Question

If a 70kg swimmer pushes off a pool wall with a force of 250N. What is her acceleration?

Answer 1

Since , F = ma 250 = 70 × a a = 25/7 m/s²

Answer 2

Newton taught us:                              Force = (mass) x (acceleration)

Divide each side by (mass) :              Acceleration = (force) / (mass) .

The only problem here is:  This formula applies when the "Force" is the
only force on the object.  When the objects in these school problems are
falling out of airplanes, shot from guns, or being hit by baseball bats, we
routinely ignore the force of air resistance against the object.  We're
comfortable with that, maybe because it's become a habit.  But now,
we're not so comfortable about ignoring the force of water resistance.

All I can tell you is that if you DO ignore the water resistance, that is,
if the water were not there, her acceleration would be

                    (250 newtons) / (70 kg) = 3.57 m/s² = about  0.36 g .

But what is it really, in the water ? 

If you've spent any substantial amount of time anywhere near competitive
swimmers, then you know that it depends on their position coming off the
wall, what they do with their knees and knuckles, how straight they hold
their body, how deep the texture of their swim-cap is, and how well they've
shaved their legs.