# Can a smooth ball be made to curve?

1. Feb 14, 2010

### harry.butts

I've been trying to find the answer to this question for a*long* time. I think I've finally come to the right place for the answer.

Can a smooth ball be thrown so it curves?
A lacrosse ball is is basically smooth. There's a small seem where the halves are glued -or melted or whatever and a small logo less than 1mm deep and maybe the size of a nickle or a bit bigger. It weighs between 5 & 5.25 oz. Hypothetically, let's say it's thrown at around 80MPH. When it's thrown, there are three forces: forward movement, gravity & rotation. There's also the Magnus effect, but I'm not sure which side would have the least resistance (and would it always be the same side?) That's about as far as I've gotten.

When I say curve, i don't mean like a slow-pitch softball (a rainbow type). I'm referring to like a baseball: left-to-right, or vice versa.

I am, in no way, related to physics. I'm a computer geek and a lacrosse coach, so take it easy on my, please If I need to supply more info, please let me know.

2. Feb 14, 2010

### Anti-Meson

The answer to your question, yes. Cricketers who bowl spin can get the ball to arc in the air before it pitches due to the reasons you have outlined.

3. Feb 14, 2010

### Staff: Mentor

There is no such thing as completely smooth, but the smoother and heavier a ball is, the less it will curve.

4. Feb 14, 2010

### harry.butts

When I say smooth, I guess I mean in comparison to a baseball (with the stitching). I know how/why it curves.

Anti-Meson, I'm not familiar with cricket. When you say arc, are you referring to an up & down curve? I'd like to know if it will curve left-to-right or right-to-left.

I don't think I said the ball is being thrown from a lacrosse stick, and there's a ton of spin. If you throw it sidearm, it'll spin L to R or R to L, depending on what hand one is using.

ress_waters, are you saying "yes", but it depends on the weight and the smoothness of the ball?

If I may be so bold, how do you know?

5. Feb 14, 2010

### Anti-Meson

When I mean arc, the ball does not only dip suddenly from release but it also moves laterally all before it hits the ground. So a combination of up/down and left/right motion. This more apparent when the ball dives.

Can't find any hawkeye trajectories though for spin bowlers as this would highlight this. You can also see this is in field hockey where players sling the ball similar to what lacrosse players do. This noticeability of the spin however is small.

Last edited by a moderator: Apr 24, 2017
6. Feb 14, 2010

### Staff: Mentor

I'm saying that your hypothetical completely smooth ball doesn't exist. If it did, it wouldn't' curve. In the real world, every ball curves.

7. Feb 14, 2010

### Anti-Meson

Even if such a hypothetically smooth ball did exist, it would curve as the Magnus effect is not constrained to balls will friction. It is the act of the ball spinning about its axis and travelling through space that causes the effect. Take a table tennis ball for example, it can be considered as almost frictionless sphere yet it is greatly affected by the Magnus effect.

8. Feb 14, 2010

### rcgldr

Within the limits of the real world, and at slower speeds, a "smooth" ball will curve more than a "rough" ball, but at higher speeds the situation is apparently reversed.

In the case of slower speeds, I know from experience with the old 38mm table tennis balls. A "Peace" ball was the smoothest, almost like a billard ball, and it curved the most. A "Barna" and maybe "Schildkrot" balls were the roughest surface balls, looking like they was painted with "flat" white instead of "glossy" and those rough surface balls curved the least. "Halex" and "Nittaku" table tennis balls were somwhere between these extremes.

In the case of full scale gliders, 600 grit sandpaper is sometimes used to rough up the wing surfaces to control where the laminar flow is tripped.

correction - Original incorrect statement: The dimples on a golf ball are there to reduce the amount of curving (Magnus Effect), although that is going beyond a "rough" surface. Corrected statement, the dimples in a golf ball reduced the drag and increase Magnus Effect. Golf balls travel fast enough that the laminar flow separates before reaching the top and bottom of the ball, resulting in a larger wake. The dimples trip up the laminar flow into tubulent flow sooner, and turbulent flow can follow a curved surface easier, reducing the size of the wake. The turbulent flow remains attached longer on the rear part of the ball, and on a spinning ball, the turbulent flow remains attached longer on the backwards moving surface, diverting the flow more, resulting in a greater Magnus Effect.

In the case of table tennis balls, they don't curve very much until drag slows down the speed of the ball, probably to the point where laminar flow remains attached a bit on the rear side of the ball, and the flow attachment at this lower speed results in the case of a smoother ball curving more than a rough ball.

So an issue here is the speed of the ball, it's mass and size (drag coefficient).

Last edited: Feb 15, 2010
9. Feb 15, 2010

### harry.butts

Thank you all for the replies. As I mentioned, I have no connection to physics. If I could go back in time, I would do something in the field. It is an incredibly fascinating science.

Unfortunately, most of it is beyond the grasp of my "feeble brain." I wish I could see things in action (through experiments or labs or film or something). It kind of baffles me how or why a smooth ball will curve more than a rough ball.

PS. The cricket thing is really cool!

10. Feb 15, 2010

### harry.butts

May I also ask what forces are applied? Is it the rotational, gravitational & velocity I already mentioned, or are there more?

I looked up info on a cricket ball. It has six seems keeping the cover on it. Are the seems what make it curve?

Again, I know there's no such thing as "completely smooth" (like there's no such thing as "perfectly" straight). I understand that part. When I say smooth, I mean with no seems, no dimples, no stitching. Just a "smoothish" rubbery surface.

So, russ_watters, (*hypothetically*) if one could throw a bowling ball (without the finger holes) fast and far, it wouldn't curve (save for the action of gravity)? A bowling ball is about the smoothest thing I can think of that's heavy. I'm sure there are other things (ball bearings maybe) that might be smoother, but I'm more of a jock, so I stick to the sporting examples.

Last edited: Feb 15, 2010
11. Feb 15, 2010

### rcgldr

A bowling ball wouldn't curve significantly unless at very high speed and moderate to high spin, perhaps if it could be shot out of some type of high pressure cannon.

Something like a hollow wiffle ball without the holes could be thrown to produce a slight curve.

Table tennis balls are very light for their size and curve easily. The table tennis paddles use a very sticky and elastic rubber surface, allowing a lot of speed and spin on the balls, so they can curve quite a bit, including truly rising from backspin. Example video with mostly top spin and side spin related curves:

http://rcgldr.net/real/tt2.wmv

The best description I could find is from this archived article:

The more recent studies agree that the magnus force results from the asymmetric distortion of the boundary layer displacement thickness caused by the combined spinning and flow past the spherer. In the case of a sphere(or cylinder), the so-called whirlpool, or more accurately the circulation, does not consist of air set into rotation by friction with a spinning object. Actually an object such as a sphere or a cylinder can impart a spinning motion to only a very thin layer next to the surface. The motion imparted to this layer affects the manner in which the flow separates from the surface in the rear. Boundary layer separation is delayed on the side of the spinning object that is moving in the same direction as the free stream flow, while the separation occurs prematurely on the side moving against the free stream flow. The wake then shifts toward the side moving against the free stream flow. As a result, flow past the object is deflected, and the resulting change in momentum flux causes a force in the opposite direction(upwards in the case shown in figure 1).

http://web.archive.org/web/20071018203238/http://www.geocities.com/k_achutarao/MAGNUS/magnus.html

The key here is the boundary layer separation at the rear of a ball.

I was wrong about golf balls and corrected my previous post. In the case of a golf ball, because of the higher speeds, the laminar flow would separate from the ball on the front side of a smooth ball resulting in a larger wake that creates a type of "wake form drag". The dimples trip the boundary layer into a higher energy laminar flow, which can follow a curved surface better than a lower energy laminar flow. There's increased "skin drag", but the turbulent flow reattaches and follows the curve of the ball to the back side of the ball, resulting in a smaller wake, and the decrease in "wake form drag" more than offsets the increase in "skin friction drag".

http://www.knetgolf.com/GolfBallDimp.aspx [Broken]

Looking at that golf ball article, if the dimples are too small, then you get more lift, but also more drag. The wake would be larger, and displaced enough so that the Magnus effect is also larger, but the large wake produces more drag. At the other extreme, if the dimples are too deep, you get less lift and less drag, but the total distance becomes less than optimal dimple size.

The competing issues here are how large the wake is and by how much that wake is deflected, and this is affected by the smoothness, speed, size, and spin of the ball. If the ball is moving slower, the laminar flow remains attached longer, eventually remaining attached on the back side of a ball, which changes the situation allowing smoother ball to curve more, which is the case for table tennis balls.

In the case of table tennis balls, the ball tends to curve less at higher speeds, then curve more as the speed decreases due to the relatively high amount of drag. My guess is that most of the curving takes place when the speed of the ball decreases to a point where the laminar flow remains attached until reaching the back side of the ball, which would explain why smoother table tennis balls tend to curve more. However, it could be related to the fact the the smoother balls slow down sooner, so you see the transition into the region where the curving becomes more noticable sooner. This Wiki article has some diagrams of the path of a table tennis ball with backspin and topspin examples:

http://en.wikipedia.org/wiki/Table_tennis#Effects_of_spin

So the smoother versus rougher surface versus Magnus Effect response depends on the speed and nature of the ball. According to some web sites, within a certain speed and spin range, and with a smooth ball, it can end up up with laminar flow on one side, and turbulent flow on the other, resulting in a reverse Magnus effect with the ball curving the "wrong" way.

Getting back to the original question, a smooth ball can curve, from this wiki article:

For a smooth ball with spin ratio of 0.5 to 4.5, typical lift coefficients range from 0.2 to 0.6.

http://en.wikipedia.org/wiki/Magnus_effect#Calculation_of_lift_force

Last edited by a moderator: May 4, 2017
12. Feb 15, 2010

### Galap

Theoretically, a frictionless sphere would not curve no matter how thrown, but since no sphere we can make is frictionless, all spheres can be made to curve to some extent.

13. Feb 15, 2010

### harry.butts

Galap, that's the best answer I've heard. Not in terms of accuracy or legitimacy or anything. It's the best answer my "'physicsly'-challenged" brain can understand it. : )

That's not true. They are ALL good answers. With some further reading, they're crystal clear! In fact, I've enjoyed reading the answers and the external reading. I've learned more since posting this question than in HS physics (in which I got a 67).

What I'm trying to say, I guess, is that it wasn't my intent to offend anyone.

Last edited: Feb 15, 2010
14. Feb 15, 2010

### rcgldr

My apologies for my originally incorrect post about the dimples on golf balls. The purpose of the dimples is to reduce drag and increase the upwards curving of golf balls (which have backspin).

Since spheres can't be made frictionless, and since the air has viscosity, you end up with those boundary layers, differences in detached flows, and diversion of the wake, coexisting with a lift force in the opposite direction to the wake diversion.

As an example of a smooth object curving, spinning bullets can be affected by crosswind components, causing them to rise or drop faster than normal by a small amount:

Spin stabilized projectiles are affected by the Magnus effect, whereby the spin of the bullet creates a force acting either up or down, perpendicular to the sideways vector of the wind. In the simple case of horizontal wind (a right to left horizontal wind) , and a right hand (clockwise) direction of rotation, the Magnus effect induced pressure differences around the bullet cause a downward force to act on the projectile, affecting its point of impact.

http://en.wikipedia.org/wiki/External_ballistics#Magnus_effect

Last edited: Feb 15, 2010
15. Feb 15, 2010

### harry.butts

I actually read that same article (http://en.wikipedia.org/wiki/Magnus_effect#Calculation_of_lift_force) yesterday, but I wasn't quite sure what I was reading.

Last edited: Feb 15, 2010
16. Feb 15, 2010

### rcgldr

spin ratio = (angular_velocity (radians/unit_time) x diameter)/(2 x linear_velocity)

Wiki defintion for a wing. I'm not sure how a sphere would affect the area component of the formula:

http://en.wikipedia.org/wiki/Lift_coefficient

There's some mis-leading Bernoulli principle explanations of Magnus effect. These state that since the air speed is faster over the backwards moving surface than the forwards moving surface, then the pressure is less because of the faster moving air. However using the air as a frame of reference, the faster moving air occurs at the forward moving surface and the slower moving air at the backward moving surface. The issue here is that Beroulli doesn't relate pressure to relative speeds of air flow, but instead notes that as air accelerates from a higher pressure area to a lower pressure area, that during this acceleration the air increases speed as it's pressure decreases, and defines an equation that relates the speed to the pressure during this acceleration (an approximation that ignores issues like turbulence).

With a moving spinning ball, the air is accelerated (forwards) more by the forwards moving surface than the backwards moving surface. The higher acceleration coexists with a higher pressure. However the layer of air that actually spins with a ball is extremely thin, so it's unlikely that it would contribute much to the Magnus Effect. The more likely explanation as mentioned above, is the difference in position (front to back) at where the attached flow separates from the surface of the ball. The flow detaches sooner on the forwards moving surface than it does on the backwards moving surface, resulting in a perpendicular diversion of flow (acceleration of air) towards the side of the ball that is spinning forwards, coexisting with an opposing perpendicular force from the air, creating the lift that curves the ball away from the side with the forwards moving surface.

17. Feb 15, 2010

### Staff: Mentor

Hey, that was almost exactly what I said in my second post!

...that said, I forgot about the laminar flow separation issue, so my first post wasn't correct.

If you're not getting that phrase "laminar flow separation", have a look at the picture on this wiki: http://en.wikipedia.org/wiki/Flow_separation

Flow wraps around an object, but on the back-side of an object, the low pressure causes the flow to separate from the object and become violently turbulent. But it just so happens that if the flow starts off turbulent, it doesn't separate as violently as if it starts off straight/laminar. And less violent separation = less drag.....and also less lift and less curving.

18. Feb 16, 2010

### harry.butts

Yes sir, but you made me *think*

I interpret the answers, except Galap's answer (sorry Russ_Watters) , as an "absolute maybe". Some people are indicating yes, while others say no.

Russ_Watters, in one word, can it be done? I ask you due to your empirical stature as a "mentor."

Last edited: Feb 16, 2010
19. Feb 20, 2010

### decklingbrain

How a reverse swing is come into play so late in the game.. Is there any science in it..???

Please clarify this..as I saw some related thing in Discovery but.. not clear in my mind..!!

Regards,
Brain..!!

.
http://www.morrant.com/cricket_equipment/cricket_balls/4736_0c.html" [Broken]

Last edited by a moderator: May 4, 2017
20. Feb 20, 2010

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