Gravity in C

[placo23]

Hi guys. Does anyone here have a small example of gravity in C? Basically I'm playing around with some stuff here and wanted to do some tests with gravity. I just wanted to have a ball for example that falls, but uses some gravity so the ball bounces a bit when it hits the floor. Just like a normall ball would do. I have no idea where to start with it.

Any ideas/samples for helping me?

Cheers

[bhunter]

Well I don't know the code but the calculation would be an acceleration downwards of 9.8M/S^2 (until reaching terminal velocity obviously) and then the bounce back would be a variable depending on the compound your ball is made out of. I would probably start it on a loss of 25% momentum (ie bounce back at 75% the velocity it hit the ground at) with a deceleration of 9.8M/S^2 until Speed = 0 and then start the routine again.

Continue until the impact velocity < 0.1 (Should be low enough).

Also note, your impact point could be made out of multiple compounds also so make sure you factor that into your code ie Grass has more absorption of energy than concrete .

[Bronx]

something like this (assumes you're using 3D, if not, get rid of the z axis):

struct RigidBody
{
     Vector3 position; 
     Vector3 velocity;
     Vector3 force;
     float mass;
};
typedef struct RigidBody RigidBody;

void Integrate( RigidBody* body, float dt )
{
     body->velocity += force/mass * dt * dt;
     body->position +=velocity;
     body->force = 0.0f;
}

void AddForce( RigidBody* body, Vector3* force )
{
     body->force += force;
}

// in main loop assuming a rigidbody has been initialized and we have the delta time
Vector3 gravity; gravity.x = 0.0f; gravity.y = -9.8f; gravity.z = 0.0f; // this shouldn't be declared in the main loop either.
AddForce( myRigidBody,  gravity );
Integrate( myRigidBody, deltaTime );

I hope this helps. I'd suggest you try downloading Chipmunk or Box2D, they're great 2D physics engine. If you're working in 3D, try giving ODE a look.

Btw, for integrating the velocity and position I'm using a John Schultz Symplectic Euler (aka Semi-implicit Euler, or Newton-Stormer-Verlet) variation. It's quite stable compared to a regular euler integration. Other integration schemes you can look at are the verlet (specifically the velocity-less verlet), midpoint, and Runga-Kutta 4 (and 3 and 2).

I hope this helps!

edit - Oh, I'd look at simple springs for the collision reaction (or better yet use Box2D or Chipmunk). Here is a decent page on the very basics of physics simulation. http://freespace.virgin.net/hugo.eli...els/m_main.htm

[JustChris]

Damn, I wish I have heard about ODE before. I'm coding my own collision engine from tutorials I've read. I knew about the Newton SDK, but that didn't seem easy to port to the PSP.

I actually have a program which is just in pre-alpha stage right now, to make a bouncing ball. It was done to debug the controls in my program. You can test the bounce against a certain y-value so that the ball reaches a certain height it bounces off the "floor" representing the y-value.

in pseudocode:

if (ball.collidesWith(floor)) {
  if (ball.velocity > velocityLimit) {
    ball.velocity = -ball.velocity * elasticity;
  } else {
    ball.velocity = 0;
    ball.airborne = false;
}

The velocityLimit and elasticity variables are set by the user. Just tweak them till they look "right". A ball with 0 elasticity will stop on a dime, and a ball close to 1 bounciness will jump very high.

If you want to check for collision against polygons that gets more involved.

I'm gonna give ODE a shot. Trying to make a physics engine on my own was educational but I need something that I know will work, and quick.

[BlackShark]

basically for very simple Gravity effect, you would have

gravity = 0.02;
momentumY = 0;

yPosition = 40;

while(bleh) 
{

yPosition += momentumY;

momentumY += gravity;


//other conditions and what not
}

[Archaemic]

Originally Posted by BlackShark

basically for very simple Gravity effect, you would have

gravity = 0.02;
momentumY = 0;

yPosition = 40;

while(bleh) 
{

yPosition += momentumY;

momentumY += gravity;


//other conditions and what not
}

Yeah, if you want the gravity to be cycle-based, and not time-based.

The whole thing will slow down and speed up based on how many times it loops. Not good. You want to base it on time between calculations.

[placo23]

Guys thank you very much for all the explnations and examples. For this one, I'll be only using 2D, as I plan to develop a small game (flash like) using the concepts, just as a proof of concept for myself.

I shall be releasing something in the next days... I've also got another game I'm developing, and shall be releasing it soon.

Thank you very much for all the comments and explanations