So the force that the gymnast with a mass m=45 kg, has to exert against the ground to stop if her acceleration is a=8*g, where g=9.81 m/s², can be obtained from the Newtons second law: F=m*a, where F is the force, m is the mass and a is acceleration.
F=m*a=m*8*g=45*8*9.81=3531.6 N.
So the force that a gymnast has to exert on the mat in order to stop is F=3531.6 N.
