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all about Wrist-spin and the Magnus effect

all about Wrist-spin and the Magnus effect

Having looked at various websites and publications with sloppy descriptions of the aerodynamic forces involved in wrist-spin bowling, the one-time A-grade A-level physicist and wannabe aerodynamicist in me has forced me to have a go at describing these effects myself.

So, the Magnus effect. Lets be clear – this isn’t the effect that swing bowlers use. In cricket it is a force mainly utilised by spinners, particularly wrist-spinners as they get the ball spinning fastest.

The history of the Magnus effect

The Magnus effect was first discovered when man invented the cannonball. When round projectiles were fired from a firearm, the ball would grip one side of the barrel, making it spin, and instead of flying straight would curve off right, left, up, down or any which way. This was rather frustrating (unless you were the person being fired at!) and there are some estimates that old unrifled firearms managed to hit their intended target less than 5% of the time.

Heinrich Magnus is the person the effect is named after, having described it in 1852 while investigating how to make German artillery more accurate. It had previously been correctly diagnosed by Isaac Newton in 1674 while watching a game of tennis, as well as being observed by British artillery scientist Benjamin Robins in 1742, which led him to suggest rifling. Somehow, though, the Magnus got the credit…

The Basics

Before you brace yourself for the science, let’s get just one vital term straight. Imagine that the Earth is a cricket ball, and that the seam is the Equator. Then imagine a line straight through the ball going in at the north pole and out at the south pole. When the ball is spun around its seam by a spinner, this line through the ball is the axis around which the ball rotates, and has a rather unoriginal name: the Axis of Rotation.

I’ll mainly deal with the axis of rotation in terms of what it means for the aerodynamic forces on the ball, but there’s one important thing I ought to make clear: When a body is spinning it is subject to gyroscopic effects that require a large force to change it’s rotational axis. The practical upshot of this is that the axis of rotation of a spinning ball as it leaves the hand will remain virtually unchanged until the ball pitches. I have sometimes observed that on TV a spinner’s delivery appears to change its axis of rotation in flight, and after much head-scratching I’m pretty sure this is an optical illusion caused by the strobe effect of the camera, as I’ve never observed this effect with my own eyes.

Getting back to Magnus, imagine a golf ball contentedly sat on its tee. Golfers have learnt over the years that they need to slice underneath the poor defenceless little ball as they tee off, so that the ball travels towards the hole with the underside moving towards the hole and the top side moving away. They do this because when a ball travels trough the air and spins around an axis of rotation that is a) horizontal, and b) perpendicular to its direction of motion, the air around the ball will push it upwards. This means the ball will take off as it leaves the tee, allowing the golfer to hit the ball further.

I use the example of golf because almost uniquely among sportsmen a golfer only ever intends to use the force of the Magnus effect in one direction – upwards. If at any point you get a bit lost and find yourself confused as to which way to spin a ball to get it to move in the intended direction, just remember: Golfers only ever intend to lift the ball up, and they only ever use backspin.

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