50kg sprinter's horizontal force and power output

[maki]

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,
in advance =o

 

[mezarashi]

Because the question explicitly states that there is a constant acceleration you cannot use the average velocity principle. You should instead refer to your kinematics equations:

$$d = v_it + \frac{1}{2}at^2$$

and solve for the acceleration. Once you get acceleration, as you have shown you understand, you may apply it to Newton's second law.

 

[maki]

works so much better that way, ty ;p