I just want that the movement of object be smooth on 30fps as on 60fps
lol - if you just have half the frames the result only can be half that smooth.
Another thing could be a missing/misplaced
time_step in your movement code.
Always take a deep look on your statistic panel (F11) to see where the time is spent.
PS:
Man are you lost,or something?
Don't be that rude to your helping hands...