Forever moving ball

Question

How do I create a body [A ball Body] that will bounce around the screen, never losing or gaining speed, regardless of what it hits in cocos2d-box2d?

Answer 1

Set restitution of the fixture to 1, and friction to 0.

Box2D manual says:

Restitution is used to make objects bounce. The restitution value is usually set to be between 0 and 1. Consider dropping a ball on a table. <...> A value of one means the ball's velocity will be exactly reflected. This is called a perfectly elastic collision.

A friction value of 0 turns off friction

Without friction and with perfectly elastic collision your ball will bounce around the screen, never losing or gaining speed in a static environment. If environment is not static, then colliding with moving object will change speed of the ball.

To solve this problem, I suggest next trick. Set contact listener, and in PostSolve method correct speed of you ball like this:

void PostSolve(b2Contact* contact, const b2ContactImpulse* impulse)
{  
   if(contact->GetFixtureA()->GetBody() == YOUR_BALL_BODY ||
      contact->GetFixtureB()->GetBody() == YOUR_BALL_BODY)
   {
       float speed = YOUR_BALL_BODY->GetLinearVelocity().Length();
       float koef = YOUR_NEEDED_SPEED / speed;
       YOUR_BALL_BODY->SetLinearVelocity(koef * YOUR_BALL_BODY->GetLinearVelocity());
   }
}

How to set contact listener see there.