# Horizontal Force

A 50kg sprinter, starting from rest, runs 50m in 7.0s at constant acceleration.
a) What is the magnitude of the horizontal force acting on the sprinter?
b) What is the sprinter's power output at 2.0s, 4.0s, and 6.0s?
I think maybe the 'magnitude' part is confusing me.
F = ma
V(avg) = (50-0)/(7-0) = 50/7
A = (50/7)/7 = 50/49
F= 50kg * 50/49
F is exactly half of what the answer in my book says, what do I need to do and why?
Thank You,
$$d = v_it + \frac{1}{2}at^2$$