(l = 1; 
x = 
          a[t] + l*Sin[φ[t]]; 
y = 
          l*Cos[φ[t]];)

xp = D[x, t]; yp = D[y, t];

L = (1/2)*(xp^2 + yp^2) + grav*y // 
            FullSimplify;

Gleichung=D[D[L,φ'[t]],t]==D[L, φ[t]];

Lsgn=Solve[Gleichung,φ''[t]]//Flatten//Simplify

Energy = (1/2)*(xp^2 + yp^2) - grav*y // 
          FullSimplify