London, April 14 : US researchers have used computer simulations to find out that in baseball, why an apparently simple "pop-up" - a ball that flies almost vertically off the bat, is tricky to catch for a fielder.

According to a report in New Scientist, by modeling forces such as the strike of the bat and air resistance, a team led by Alan Nathan at the University of Illinois, Urbana-Champaign, and Terry Bahill at the University of Arizona, Tucson, calculated the various trajectories that a pop-up can take.

What they found was that while a normally struck baseball follows a parabolic path, a pop-up's path is influenced by the huge backspin it acquires by hitting the top of the bat.

This generates a rotating sheath of air around the ball, which makes its trajectory curve.

In the worst case for the fielder, a backspinning ball begins by flying forwards at a steep angle, before the backspin forces its path to vertical, and then eventually sends it looping back on itself.

Once it reaches its apex and begins to drop, the spin will cause the ball to cross back over its upward path.

When the team ran a computer simulation of how a fielder would react to such an unexpected trajectory, they found it matched the to-and-fro dance that players tend to make when faced with such a tricky catch.

ANI