from typing import List


def _update_velocities(v:     List[float],
                       new_a: List[float],
                       a:     List[float],
                       dt:    float
                       ) -> None:
    """Update velocities using current and new accelerations in place"""

    n_particles = len(v) // 3

    for i in range(n_particles):
        for k in range(3):  # x, y, z

            idx = 3 * i + k
            v[idx] += (new_a[idx] + a[idx]) * (dt / 2.0)

    return None