Why is p=fv different than calculatingit manually

[jiggleswiggly]

A 49.0 sprinter, starting from rest, runs 45.0in 7.10 at constant acceleration.
What is the magnitude of the horizontal force acting on the sprinter?


okay so I got 87.5N which is correct

Then it asks:
What is the sprinter's power output at 1.70 , 3.60 , and 5.50 ?


why is it when you use d=1/2*a*t^2
it is wrong

see:
(1/2)*(1.78)(1.7)^2 = 2.57 for distance

then 2.57*87.5= 224 joules

then for power it is 224/1.7 = 131

but the answer is exactly twice that

if you use p=f*v you get the correct answer straight away

shouldn't these be the same?

 

[cjl]

Because the power output isn't constant, and when you're calculating the power using the distance traveled and the time elapsed, you're finding the average power up to that point, not the instantaneous power.