Question

http://prntscr.com/tkazrg

Answer 1

$F=600-2\cdot10^5t$

if $F=0$ then $t=600/(2\cdot10^5)=0.003s$

$F=\frac{dp}{dt}\to\Delta p=\int(600-2\cdot 10^5t)dt=600t-10^5t^2=$

$=600\cdot0.003-10^5\cdot (0.003)^2=0.9 (N\cdot s)$