Calvin's dad is explaining the difference between rotational speed and tangential (linear) speed. 1. When Calvin's dad says "two points on one disk move at two speeds", is he referring to rotational speed or tangential speed? PLAYING A RECORD? I'LL SHOW YOU SOMETHING INTERESTING. COMPARE A POINT ON THE LABEL WITH A POINT ON THE RECORD'S OUTER EDGE, THEY BOTH MAKE A COMPLETE CIRCLE IN THE SAME AMOUNT OF TIME, RIGHT? YEAH... M BUT THE POINT ON THE RECORD'S EDGE HAS TO MAKE A BIGGER CIRCLE IN THE SAME TIME, SO IT GOES FASTER. SEE, TWO POINTS ON ONE DISK MOVE AT TWO SPEEDS, EVEN THOUGH THEY BOTH MAKE THE SAME REVOLUTIONS PER MINUTE!