Help in character jump..............................

i am using box2d in my game and using the following code to make a player jump….

body~~>ApplyForce * 20), body~~>GetWorldCenter());

it works well in iphone and the frame rate is 59-60.0

but while deploying it in different devices this line of code is not working as an example while in iPad the frame rate goes down to 19-20.0 and the character doesnot jump that same distance and it is also goes for all the devices

so the problem is i am having differnt jumps on different devices, so what are the odds……. anybody there???