One more problem to solve... A soldier on skis fires a bullet from rifle horizontally. After what distance will soldier stop moving if coefficient of friction between surface of skis and snow is 0,01. Mass of bullet is 0,01 kg, initial speed of fired bullet is 900 m/s and mass of soldier, rifle and skis is 60 kg? Correct solution is: s=2,9 cm With momentum conservation I have calculated initial horizontal speed of soldier: m1*v1=m2*v2 60*v1=0,01*900 v1=0,15 m/s Kinetic energy is wasted on friction: Ek=W Ek=F*s 1/2*mv^2=m*g*u*s (u is friction coef.) s=11,47 cm which is incorrect. Why?