ios - make physics body jump in cocos2d-Box2d

Question

I am a newbie to the gaming world i am stuccoed to make a physics body jump..
here is how i defined the body

Cycle = [CCSprite spriteWithFile:@"Panda.png"];
        [self addChild:Cycle z:3];

    // Create a world
    b2Vec2 gravity = b2Vec2(0.0f, -8.0f);
    world = new b2World(gravity);

    // Create edges around the entire screen
    b2BodyDef groundBodyDef;
    groundBodyDef.position.Set(0,0);

    b2Body *groundBody = world->CreateBody(&groundBodyDef);
    b2EdgeShape groundEdge;
    b2FixtureDef boxShapeDef;
    boxShapeDef.shape = &groundEdge;

    //wall definitions
    groundEdge.Set(b2Vec2(0,0), b2Vec2(screenSize.width/PTM_RATIO, 0));
    groundBody->CreateFixture(&boxShapeDef);

    groundEdge.Set(b2Vec2(0,0), b2Vec2(0,screenSize.height/PTM_RATIO));
    groundBody->CreateFixture(&boxShapeDef);

    groundEdge.Set(b2Vec2(0, screenSize.height/PTM_RATIO),
                   b2Vec2(screenSize.width/PTM_RATIO, screenSize.height/PTM_RATIO));
    groundBody->CreateFixture(&boxShapeDef);

    groundEdge.Set(b2Vec2(screenSize.width/PTM_RATIO, screenSize.height/PTM_RATIO),
                   b2Vec2(screenSize.width/PTM_RATIO, 0));
    groundBody->CreateFixture(&boxShapeDef);

    // Create ball body and shape
    b2BodyDef ballBodyDef;
    ballBodyDef.type = b2_dynamicBody;
    ballBodyDef.position.Set(300/PTM_RATIO,100/PTM_RATIO);
    ballBodyDef.userData = Cycle;
    body = world->CreateBody(&ballBodyDef);


    b2PolygonShape dynamicBox;
    dynamicBox.SetAsBox(.5f, .5f);//These are mid points for our 1m box

    // Define the dynamic body fixture.
    b2FixtureDef fixtureDef;
    fixtureDef.shape = &dynamicBox;
    fixtureDef.density = 1.0f;
    fixtureDef.friction = 0.3f;
    body->CreateFixture(&fixtureDef);

and in touches began i am applying linear impulse as

b2Vec2 force = b2Vec2(30, 30);
body-> ApplyLinearImpulse(body->GetPosition(),force);

so can any body tell me what am i doing wrong..
thanks in advance..