Question: F=ma solving

How does one get Maple to solve F=ma problems like r:=t->; When I try diffeq:=Diff(r,t$2)+(r.r)^(-3/2)=0; I get > sol:=dsolve(diffeq,r(t)); Error, (in ODEtools/info) expected an ordinary differential equation. Received: 1/(r^2)^(3/2) = 0
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