The Float Serve(volleyball) help in explaining the odd trajectory...

[scalpitch]

The Float Serve

 

I am a volleyball player and also currently taking Physics in my last year of high school. I am trying to explain the odd trajectory that a volleyball follows when it is served a certain way.

 

The Main Idea:

A float serve is implemented in the game of volleyball to make it as difficult as possible for the opposing team to effectively receive and play the ball. By contacting the center of the ball with a hard hand and not following through with the swinging arm, the ball follows a very odd trajectory through the air, making it appear as if it were floating through the air (traveling side to side, up and down, and curving) WITHOUT any kind of spin. It's pretty cool!!

 

The Factors:

To explain this odd trajectory in it's entirety is quite a task. It can be explained by the fact that the hand-to-ball contact alters the ball's form and the air resistance changes it's otherwise straight trajectory as it is no longer a perfect sphere. The recoil of the ball (depending on it's material) continues to change it's form throughout it's trajectory causing it to suddenly change directions. To make the analysis more complete, we must analyze the ball's weight distribution and material complexion. This analysis would emphasize the extra material that is needed to create the air valve. We must also take into consideration the surface of the volleyball, the angle to which the ball contacts the ball, and the air pressure on the inside of the ball. A common instruction in the volleyball world about contacting the ball during a float serve is to aim for the valve in the center of the ball. The valve may only serve it's purpose as a target or it may add to the new shape of the volleyball by adding air pressure: the air that is compressed between the palm of the hand and the valve may force it's way into the ball.

 

Conclusion??

Would the minds of you physicists out there think it necessary to include any more factors into the explanation behind this serve's odd trajectory?

[swansont]

I don't think the deformation of a volleyball continues along its whole trajectory; I doubt it lasts more than a few inches of travel. I also don't think you're forcing air into the ball — if that were true each time you hit it, the pressure would continually increase. Eventually you wouldn;t be able to hit the float serve you describe, because the pressure in the ball was too high. That's something you can measure to test your idea.

 

I think you're overanalyzing the problem, and ignoring a more basic effect. The lack of spin is known to have very strange effects on a thrown baseball, and this does not rely on deformation. The "knuckleball." It's all (or at least mostly) about fluid dynamics.

[Skye]

I'm not sure air resistance is that important. What's important is that the ball is like a spring and oscilates when you hit it. Hitting the valve is like hitting a mass on the end of a spring, so there are going to be effects due to the momentum of that. But overall the ball will move about because of the changing CoM.

[swansont]

No, the COM will move in its ballistic trajectory once you've hit it (modified by air resistance, so it won't be parabolic). The only way to make the COM deviate from that path is another external force; the only way for the COM to deviate from the center is a deformation that lasts for a time during the flight. Oscillations don't move the COM — anything that's not a standing wave will quickly get damped out.

[Skye]

Yeah, you're right. The CoM will move through it's roughly parabolic path. So the ball itself would move about the CoM as it was travelling along this path. And that's what makes it difficult to track the ball, as it would be oscillating around that CoM, that is moving along this path.

[swansont]

Yeah, you're right. The CoM will move through it's roughly parabolic path. So the ball itself would move about the CoM as it was travelling along this path. And that's what makes it difficult to track the ball, as it would be oscillating around that CoM, that is moving along this path.

 

Not sure what you're claiming here (I can read this two ways). A ball that is vibrating will not have its CoM moving compared to a trajectory that has no vibration. The difficulty of hitting the knuckleball is that the CoM is not moving along the normal, predictable path — it deviates. The only way to get a deviation from that path is with an external force.

 

A deformed ball will present some difficulty in hitting it, but the CoM trajectory is predictable. The problem becomes one of phase: which part of the oscillation are you hitting when the ball arrives.

[Norman Albers]

I have a lovely niece who is currently third year at Harvard and a pitcher on their softball team. She is maybe 5'1" tall, of slight build, and in high school, pitched softballs clocked at 60 miles per hour!!! We had some great discussions. You can understand a curveball: say you give the ball counterclockwise spin looking down on it. The spin drags some air around with it, and on the right side this bucks the air trying to slide by, and pressure rises. The opposite happens on the left side, so the ball goes left. Same for right, up, or down. The knuckball has no spin (a boson), so there is no consistent pressure gradient. I'll guess and say, if it happens to wander rightward, there is pressure buildup back the other way, and no clear trajectory.

[Bignose]

Norman in on the right track here. It is a knuckleball effect. To first understand it, you probably need to look up the Magnus effect, the lift force that affects a rotating object. Then, think about what happens when an object, that is not perfectly symmetrical and smooth, rotates: effectively, the non-smoothness gets "blurred" out, because the rotation ensures that all the imperfections in the ball gets exposed to the different parts of the flow field. I.e. the rotation brings a seam to the leading edge, then a non-seam part, and then another seam, etc. Now, think about what happens when a non-smooth ball does NOT rotate. The leading edge will be a seam until a little gust of air pushes that aside, then the leading edge is a non-seam part. The ball seems to tumble, or as you described it, float. In baseball, a good knuckleball doesn't have zero spin. A good knuckleball actually will complete about one and half rotations before it crosses the plate -- the idea again being to expose different leading edges to the front of the flow to cause more tumbling and knuckling action.

 

But, at the heart of the matter is drag, Magnus lift, and the fact that no air flow is ever perfectly uniform -- there will always be little vortices to cause the balls to tumble.

[Norman Albers]

Great fun, Bignose, I like it! Golf balls are made with a certain amount of stipling for this reason. I had my niece show me one of her curves, and how she gripped the ball. I showed her that there were not as many seams crossing the airstream, and she agreed to try it at right angles. Never got feedback, but this is fun physics.

[Skye]

Not sure what you're claiming here (I can read this two ways). A ball that is vibrating will not have its CoM moving compared to a trajectory that has no vibration. The difficulty of hitting the knuckleball is that the CoM is not moving along the normal, predictable path — it deviates. The only way to get a deviation from that path is with an external force.

 

A deformed ball will present some difficulty in hitting it, but the CoM trajectory is predictable. The problem becomes one of phase: which part of the oscillation are you hitting when the ball arrives.

Yeah, I'd had about ten beers so I was having enough trouble keeping my own CoM above the floor, let alone making any sense.

 

But I meant the second way of reading it. It's more noticeable with other balls though, like the really cheap plastic inflated balls. If you really want to see it happen get a beach ball with a little water in it.