# Question for TW Prof on ball elongation

#### sureshs

##### Bionic Poster
There are 5 explanations being put forward to explain the "oval" shape of the ball when hit with extreme spin:

1. Deformable spinning body attains this shape due to centripetal force requirements being greater at larger radii (the force being supplied by the ball skin itself)

2. The Magnus effect producing the downward force on top spin balls also causes the elongation

3. Elongation is due to there being more side spin than top spin on the ball

4. Yet another theory has been put forward that the elongation is an optical illusion based on the spin on the ball

5. The elongation is just a continuation of the deformation of the ball at impact or bounce (but can that cause continue to have its effect for so long?)

Which is correct?

Last edited:

#### TW Staff

sureshs,

Your question has been passed along to the Professor.

Brittany, TW

#### sureshs

##### Bionic Poster
Thanks...............

#### sureshs

##### Bionic Poster
Yet another theory has been put forward that the elongation is an optical illusion based on the spin on the ball

#### Dave M

##### Hall of Fame
Have you ever seen a rugby ball when it's been kicked or as it bounces in slow motion?

#### prostaff junkie

##### Rookie
There are 3 explanations being put forward to explain the "oval" shape of the ball when hit with extreme spin:

1. Deformable spinning body attains this shape due to centripetal force requirements being greater at larger radii (the force being supplied by the ball skin itself)

2. The Magnus effect producing the downward force on top spin balls also causes the elongation

3. Elongation is due to there being more side spin than top spin on the ball

Which is correct?
I think there is a direct correlation between how egg-shaped the ball becomes and what phase the moon is in at any given time.

#### Ash_Smith

##### Legend
it's actually due to the "fluff" on the ball - as the ball spins the fluff is pulled outwards due to centrifugal force and the ball appears egg shaped

#### Dream_On

##### Rookie
Have you ever seen a rugby ball when it's been kicked or as it bounces in slow motion?
I thought tennis balls were the same shape as rugby balls but when hit turn into spheres due to the distortion from racquets that are stiffer than 62 ra?

#### Dave M

##### Hall of Fame
I think there is a direct correlation between how egg-shaped the ball becomes and what phase the moon is in at any given time.
It's tidal, I should of realised the effect that should have living on the coast. I played on Poole Park courts by the water and as the tide came in I'm sure I was getting more backspin, that explains it.
it's actually due to the "fluff" on the ball - as the ball spins the fluff is pulled outwards due to centrifugal force and the ball appears egg shaped
Well that's a given but I wonder if the same happens with a treton ball?
I thought tennis balls were the same shape as rugby balls but when hit turn into spheres due to the distortion from racquets that are stiffer than 62 ra?
I believe the results can be skewed somewhat depending on the type of rugby ball used in comparison and if the leather of the kickers boots is kangaroo hide.

#### sureshs

##### Bionic Poster
And yet another theory has come forth: that the elongation is just a continuation of the deformation of the ball at impact or bounce (but can that cause continue to have its effect for so long?)

#### sureshs

##### Bionic Poster
it's actually due to the "fluff" on the ball - as the ball spins the fluff is pulled outwards due to centrifugal force and the ball appears egg shaped
That would be part of cause #1

#### sureshs

##### Bionic Poster
Have you ever seen a rugby ball when it's been kicked or as it bounces in slow motion?
Not really. I have rarely seen rugby played. Unfortunately, the American version (football) already uses elongated balls.

#### sureshs

##### Bionic Poster
sureshs,

Your question has been passed along to the Professor.

Brittany, TW
Please inform him that 2 more causes have been proposed and point him to post #1 again.

#### prostaff junkie

##### Rookie
Perhaps your balls are flat? :twisted:

#### sureshs

##### Bionic Poster
Perhaps your balls are flat? :twisted:
Yes sometimes they are flat.

#### prostaff junkie

##### Rookie
Yes sometimes they are flat.

#### TW Professor

I've seen the ellipse occur two ways in the lab. First, impact causes the ball to vibrate back and forth in the direction of compression, which alternately makes the ball appear long and skinny horizontally and then vertically (spinning would complicate the visual, either amplifying or diminishing the observed effect). But this lasts only for a few milliseconds after the ball leaves the racquet. The other effect is camera distortion. If the camera's shutter exposes the image successively from top to bottom, the bottom of the ball will have moved relative to the top by the time it is exposed--even if the shutter speed is just 1/8000 sec with a frame rate of 300 fps, as in the photo below.

This distortion is known as the rolling shutter effect and the distortion increases with the zoom level, since the time between exposing the top and bottom of the ball will increase as the image gets larger.

#### sureshs

##### Bionic Poster
Does the eye also behaves like the camera in this regard?

#### Dave M

##### Hall of Fame
Are you able to see the ball do that with the naked eye?

#### sureshs

##### Bionic Poster
Are you able to see the ball do that with the naked eye?
Are you asking me or TW Prof? Yes, I can see the elongation, but want to know if it is an optical illusion.