Question

A basketball is dropped from 140 meters height with back spin. It goes straight down and then it gains speed as it curves 70 meters horizontally away from where it is dropped. That is a spectacular example of the Magnus effect. A paper cylinder is dropped down the inclined plane and rolls backward. This is another example of Magnus affect. Why is the example of the basketball more extreme than that of the paper cylinder ?

Answer 1

Basically, the reason for magnus effect is that, when there is a difference in pressure developed on the two sides of the object due to various reasons (rotation, shape, or smoothness), then the object sways on the lower pressure side.

Hence in case of basketball, when the basketball is dropped with back spin, the back side of the basketball is moving down and the front side is moving up, now as the ball falls down, it cuts the air, and air blows from both the sides, but since front is moving up, therefore the air from the front side passes, swiftly. Whereas the air moving from the backside of the ball is slower in comparison.

So using the bernaulli's theorem, We can say that pressure is lower where the speed of air is high. And so, the basketball sways on the lower pressure side (i.e. in the front of basketball)

Now, for cylinder, dropped from an incline.

Here the cylinder first rolls on an incline, producing a forward spin, therefore the back side of cylinder is moving up and front side moving down, resisting the air to pass through front, hence the pressure is higher on the front side, and the cylinder sways backwards.

Now in comparison to basketball, cylinder sways very less, this happens because of the forces applied by air, (buoyant & resistive) the force applied on cylinder is comparable to its weight hence it's speed doesn't increase appreciably, therefore the difference in pressure on both the sides is not that significant. Whereas in case of basketball, the force by air is very less in comparison to weight of the basketball, hence the basketball gains high speed, producing high pressure difference on both sides, and effectively showing extreme results of swaying due to magnus effect.