[Discraft] soft/gummy plastic and centrifugal force?

[jubuttib]

Do you need a high framerate video of top fuel dragsters to see their tires balloon out when accelerating?

There will be warping at some force level for every type of disc plastic. Somebody smarter than I am can probably run an analysis of the plastic consistency and figure this out with a formula or two.

Contrary to popular interwebz belief... pics/vid or it didn't happen isn't necessary all the time.

The sort of deformation you get in dragsters is a few orders of magnitude higher than what we're looking at...

I was mainly referring to how low shutter speed, low framerate video (many cameraphones only record at 15-25 fps, we'd need at least 30), especially if it's done with a rolling shutter, will itself cause warping in the images. We can see that the disc warps, sure, but how it warps is a guess unless we can get a good quality source. A high shutter speed is needed, high fps isn't totally necessary but preferable because it in itself requires a high shutter speed, and gives us a clear picture of what's happening.

 

[Anselm]

CENTRIPETAL force would have very little effect on the shape or flight of the disc. Discs are symmetrical, with only minor irregularities, so that when one point on the discs feels a centripetal force due to rotation, there will be an identical point on 180 degrees opposite it on the other side of the disc that feels an identical force. This holds true for all points on the disc. By Newton's third law, this results in zero net force on any point of the disc.

Of course, any asymmetries in the disc (plastic density irregularities, dings, scratches, etc) would lead to minor asymmetries in the disc's internal forces, but the mass of such asymmetries would be so small as to make these irregular forces negligible.

As others have said, deformations of the disc would only be caused by the force of your grip on it.

 

[jubuttib]

CENTRIPETAL force would have very little effect on the shape or flight of the disc. Discs are symmetrical, with only minor irregularities, so that when one point on the discs feels a centripetal force due to rotation, there will be an identical point on 180 degrees opposite it on the other side of the disc that feels an identical force. This holds true for all points on the disc. By Newton's third law, this results in zero net force on any point of the disc.

Of course, any asymmetries in the disc (plastic density irregularities, dings, scratches, etc) would lead to minor asymmetries in the disc's internal forces, but the mass of such asymmetries would be so small as to make these irregular forces negligible.

As others have said, deformations of the disc would only be caused by the force of your grip on it.

First things first: Looking at the wiki, centripetal force would be, more or less, "that by which bodies are drawn or impelled, or in any way tend, towards a point as to a center", and "centrifugal force" for would be, more or less, "the effects of inertia that arise in connection with rotation and which are experienced as an outward force away from the center of rotation." So in our case "centrifugal force" would be trying to spread the disc apart and centripetal force would be the structural integrity of the plastic trying to hold it together (that's the only force acting towards a center).

Secondly, the points on opposite sides of the disc do feel an equal "centrifugal force" (in the sense described earlier) that pushes them away from the center, but in opposite directions, meaning the forces are pushing them away from each other. The centripetal force will keep the disc from breaking up. The opposing forces are equal (as per Newton's 3rd), but that doesn't stop them from moving (if it did, nothing could move). Given a soft enough structure the points will move away from the center of the disc, stretching it, increasing the diameter of the disc (and since we're talking about golf discs, also pushing the hanging rim of the disc outwards, changing the wing angle and raising PLH). That's if the structure is homogenous (in the scale we're working with), if the structure isn't homogenous (some parts are softer/stretchier than others) it'll deform in an asymmetric way.

So no, deformations of the disc would'nt only be caused by the force of your grip on it, the spin of the disc can definitely also deform it.

 

[GT Chris]

As per my relating any physics-related topic to xkcd: Centrifugal Force

 

[Anselm]

Jubuttib, I stand corrected.

I did run a quick, rough calculation, though, which I think demonstrates that the centripetal force, and corresponding centrifugal "force," on any point in the disc is quite small.

Assuming a rate of rotation of 300 rpm (pure guesstimation), and further assuming all of the mass (175 g) of the disc to be concentrated in a single point on the rim, the centripetal force on that point is 4.2 pounds, which is a HIGH upper bound on the centripetal force felt by any point, because the mass at any "point" is much smaller than 175g.

Now, if the disc does deform, the centrifugal "force" will be slightly higher than the centripetal force, but as long as the disc doesn't rip apart they will be approximately equal.

Maybe something like one of Gateway's SSS putters would deform under that kind of load. My guess is that it wouldn't, and that anything stiffer than that would hardly deform at all.

 

[crazypep]

...make a mountain out of a micron...

 

[jubuttib]

Assuming a rate of rotation of 300 rpm (pure guesstimation), and further assuming all of the mass (175 g) of the disc to be concentrated in a single point on the rim, the centripetal force on that point is 4.2 pounds, which is a HIGH upper bound on the centripetal force felt by any point, because the mass at any "point" is much smaller than 175g.

Remember also that since the points are connected to each other, for any given point all of the parts of the disc that are further towards the edge from any point will be pulling said point (the points closer to center will also be pushing it and those to the side will also affect it).

The forces are indeed pretty small, but we're talking about very floppy discs (think Soft X-Link, which you ran fold into quarters or even eights, roll up, etc.) so the forces required aren't huge.

 

[kachtz]

if it helps any, i can feel discs like a KC Pro ROC and star eagle bend in my hand before it comes out. i dont feel wider rim discs bend much at all, like Teerex / xcaliber

 

[psudgc17]

centrifugal Agreed. Centripetal FTW. I won't get into that argument...


The problem with this is that the stretching also messes with how hard you can hold onto the disc. I've noticed with flx plastic on forehands, that my grip will actually slip out of the disc because the rim gives way. Haven't really done it on bh's (I stopped throwing flx), but either way, usually firmer allows for better grip and transfer of energy.

I'd also argue that deformation of the disc upon release and flight causes a loss of energy. I've thrown a lot of floppy Pro Rhynos in my day and I'm switching to Medium Voodoos because I can put more behind it before it "wobbles out." Ignoring all issues of it being a different stabiltiy/disc/etc. Whatever the real physics are (I'm an EE, not physics student), I'd be willing to bet that the gains/drawbacks are not very noticeable if at all.

 

[tarel]

just play the gummy stuff in no wind and in the woods.

 

[kerplunk]

You can tune some discs (Epic, Quest Double D) by bending the rim up or down, maybe I will try to make my soft Bangers more stable by pulling outward on them before I throw

Seriously though, my opinion is that this centrifugal force would be negligible, especially compared to the likelihood of the disc bending in your hand and coming out funny, which I think would likely cause OAT and less stability. But I do suck at throwing backhands.

 

[jubuttib]

You can tune some discs (Epic, Quest Double D) by bending the rim up or down, maybe I will try to make my soft Bangers more stable by pulling outward on them before I throw

Correction: You can tune almost all discs by bending the rim up or down. Some really floppy ones that don't have much structure won't hold it, and some premium plastics that have great memory will work themselves back from it, but you can tune pretty much any disc by bending them. Since what you're actually doing is changing the wing angle, hence lowering/raising the PLH, it's not a specific property of the discs, but common to all discs.