Use "compare two values" from the system condition. Since you probably have only one ball, you don't have to worry about picking the right instance, so simply compare ball.physics.yourVariable and 50.
I used System -> ball.Physics.VelocityX = 0.05 -> Ball -> Apply Physics impulse 0.08 toward (0,0) at image point 0.
It seems to use decimals instead of big numbers.
It seems to do the job, in an awkward way ^^
Please let me know if it needs tweaking.