P. 4098. A spaceship gets into an interplanetary dust cloud. The average mass of the grains of the cloud in a cubic metre is is 0.02 mg. The speed of the spaceship with respect to the cloud is 10 km/s, the area of its cross section which is perpendicular to its speed is 50 m2. The collision between the grains and the spaceship is totally inelastic. What must the thrust of the engine be in order not to change the speed of the spaceship?