box2d - libgdx

///// body construction
        BodyDef bodyDef = new BodyDef();
        // We set our body to dynamic, for something like ground which doesnt move we would set it to StaticBody
        bodyDef.type = BodyType.DynamicBody;
        // Set our body's starting position in the world
        bodyDef.position.set(100, 200);

        // Create our body in the world using our body definition
        Body body = world.createBody(bodyDef);

        // Create a circle shape and set its radius to 6
        CircleShape circle = new CircleShape();
        circle.setRadius(20f);

        // Create a fixture definition to apply our shape to
        FixtureDef fixtureDef = new FixtureDef();
        fixtureDef.shape = circle;
        fixtureDef.density = 500f;
        fixtureDef.friction = 0f;
        fixtureDef.restitution = 1f;

        // Create our fixture and attach it to the body
        fixture = body.createFixture(fixtureDef);

//// apply force on press of keys
        Vector2 impulse = new Vector2();
        impulse.x = ( Gdx.input.isKeyPressed( Input.Keys.LEFT ) ? - 1000
                : ( Gdx.input.isKeyPressed( Input.Keys.RIGHT ) ? 1000 : 0 ) );
        impulse.y = ( Gdx.input.isKeyPressed( Input.Keys.UP ) ? 1000
                : ( Gdx.input.isKeyPressed( Input.Keys.DOWN ) ? - 1000 : 0 ) );


//// painting using the body pos
    Actor e = (Actor) b.getUserData();

    if (e != null) {
        // Update the entities/sprites position and angle
        e.setPosition(b.getPosition().x, b.getPosition().y);
        // We need to convert our angle from radians to degrees
        e.setRotation(MathUtils.radiansToDegrees * b.getAngle());
    }