In a central force problem, angular momentum is conserved. we quantized one of the component of L, say Lz. Also, we quantized the angular momentum, L = √l(l+1)h_bar If we know Lx and Ly without uncertainty, then we know the direction of L. Hence we know the motion of the particle is confined in a plane(let say x-y plane). Then we know exactly the position in z direction, which contradicts the uncertainty principle. Hence, we can't know Lx and Ly without uncertainty. Does that mean the direction of the angular momentum is uncertain ? So, can we say angular momentum is conserved ? One more Thing (particle in a box) Can I say E = (p^2 /2m) ? coz E is quantized, so quantization of p violates the uncertainty principle. What's wrong?