physics please help
posted by Emma .
Worldclass sprinters can accelerate out of the starting blocks with an acceleration that is nearly horizontal and has magnitude 15m/s^2 .
Part a)How much horizontal force F must a sprinter of mass 60kg exert on the starting blocks to produce this acceleration? express your answer in newtons
Part b)
Which body exerts the force that propels the sprinter, the blocks or the sprinter?
question # 2: net force of 225 N accelerates a bike and rider at 2.65m/s^2. What is the mass of the bike and rider together ?
Thank you .

A : F=ma
B : m= F/a