World-class sprinters can accelerate out of the starting blocks with an acceleration that is nearly horizontal and has magnitude 15m/s2.   1) How much...

World-class sprinters can accelerate out of the starting blocks with an acceleration that is nearly horizontal and has magnitude 15m/s2.

1) How much horizontal force 

F must a sprinter of mass 51kg exert on the starting blocks to produce this acceleration?

2) Which body exerts the force that propels the sprinter, the blocks or the sprinter?