H=0.5*a*t^2得a=2,F=ma求F=4,阻力:28-20-4=4
前6s:H=0.5*a*t^2,H1=36,v=a*t得v=12;后:F=ma得a2=12,v^2=2*a*s得H2=6,故h=42
F=ma得a1=8,a2=6,v^2=2as可知a1*s1=a2*s2,且s1+s2=42,故s1=18;即t1=2.12
H=0.5*a*t^2得a=2,F=ma求F=4,阻力:28-20-4=4
前6s:H=0.5*a*t^2,H1=36,v=a*t得v=12;后:F=ma得a2=12,v^2=2*a*s得H2=6,故h=42
F=ma得a1=8,a2=6,v^2=2as可知a1*s1=a2*s2,且s1+s2=42,故s1=18;即t1=2.12