Drag of a polished car?

More complex than that. The behaviour of the flow of water over a small object is similar to the behaviour of the flow of air over a larger object or at a higher velocity.

The top of the ball is not really moving with the airflow. When the surface speed of the top of the ball is greater than the spinning balls motion thru the air the rotation of the ball creates a relative airflow in the same direction as its linear movement. The surface drag on top of the ball is in the direction of its linear motion. Drag is in the direction of the relative airflow that caused it. Pressure differentials do not prove the presents of lift because they are also a characterization of dynamic drag. The dimples increase surface drag and the Magnus affect not lift. The only way the ball can be said to generate lift as a result of it spinning is if you totally and intentionally ignore the large fact that the ball is spinning when determining lift.

Dimples increase friction drag, also called surface or viscous drag. When a golf ball is hit without a spin the dimples promote a stickier turbulent flow. This enhances friction drag. A turbulent boundary layer can better follow the curvature of the ball's profile. It travels farther around the ball before separating, which creates a much smaller wake, and less dynamic drag. Because the dynamic drag is much more prevalent compared to friction drag reducing it reduces over all drag.

The message from Steve Walker contains these words:

Oh gawd, Reynolds numbers!

In message , Guy King writes

Only bit of physics I ever found interesting (I was an aquatic biologist).

snipped-for-privacy@strato.net wrote: The only way the ball can be said to generate lift as a result of it spinning is if you totally and intentionally ignore the large fact that the ball is spinning when determining lift.

Er, that doesn't make sense - ignore that the ball is spinning, then the ball can be said to generate lift as a result of it spinning?

Biggles

The Magnus effect is the force perpendicular to the forward motion of the spinning ball through a fluid. If a non-spinning ball were to generate a force perpendicular to the forward motion of the ball through a fluid it would most certainly be lift. Lift is perpendicular to the relative airflow that caused it and the relative airflow that is causing the aerodynamic force on the non-spinning ball is caused by its motion thru the air.

Moving thru air is just one of three ways to cause a relative airflow. If you move a ball while in the air (rotation) you can also cause a relative airflow around the ball. The aerodynamic force from this flow is reflected in an even surface drag force around the ball that opposes rotation. If this drag force were to become uneven it will result in a more linear motion of the ball. A good way to make the friction drag around the spinning ball uneven is to move it thru the air. A good way to enhance the effect is to apply a surface treatment that increases surface drag like dimples.

Determining lift only from that part of the relative airflow caused by the rotating balls motion thru the air does not make sense because the relative airflow that is causing this aerodynamic force is caused by its rotation as well as its motion thru air. Calling it lift is based on the false premise that the ball is not spinning.

The message from Biggles contains these words:

Of course, it only generates lift if it's spinning /and/ moving through the air at the same time. Just spinning won't do it.

In summary "yes what I originally said does not make sense"?

Biggles

As an aside, the USAAF didn't paint their aircraft late in WWII, they polished the bare metal. Perhaps the performance increase was due to the lack of the weight of the paint, rather than the "slippery" polished surface?