Take the tour ×  I was told that the total integral of the
stress over the surface of a swimmer (i.e. the total force
exerted by the swimmer on the fluid) always vanishes, because
there are no external forces applied on it. That seems fair by
the 3rd Newton Law.  But, how does it take into account the
effects of the 2nd Newton's Law? If, for example the swimmer
starts from a stationary state, and at the time $t_1$ reaches a
velocity $v_1$, where does the force generating the acceleration
$v_1/t_1$ come from?  I was told that this can happen just
because in the case of a swimmer with density different than
that of the fluid, so gravitational forces are present. But it
seems bulls**t to me. Could you help me to clarify all this?
share|improve this question add comment  1 Answer  up vote 3
down vote accepted  Let's imagine a swimmer in a swimming pool
and approximate the Earth as an inertial frame. The swimmer can
certainly accelerate relative to the Earth frame (in the
direction parallel to the Earth's surface