Re: can a batted baseball pick up speed on the second or third bounce?

The answer is simply: no.  The ball may appear to pick up speed because it
doesn't slow down as much as one might expect.  I recently answered a similar
question about a volleyball hitting a beam (927551792.Ph).  The "skipping" of a
ball on a bounce is very similar to skipping stones on water.  

Baseball fans, players, and commentators are often guilty of claiming that 
"the ball took off."  This is especially common on astroturf where a low line
drive can skip off the turf creating the illusion of speed gain because the spin
of the ball does not change the resulting direction much at collision because of
a lower frictional effect at ball-turf impact.  The result is that the ball
leaves the turf after the bounce at a different angle and with more energy than
is experienced on grass or dirt.  

A key point to a batted ball:  the spin of the ball in flight after being hit is
almost entirely determined by the bat.  When the ball is struck by the bat it
compresses significantly and much of the spin of pitch is lost.  In racket
sports is often possible to hit the ball with significant top spin so that when
the ball hits the ground the topspin gives the ball a little "boost" from the
grab of the ball on the surface creating a force in the same direction as
general motion of the ball.  This is especially cool in ping pong.  This topspin
happens only in a minor way in baseball.  Sometimes a "hump back liner" occurs
in which a ball is hit in the air with top spin and it seems to die (descend)
much quicker than expected.  More common is a baseball in the air with backspin
or more uncommon a liner with "no spin" that "knuckles" through the air.  A
knuckling line drive is more common in softball.  Most baseball/softballs that I
have observed skip started out as low line drives which probably means that the
ball after hitting the bat has a small amount of top spin.

The "skipping" effect is more pronounced when the field is wet.  So it is
possible to see a "skip" on short wet grass on a hard field or wet astroturf.

On rare occassion, the "skip" effect can happen on the second or third bounce. 
I have experienced this playing softball on a very humid night.  A sharply hit
line drive to the outfield will bounce normally (more common to experience) and
then hit a hard spot of short, wet grass and "skip" away.  

The absolutely most extreme example of this "skip" effect is skipping stones
across water.  If one throws the stone with a flat enough trajectory it will
bounce off the water.  If the stone hits the water either too slowly or to close
to vertical -- "ploop" the stone falls into the water because the surface
tension of the water cannot support the incoming impact of the stone.  If the
stone is coming close to the surface of the water (grazing angle below some
critical angle that depends on a lot of things) the impact force straight down
into the water from the stone can be supported by the surface tension.  Often
flat stones thrown like a frisbee or discus skip best because the impact force
is spread out over a bigger surface area.  The skip of the stone often astounds
people who haven't seen it before (outside of their experience), but that is
what makes physics fun.

Sincerely,

Tom "Gold Glove, Rusty Legs" Cull