with a > 0 . Physically, this equation can be motivated as follows: a particle of mass m with velocity V t moving in a medium, e.g., a fluid, will experience a friction force which resists motion whose magnitude can be approximated as being proportional to particle's velocity − − a V t with a = c o n s t a n t . Other particles in the medium will randomly kick the particle as they collide with it and this effect can be approximated by a white noise term; σ σ ( d W t / d t ) . Newton's second law is written as