Answer:
The kinetic energy of an electron in the 1s orbital of a hydrogen atom is due to its motion along the radial direction, not its angular momentum. While the angular momentum of an electron in the 1s orbital is zero, it still has kinetic energy due to its motion along the radial direction, which is determined by the probability density of the electron's wavefunction. This is known as the uncertainty principle, which states that the position and momentum of a particle cannot both be known simultaneously with perfect accuracy. So, even though the angular momentum of an electron in the 1s orbital is zero, it still has some uncertainty in its position, which results in its kinetic energy.