A wide hose pipe is held horizontally by a fireman. It delivers...

Rate of change of mass, $\frac{d m}{d t}=a v \rho$ $\therefore F=\frac{v d m}{d t}=(a v \rho) v=a v^2 \rho$ Volume of liquid flowing per second $=a v$ This is proportional to velocity. By doubling the volume of liquid flow per second, the force becomes four times. When liquid flows forward, the hose pipe tends to come backward. So, to keep it intact, it should be pushed forward. Thus, the hose pipe should be pushed forward four times.