1. The problem statement, all variables and given/known data A 50 kg sprinter, starting from rest, runs 50 m in 7.0 s 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 2.0 s, 4.0 s and 6.0 s? 2. Relevant equations xf= xo + vt+ 0.5*a*t^2 F= ma vf= vi + at P= (delta Work)/(Delta t) P= f*d*cos angle 3. The attempt at a solution Using the distance, initial velocity, and time I solve for acceleration xf= xo + vt+ 0.5*a*t^2 50 = 0 + 0+ 0.5*a*(7^2) a= 2.04 m/s^2 Therefore, F=ma=50 (2.04)=102 N This, I got right. But when I calculate the power, I'm not getting the right answer when i use the first equation. I calculate the velocity at t= 2s, and I get vf = vi + at vf= 2.04 ( 2) vf= 4.08 m/s Therefore work=delta kinetic energy= 0.5 m (vf^2 - vi^2) K= 0.5 ( 50) ( 4.08^2) K= 416.16 J Therefore delta work=delta kinetic energy=416.16 J Therefore Power= 416.16/ 2 s = 208.08 W. Apparently the answer is just 416.16 J. When I use the second equation of P= F*v=102*4.08=416 J, I get the correct answer. Why are these two equations giving me different answers?