F = ma = mΔv/t = m(vf - vi)/t = mvf/t (since vi = 0)
⇒ a = vf/t
While accelerating from rest to vf, the distance d the swimmer traveled satisfies
d = (1/2)at2
Then
d = (1/2)(vf/t)t2 = vf t/2 ⇒
t = 2d/vf
F = ma = mvf/t = mvf / (2d/vf) = mvf2 / (2d)
F = [45 * 122 / (2 * 20)] N = 162 N
If you also want to know how long the swimmer took to accelerate to vf,
t = 2d/vf = (2 * 20 / 12) s = 3.33 s
