![alt text][can you please help me my ball not jumping at one fixed speed some time bal jump very high why?]
Hey there and welcome to the forum,
first up: Please use the built in code formatting to post code and do not attach screenshots of it. Someone who might want to test your code will not retype it from an image.
About your problem:
You add a force that you multiply by Time.deltaTime. This is needed when you handle Forces that are supposed to be evenly distributed over a given sequence of time since it will make sure that the sum of force over this time is not dependent on the amount of frames. In your case you only apply this force for exactly 1 frame. When you do this you should not use Time.deltaTime since it gives you the length of the current frame. On a faster machine your ball will always bounce lower than on a slow one since the frames will be longer/shorter accordingly.
TL;DR: Force over time, use Time.deltaTime; Force applied in only one frame, do not use Time.deltaTime
If that was unclear in any way feel free to let me know, i will try to clarify.