TKOF PRIOR TO CABIN ATTENDANT BEING SEATED AND STRAPPED IN.

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AS WE APCHED RWY 10L I MADE THE BEFORE TKOF PA. AT THIS TIME THE #1 FLT ATTENDANT CAME UP AND SAID THEY HADN'T YET FINISHED THEIR DEMONSTRATION. ABOUT 30 SECONDS LATER SHE RETURNED AND SAID 'OK; WE'RE READY NOW.' I RANG THE CHIME; TURNED THE CORNER ONTO THE RWY; AND THE FO BEGAN THE TKOF. SOON AFTER LEVELOFF OUR #1 FLT ATTENDANT ADVISED ME THERE WAS AN FAA SAFETY INSPECTOR ON BOARD WHO WANTED TO DISCUSS MY TAKING OFF BEFORE THE #4 FLT ATTENDANT WAS SEATED. THIS WAS THE FIRST I KNEW THIS HAD OCCURRED. THE INSPECTOR; NAMED MR. X; CAME UP AND SAID THE #4 DIDN'T GET SEATED AND STRAPPED IN UNTIL JUST PRIOR TO ROTATION. THIS OBVIOUSLY CONCERNED HIM; BUT NOT UNDULY; AND HE SEEMED CONTENT TO LET US OFF WITH A WARNING. SUBSEQUENT DISCUSSION WITH THE 2 FLT ATTENDANTS INVOLVED REVEALED HOW THIS HAD HAPPENED. FIRST; I NEGLECTED TO BRIEF OUR #1 FLT ATTENDANT ON THE SHORT TAXI; WHICH TURNED OUT TO BE 8 MINS BUT INVOLVED A FAIRLY SHORT DISTANCE. SO SHE BEGAN THE DEMONSTRATION LATER THAN SHE MIGHT HAVE; AND THE PROCESS WAS FURTHER DELAYED BY THE LACK OF A DEMONSTRATION BAG FOR THE EXTRA FLT ATTENDANT. 1 BAG EVIDENTLY HAD TO BE USED FOR FIRST CLASS FIRST; THEN COACH. FLT ATTENDANT #1 MAY HAVE MISJUDGED HOW LONG IT WOULD TAKE ME TO GET ROLLING; OR HOW LONG #4 WOULD NEED TO PUT AWAY THE DEMONSTRATION BAG AND RETURN TO HER AFT JUMPSEAT; BUT THE RESULT WAS THE #4 BEING SURPRISED BY THE UNEXPECTED ACCELERATION AND HAVING TO RUN TO HER SEAT AND STRAP IN. UPON ARR IN CHICAGO WE REQUESTED ANOTHER DEMONSTRATION BAG.