# PhysicsFilter: The Great Racquetball-in-the-Eye DebateMay 25, 2006 1:59 PM   Subscribe

PhysicsFilter: The Great Racquetball-in-the-Eye Debate

Welcome to the biggest, knock-down, drag-out argument I've ever had with my best friend (we're still friends, btw, but have agreed to Never Speak of This Again). Needless to say, I want an answer.

The setup: His Dad says we should wear eye protection when playing racquetball. He says NOT ONLY is eye protection a good idea in general (i.e. don't get hit in the eye by your opponent's swinging racquet), BUT THAT when a ball strikes a solid object like your head, the ball will compress against the object and BEFORE IT REBOUNDS will decompress and deform such that a segment of the ball will now form itself into a "rounded spike," with a force capable of entering your eye socket and doing significant damage.

Bullshit, I say. A ball wouldn't deform like that. It would compress when it hit the object, then decompress to its original shape when it rebounded off the wall. It may ripple further, but by that time, the ball is no longer in contact with the object. It's not going to "ripple itself into a spike" that drives further into the object. I think Dad probably means that the ball will be deformed by hitting your naturally uneven skull and eye socket bones and possibly drive through the soft tissue of your eye. Like hitting a deer with a car -- the deer can't go through the engine block, but it will go through the windshield just fine.

No, my friend says. It'd be ball, skull, compress, SPIKE and THEN rebound from the object, even against on a flat wall. The ball starts rippling in three dimensions before it leaves contact with the object. That spike will go RIGHT INTO YOUR SKULL.

Hilarity ensues. We literally argued for hours on this.

Stipulations:
* We both agree eye protection is a good idea. The argument is specific to the three-dimensional "ball spike after leaving the wall" concept.
posted by frogan to Science & Nature (21 answers total)

Depending on the velocity of the ball and the elasticity of the material, your friend may be correct to some extent. Assume the ball hits the eye. The ball is bigger than the eye socket, yes? The part of the ball that hits your eye socket will stop, but the rest of the ball will travel forward. That is his thinking behind the spike, but the ball would need to be either very malleable or traveling at incredible speeds for it to actually puncture your eye.

IANAPhysicist (give me a year)
posted by Loto at 2:03 PM on May 25, 2006

I misread the question, apologies. If the ball is traveling fast enough from the wall, it will deform, but not in the way he thinks. The ball will flatten in the air, but not like a spike... more like a fat pancake. Think of it this way. The force on the ball at a point will be greatest when the tangent of that point is perpendicular to the force. So, if you throw a ball along the +x axis, the force will be greatest for a point on that axis, meaning more deformation along that axis. Make sense?
posted by Loto at 2:10 PM on May 25, 2006

I'm not sure about the physics of this but I once was a reckless, non-goggle wearing rball enthusiast. My eye socket made acquaintenace with a ball head-on and, let me tell you, that's the worst pain I've ever experienced. I immediately screamed like a little girl and fell to the floor. I expected permanent eye damage, perhaps blindness. I guess I got off easy, with only a week of pain and impaired vision.

So, physics aside, PLEASE wear eye protection. The fine points of compression and shape will be lost when you can no longer perceive three dimensions.
posted by lfaren at 2:19 PM on May 25, 2006

Here is a series of high-speed photographs showing a racquetball hitting a wall. There's definitely a "spike", or protrusion, that forms, visible pointing out from the wall in the 1.57 ms frame. If you look at the full series of .25 ms-interval images, you can see that this protrusion moves to the right, towards the wall, and it disappears into the ball before the ball rebounds from the wall. One could definitely imagine the protrusion pushing into the wall before the ball rebounds.
posted by mr_roboto at 2:21 PM on May 25, 2006

Regardless of whether it assumes the shape of a spike, you do NOT want a raquetball hitting the globe of your eye. Wear the glasses. Unless you think wearing an eyepatch is cool.
posted by megatherium at 2:21 PM on May 25, 2006

Looking at that series of images again, I think you're definitely going to lose this bet, frogan. That thing sends a concentrated spike of high-speed rubber straight into the object it's striking.
posted by mr_roboto at 2:27 PM on May 25, 2006

Try the following thought experiment: imagine a wall of titanium two feet thick, with a perfectly circular indentation one foot deep and one foot in diameter. Now imagine a water baloon made of stretchy but unbreakable plastic, spherical and one foot in diameter. Shoot the water baloon towards the centre of the hole with tremendous speed. It will hit the back of the hole. Now shoot a similar baloon, but with diameter 13 inches, towards the centre of the hole. If the speed is great enough, the ball will sort of squeeze into the hole. Keep doing the same with balls of progressively larger diameter. What will happen?

I think if the speed is large enough, no matter what the size of the ball, some of the ball will squeeze into the hole and fill it up.
Same with your eye. Of course, the speeds required might also shatter your skull, because your skull is not made of titanium and the raquetball is not so stretchy.
posted by louigi at 2:28 PM on May 25, 2006

I heard something similar but I don't remember where. I think it might have been in literatur at my fitness club.

I can't solve the physica on my own, but I went looking for more information by searching 'squash ball deform eye'. I found this (towards the bottom):

Most people do not realize that deformation of the ball upon impact will allow most balls to penetrate smaller areas than would be possible with the ball at rest. At a high impact, the ball flattens and becomes more elastic so that a portion of it can protrude into a small space. If the ball is larger than the orbital opening, the forehead and the orbital rim will absorb part of the impact, but some of it will reach the eye. This is how a soccer ball or volleyball is capable of causing significant eye injury.
.
posted by PercussivePaul at 2:35 PM on May 25, 2006

I'm going with friend and frDad. I think the protrusion mr_roboto talks about is formed when the ripple in the surface of the ball as it strikes the wall runs around the ball to the antipode (think tsunami on the world ocean, here). The protrusion disappears because the ripple continues to travel around the ball, but that means it will reform at the original point of impact: the wall (aka your eyeball!).

If conditions were just wrong, I bet this could rupture an eyeball. If frDad has the physical intuition to see this directly, bow down before him.
posted by jamjam at 2:44 PM on May 25, 2006

Imagine the ball as a bunch of tiny springs all held together in a spherical shape. Upon first impact the front springs will be compressed exerting a force on the ones behind them. At the same time however the behind springs are still moving forward at about the original velocity. The force of the springs must be great enought to change the momentum of the behind springs in the amount of time it would take for the back part opf the ball to catch up to the front part and also hit the wall. From the sequence of photos by mr_roboto, you can see this is clearly the case. In order for a "spike" to hit you the change in momentum of the ball would have to be greater than spring force exerted by the springs in the ball. To test it out see if you can press the back of the raquetball into the front of it. Its very tough to do I expect, but not impossible. In my opinion the greatest damage done to the eye would be the initial momentum transfer from the tip of the raquetball to the eye, the secondary spike if it occurs would be less forcefull by a difference of the springiness of the ball.

Sorry for the somewhat wordy and confusing descriptions here.
posted by ozomatli at 2:49 PM on May 25, 2006

Upon looking at jamjam's post I should clarify: I am talking about that indentation in the raquetball as the "spike". In jamjam's case he is talking about the "shock wave" traveling around the surface of the raquetball and reforming at a maximum at the opposite pole of the ball. In this case as well the size of the force of this shock wave will be much less than the initial impact, I also suspect that it will arrive after the ball left contact with the face, but I am not sure how fast that would travel around the ball (a function of the springiness of the ball again).
posted by ozomatli at 2:58 PM on May 25, 2006

It's not going to penetrate your eye.

What happens is a "blow out fracture." This is the cause of the trauma. The globe is typically not injured (or penetrated), but rather the bony architecture holding your eye in place.

You do NOT want this to happen to your eye socket.
posted by i_am_a_Jedi at 3:58 PM on May 25, 2006

Everyone above is right - the ball will deform in such a way as to enter your eye socket. The conceptual problem you're having is right there in your question: when a ball strikes a solid object like your head, the ball will compress against the object.

You're mistaking 'solid' for 'inelastic', and the contents of your eye socket are in fact neither one. (Most of the interior of the eyeball is filled with gel, and the eyeball is extremely elastic and deformable.)

Recall that stress (the impact of the ball) is proportional to strain (the deformation of the eye globe) within the elastic limit. That's Hooke's Law.
posted by ikkyu2 at 5:05 PM on May 25, 2006

Response by poster: Let me refocus the question, as a point seems to be getting missed...

There's no question that a ball would cause damage as the result of the high-speed ball being pressed into a smaller hole (your eye socket).

The question is about high-speed deformation of the ball shooting a "spike" onto any surface it hits and compresses against. What's being contemplated is akin to a "second impact." Ball hits object, ball compresses. As natural part of compression, there's a secondary "spike" effect that imparts an additional force on the object.

I'm calling bullshit on the "second impact," but evidence above is making me not so sure.

Anyone else? This is great stuff, especially the high-speed photos.
posted by frogan at 5:21 PM on May 25, 2006

It all comes down to your definition of the word "spike" I personally would not consider the deformation that appears in that picture a "spike." I think the term spike defines something that would penetrate the flesh and create an open wound. I seriously doubt that the raquetball deformation would do this. In more likelyhood it would cause blunt force trauma to the eye socket.

I dont wear glasses when i play either...i think it affects my game since they get all foggy as i get schweaty. Also i think the probablilty of the ball having such a trajectory to cause damage to what is essentially a .5cm radius circle on your face is statistically insignificant.
posted by I_am_jesus at 5:24 PM on May 25, 2006

I've thought a bit about the physics of this, and I think that when the ball squishes against the wall, it might be possible to approximate it as a 2D membrane. In this case, the "spike" would be in the center of the membrane, with a nodal circle around it. This corresponds to the (0,2) vibrational mode of the membrane: the second radial mode. The squishing process excites this mode, and drives the spike right into your eyeball.

In a rough approximation, at least.
posted by mr_roboto at 6:16 PM on May 25, 2006

Response by poster: This corresponds to the (0,2) vibrational mode of the membrane: the second radial mode. ... The squishing process excites this mode, and drives the spike right into your eyeball.

This seems to be the heart of the claim being made by frDad. The question is -- can the second radial mode create a "second impact?" Seems to me that either a) the ball has already rebounded before this spike occurs or b) our pal Newton says the secondary spiking action cannot reach farther than the leading edge of the outside ring.

???
posted by frogan at 6:29 PM on May 25, 2006

Response by poster: I think the term spike defines something that would penetrate the flesh and create an open wound.

posted by frogan at 6:30 PM on May 25, 2006

This doesn't answer your question exactly but I know someone who had a detached retina as a result of a racquetball to the eye. I can't quite figure out which of the two scenarios this would fall under, and I don't know the details of the impact, unfortunately. But I wouldn't want to play without eye protection after hearing this story.
posted by advil at 12:25 AM on May 26, 2006

I think the term spike defines something that would penetrate the flesh and create an open wound.