  The Langevin equation includes a random force to maintain equilibrium and
prevent friction from bringing motion to a standstill; but for ballistic
motion, the random force is often neglected. Here, we use the Langevin equation
for molecular dynamics simulations of 2.76 eV H-atoms experiencing electronic
friction in collisions with 300 K metals, where a random force arises from
thermal electron-hole pairs. Simulations without the random force fail
dramatically to reproduce experiment, although the incidence energy is much
larger than $k_\text{B}T$. We analyze the Ornstein-Uhlenbeck process to show
that this is a general property of ballistic particles experiencing friction
under the influence of thermal fluctuations.
