an experimental rocket sled on a level , frictionless track has a mass of 1.4*10^4 Kg. It propels itself by expelling gases from its rocket engines at a rate of 10 Kg/s, at an exhaust speed of 2.5*10^4 m/s relative to the rocket. For how many seconds must the engines burn if the sled is to acquire a speed of 50m/s starting from rest? You may ignore the small decrease in mass of the sled and the small speed of the rocket compared to the exhaust gas.
ps: i am learning conservation of energy.. this question confused me coz the reading.. somebody help. thanks_