Ball Rolling

[gizzerd91]

So I've gotten into chipmunk a bit in cocos2d, and have a working scene, with a ball bounding around a bunch of static land blocks. The ball bounces and falls beautifully, but it never rolls. I've been updating the rotation of the ball correctly, because when I manually input a new rotation for the rigidbody, the sprite follows. The trouble is the rigidbody doesn't seem to want to roll on its own. When it collides, it just bounces like it would in a physics sim if the rotation were locked. I was wondering if there is a default lock on rotation, or some other code I was missing to make this work. Here's my (simple) code, if that helps anyone figure out why there's no rolling:

To create my ball:

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	cpBody *ballBody = cpBodyNew(100.0,INFINITY);
	ballBody->p = cpv(160,300);
	cpSpaceAddBody(space, ballBody);
	ballShape = cpCircleShapeNew(ballBody, 10.0, cpv(1.0,4.0));
	ballShape->e = 0.5; // Elasticity
	ballShape->u = 1.0; //Friction
	ballShape->collision_type = 1;
	cpSpaceAddShape(space,ballShape);

To create my floor:

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cpVect blockVerts[] = { cpv(blockOffsetX, blockOffsetY), cpv(blockOffsetX+blockWidth, blockOffsetY), cpv(blockOffsetX+blockWidth,blockOffsetY-blockHeight), cpv(blockOffsetX, blockOffsetY-blockHeight) };
	cpShape *floorShape = cpPolyShapeNew(floorBody, 4, blockVerts, cpv(320.0f / 2, 81.0f / 2));
	floorShape->collision_type=0;
	floorShape->u = 0.5;
	cpSpaceAddStaticShape(space, floorShape);

And finally to move my ball:

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	cpBodyApplyImpulse(ballShape->body, cpv(walking*20,0.0), cpv(0.0,10.0));
	cpSpaceStep(space, 1.0f/30.0f);
	posX = ballShape->body->p.x; //The ball sprites position is set elsewhere, these are value for the enemy AI to use
	posY = ballShape->body->p.y;
	CGFloat radians = ballShape->body->a;
	[ballSprite setRotation:radians*360/3.1415];

Why wont it roll?

[mobilebros]

It doesn't roll because your moment of inertia is set to INFINITY, this line:

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cpBody *ballBody = cpBodyNew(100.0,INFINITY);

Try doing this instead:

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cpBody *ballBody = cpBodyNew(100.0, cpMomentForCircle(100.0, 10, 10, cpvzero));

[DH01]

Thanks! I was having this same problem and couldn't find a resolution.. You made my day!