How’d you explain the phenomenon of the “curve ball” in sports like tennis, baseball, golf or cricket?
In many games like cricket, golf, tennis and baseball, we notice that a spinning ball deviates from its parabolic trajectory as it moves through air.
I know that this deviation can be partly explained on the basis of Bernoulli’s principle, I already have text regarding this topic, I was just wondering if someone could explain the phenomenon in simpler words.
Thank you very much.
The “curve ball” is called “swing ball” too
it all comes down to the difference between laminar flow and turbulent flow. think about a non-spinning ball moving through the air:
if the surface of the ball is perfectly smooth, then the motion of the air just near the surface (this is called the boundary layer) will be perfectly smooth too. as the air moves around the sides towards the back of the ball, the angle between the direction of the air’s motion and the surface of the ball will get bigger and bigger.
at some point the boundary layer won’t be able to “stick” to the surface anymore and will detach. (this phenomenon is called boundary layer separation.) the faster the air is flowing, the separation point will be closer to the place at which the ball is widest. this separation means that, behind the separation point, the air near the ball’s surface will be turbulent. it is essentially what happens to an airplane’s wing during a stall.
because the ball is symmetric, the turbulent area will be equally spread between the left and right sides of the ball.
(a brief aside: laminar flow exerts a greater pressure on the ball’s surface than turbulent flow does. this is why aircraft engineers go to great lengths to make the wings of a plane very smooth – if the airflow around the wing is very smooth too, then the wing can generate greater lift.)
anyway, so everything’s symmetric for a ball that’s not spinning. now think about a ball that’s spinning around it’s vertical axis. the relative velocity of the air to the ball’s surface is different on the left and right side because of the spinning. this velocity difference means that the boundary layer separation happens at a different place on the left or right side.
this means that the left/right side of the ball will have a larger/smaller area with turbulent flow at the surface. the symmetry is broken and the resulting pressure difference causes a net left/right force on the ball, which makes it curve.
there is a nice picture of the non-spinning ball at the bottom of this page:
http://astron.berkeley.edu/~jrg/ay202/node97.html
here’s a paper about it, with a nice picture that shows the laminar and turbulent asymmetry:
http://www.wooster.edu/physics/JrIS/Files/nowicki.pdf
more pictures near the bottom of this page:
http://www.aerospaceweb.org/question/aerodynamics/q0215.shtml
cheers!
Mario Golf and Tennis
