Clear["Global`*"]

DSolve[{f'[x]==k*f[x],f[0]==f0},f[x],x]

% /.{k->-.5,f0->1000}

Plot[Evaluate[f[x]/.%],{x,-1,3}]

DSolve[{f'[x]==-0.5*f[x],f[0]==1000},f[x],x]

Plot[Evaluate[f[x]/.%],{x,-1,3}]

Clear[phi,smallanglephi]

DSolve[{phi''[t]==-(g/R)phi[t],
      phi[0]==phi0},
	phi[t],t]

% /. C[2]->0

smallanglephi[t_]:=Evaluate[phi[t] /. % /. {g->9.8,R->5, phi0->20*Pi/180}]

Plot[smallanglephi[t],{t,0,10}]

s= NDSolve[{y''[x]==-(9.8/5)*Sin[x],y[0]==20*Pi/180,y'[0]==0},y,{x,0,30}]

Plot[smallanglephi[t],{t,0,30}]