Small SpriteKit demo

MyScene.m

#import "MyScene.h"

@interface MyScene () <SKPhysicsContactDelegate>
@end

@implementation MyScene {
  NSMutableSet *_notCrashedPhysicsBodies;
}

static const uint32_t carCategory = 0x1 << 0;

const float worldScale = 10; // 1m == 10pt
const float carLength = 4;
const float carWidth = 2;
const float carMass = 1000;
const float speedLimit = 30 * 1.609 * 1000 / 3600; // 30 mph, in ms^-1
const float accelerationForce = 10000; // 10 kN

- (id)initWithSize:(CGSize)size {
	if (self = [super initWithSize:size]) {

		_notCrashedPhysicsBodies = [NSMutableSet set];

		self.physicsWorld.contactDelegate = self;
		self.physicsWorld.gravity = CGPointZero;

		CGPoint location = CGPointMake(size.width / 2, size.height / 2);
		SKNode *parkedCar = [self addCarAtLocation:location];
		parkedCar.physicsBody.linearDamping = 1;
	}
	return self;
}

- (SKSpriteNode *)addCarAtLocation:(CGPoint)location {
	CGSize size = CGSizeMake(carLength * worldScale, carWidth * worldScale);

	SKPhysicsBody *physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:size];
	physicsBody.mass = carMass;
	physicsBody.categoryBitMask = carCategory;
	physicsBody.collisionBitMask = carCategory;
	physicsBody.contactTestBitMask = carCategory;
	physicsBody.angularDamping = 10;

	SKSpriteNode *carSprite = [SKSpriteNode spriteNodeWithImageNamed:@"car.png"];
	carSprite.color = [UIColor colorWithRed:(float) rand() / RAND_MAX green:(float) rand() / RAND_MAX blue:(float) rand() / RAND_MAX alpha:1];
	carSprite.colorBlendFactor = 0.8;
	carSprite.position = location;
	carSprite.physicsBody = physicsBody;

	[self addChild:carSprite];
	return carSprite;
}

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
	for (UITouch *touch in touches) {
		CGPoint location = [touch locationInNode:self];
		SKSpriteNode *sprite = [self addCarAtLocation:location];
		[_notCrashedPhysicsBodies addObject:sprite.physicsBody];
	}
}

- (void)update:(CFTimeInterval)currentTime {
	for (SKPhysicsBody *carPhysicsBody in _notCrashedPhysicsBodies) {
		CGFloat velocity = carPhysicsBody.velocity.dx;
		NSLog(@"%f mph", velocity * 3600 / (1000 * 1.609 * worldScale));
		if (velocity < speedLimit * worldScale) {
			[carPhysicsBody applyForce:CGVectorMake(accelerationForce * worldScale, 0)];
		}
	}
}

- (void)didBeginContact:(SKPhysicsContact *)contact {
	for (SKPhysicsBody *body in @[contact.bodyA, contact.bodyB]) {
		body.linearDamping = 1;
		[_notCrashedPhysicsBodies removeObject:body];
	}
}

@end

http://www.youtube.com/watch?v=Q_5iyyl_wKU