A particle moves in one-dimensional field with total mechanical energy E. If potential energy of particle is U(x) then :
(i) particle has zero speed where U(x) = E.
(ii) particle has zero acceleration where U(x) = E 
(iii) particle has zero velocity where $\frac{\mathrm{dU}\left(\mathrm{x}\right)}{\mathrm{dx}}=0$
(iv) particle has zero acceleration where $\frac{\mathrm{dU}\left(\mathrm{x}\right)}{\mathrm{dx}}=0$