public void update(UserBall b){
		BPx = b.getPx();
		BPy = b.getPy();
		BVx = b.getVx();
		BVy = b.getVy();
		BRad = b.getRadius();
		
		if  ((BPy+BRad > Py) && (BPy+BRad < Py+BRad) && (BPx+(0.5)*BRad > Px) && (BPx-(0.5)*BRad < Px+width)){
			// ball is on top of platform
			BPy = Py-BRad;
			BVy = -60;
		}
		
		b.setPx(BPx);
		b.setPy(BPy);
		b.setVx(BVx);
		b.setVy(BVy);
	}