Ans) In the first one..... F=ma...the only force which acts is F...which causes an acc. of ''a'' to the body of mass m...
and as the force=>F=ma
but on the second...
let ....ma=F(2) ....then we have ..F - F(2)=0
which shows two forces acting on the particle or system whose resultant is zero..
As only 1 force acted in the first case and two forces acted on the second case if we consider the system only.