work correctly, because
            // the value of oldxpos(which is integer value) 
            // will always be decremented by the xspeed,
            // this will force the newxpos also to be 
            // always decremented by xspeed and
            // hence ball.left will always decremented 
            // by 1 (int) casting, and hence the ball will never stop.

            if(xspeed > -0.5 && xspeed < 0)
                xspeed = 0;
            // Update the ball position
            xpos = (int)newxpos;
            ypos = (int)(ground - newypos);
            // Increase the time
            t += 0.3;


The project is not completed yet. I was thinking of creating some obstacles to see how the balls will collide them, seems funny :). You also can improve the way it looks and make it more usable if you write a routine to drag and drop the ball by grapping it, which I can't find out how to do in C#!!

I'd like to thank Anas Trad and Du3a2 Al-ansari, my friends, for their contributions to help finish this simulation.


  • Gravity simulation version 1.0.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Suhib N. Rawshdeh
Engineer WSU
United States United States
No Biography provided

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Comments and Discussions

GeneralFunny =) Pin
Member 40864544-Jan-08 2:01
memberMember 40864544-Jan-08 2:01 
JokeRe: Funny =) Pin
Souhieb N. Rawashdeh6-Jan-08 22:10
memberSouhieb N. Rawashdeh6-Jan-08 22:10 
GeneralExcellent Pin
Mohammad A Gdeisat31-Dec-07 22:13
memberMohammad A Gdeisat31-Dec-07 22:13 
GeneralNice Project [modified] Pin
GuyThiebaut26-Dec-07 4:29
memberGuyThiebaut26-Dec-07 4:29 
GeneralRe: Nice Project [modified] Pin
Souhieb N. Rawashdeh28-Dec-07 6:13
memberSouhieb N. Rawashdeh28-Dec-07 6:13 

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