English: Numerical approximation of the trajectory of a particle. Differential equation: If 100 fans are set up to blow air from one end of the room to the other and then a feather isdropped into the wind, what happens? The feather will follow the air currents, which may be very complex. One approximation is to measure the speed at which the air is blowing near the feather every second, and advance the simulated feather as if it were moving in a straight line at that same speed for one second, before measuring the wind speed again. This is called the Euler method for solving an ordinary differential equation.