# How does the curve ball drag air around it?

In cricket or baseball there is a type of ball called the curve ball. This is the top spin of the ball.I read that due to spin the ball drags the air around it due to friction in the way shown above.Can you please explain why??

• More Phys.SE posts on Magnus effect: physics.stackexchange.com/search?q=magnus+effect Commented Oct 25, 2013 at 6:07
• @soumyadeep: it certainly looks as if the results Qmechanic suggests address your question. Can you edit your question to indicate precisely what you think has not been answered by the existing questions. Commented Oct 25, 2013 at 7:08
• @John Rennie can you pls just explain the word "aerodynamic drag"and how it works?i found it in one of the answers. Commented Oct 25, 2013 at 17:38
• We don't have a curve ball in cricket. There's something called "flight", but it isn't quite the same thing. And "swing" certainly is something completely different. Commented Oct 25, 2013 at 19:34
• @aufkag OK.but can you pls tell me what is 'aerodynamic drag'? Commented Oct 26, 2013 at 9:13

In this case, fluid molecules near the surface of the ball, to good approximation, are essentially dragged along with the local surface motion. The reason the fluid is dragged along is that the ball surface on a molecular level is not perfectly smooth. You can read about Brownian motion if you want to understand this more in detail. Near the surface, in the boundary layer, the physics driving fluid motion is dominated by viscous forces. The arrows that are drawn in your diagram are more appropriate for describing the fluid motion outside of the boundary layer, which is governed by pressure and momentum.

• How is the fluid motion governed by pressure and momentum? Commented Oct 26, 2013 at 9:16
• @soumyadeep: he just means that outside the BL, to a good enough approximation, frictional forces can be neglected. Commented Nov 4, 2013 at 3:25

Extending on the answer of SimpleLikeAnEgg;

Magnus effect(http:/en.m.wikipedia.org/wiki/Magnus_effect) would work for rotating body and like friction the viscous force (also a type of fricton) would cause the effect of curve ball dragging force.

What happens is that the part of the ball having velocity(tange bntial velocity due to rotation) in direction of motion of ball would then have a resulting velocity greater than that on the other side where there is velocity in opposite directions, due to this effect the air friction (viscous drag/air drag) would be different on the two parts of the ball, similarly due to action-reaction force the friction on nearby air would also be different. Since air is not fixed like ground, it moves under the influence of this friction and as a result we get magnus effect resulting in curve balls dragging air, and producing little alterations in vertical acceleration (due to lift related phenomenon)

• Outstanding!I think i have got it .I am extending you answer a bit.Suppose my ball moves in the right.So friction on upward part is less as relative velocity of wind w.r.t to ball is towards left and it decreases the velocity of air on upward portion and similarly increases velocity in the downpart.Now by Bernoulli equation the pressure increases on upward portion and ball is forced to follow a curved path.Correct me if iam wrong. Commented Oct 27, 2013 at 18:37
• The ball already has forward velocity, and the path of ball would be a little bit more complex then simply saying curved, it definitely would be curved but type of curve would depend on various factors such as speed, material of both ball and reynolds number of air at that particular instant etc. Commented Oct 27, 2013 at 18:46
• OK,but the reason is correct isn't it? Commented Oct 27, 2013 at 18:48
• Maybe at a particular instant, but when it would curve what would you say about its motion. I know that it would be hard to predict, we do computer simulations for such motions as they are too hard to mathematically interpret Commented Oct 27, 2013 at 18:53
• Hmm,that's a point.and another thing is i cannot simply apply bernoulli equation as it is not laminar flow!The whole thing is becoming amazingly complex(and more interesting) now. Commented Oct 27, 2013 at 18:57