Solution to task 341 from Chertov´s for extramural stud

Solution to task 341 from Chertov´s for extramural stud

341. A grain of dust with a mass of m = 200 μg, carrying a charge of Q = 40 nC, flew into the electric field in the direction of the lines of force. After passing the potential difference U = 200 V, the dust grain had a velocity v = 10 m / s. Determine the speed of a speck of dust before it flew into the field.