Why a backspin ball curves down?!

I'm sorry that article made little sense to me. Though I usually use the Bernoulli effect to explain trajectories. Although the Magnus effect is more appropriate, however I'm led to believe the conditions required for this is a theoretical cylinder. Much better is to look up Robins effect, where a theoretical sphere is used. 

Well, duh, might as, might as well ask why is a tree good? Why is the sunset good? Why are boobs good?

yes ffx-me, that is the current understanding we share here. But the article is attempting to explain how a low velocity ball with heavy backspin would have laminar flow on top, but turbulent flow below with slip (ie, low friction) causing a BACKSPIN ball to curve DOWN.

However after re-reading it many times, I'm still a point where I accept the premise, but reject the conclusion.

...and now my brain hurts. Iz cannot brain today. I haz a dumb.

Right now, I don't care why something curved down or up, as long as I can loop it back strongly.

ffx-me wrote: lol the slow backspin ball thing seems like crap to me

  
      I agree, I'd like to know the ball speed and rpm at which this would actually occur.

Absolutly not.  If you chop smash a ball, it will curve up, the forces from the spinning ball over power gravity.

danhs wrote:      I have never seen a backspin ball curve down under any conditions.  The theory may be right but it must call for conditions which basically never occur in table tennis.

Me either but I have seen very strong choppers be able to skid the ball so it hardly bounces (that's very hard to play :S) I've only seen one person be able to do it often (a girl too)

I have found it - 2.7 grams :)

ppmax wrote: Once the relative velocities cross some threshold, a different force takes over to curve the ball the other way.

That doesn't make sense at all.  Ma Lin's ghost serve drops quickly simply because it's very low, and it has very little horizontal velocity after the 1st bounce where the heavy backspin slow the ball down a lot, which is also why the serve's very short and the ball comes back.

Hookshot wrote: Absolutly not.  If you chop smash a ball, it will curve up, the forces from the spinning ball over power gravity.

Right. Playing "hollywood doubles" I know this to be absolutely true... up until gravity and friction slow down the ball and it starts arching or falling down again.

I guess I had a hard time understanding the premise of the downward spinning physics in the original link.

Let me get this right... The spin is the same direction, but the lateral force (i.e. the swing as the ball is stricken) is what determines the vertical direction of the ball -- like whether ball drops on a loop, or it curves upward on a forceful chop. Spin is the same but on a chop the ball is traveling in the opposite direction of the spin -- so it goes up - makes perfect sense. (this that floating attribute on chops) On a loop, ball spin is going same direction so the spin is allowed to contribute to a downward arc.

So could someone who is much smarter than I am explain why and how a backspin ball would curve downwards ... and especially how it could happen in a game of table tennis? Seems like a powerful mechanical device would have to spin the ball with tons of "spin" force and very little lateral force. Seems it is impossible for a human to do this.

I FINALLY GOT IT!

jphillips348 wrote: Well, duh, might as, might as well ask why is a tree good? Why is the sunset good? Why are boobs good?

"This IS  the good stuff, snakes and sparklers"

I don't think the explanation given in this article is fully accepted by the scientific community and is not taught in schools or universities. From what I can tell it is an attempt to redefine the status quo. Most of the "findings" seem legit but the slow underspin curving down is definitely incorrect just from actual observation.

To say an under spin ball can curve down is the same as saying a topspin ball can curve up!

Well that's the wisdom in my simple logic :P

no no no sido, I'm pretty sure Recanter was being sarcastic. Topspin balls do not curve up and backspin does not curve down. Plus your statement about backspin balls going straight is correct IMO, but contradictory to what the article is saying (i disagree with that part of the article).
In order to impart large amounts of topspin to a ball while keeping it slow you have to use a more upward stroke which will send the ball along a higher trajectory. after the bounce the topsping will kick the ball forward at a faster rate than it was travelling before.

My previous statement wasn't in reference to whether or not people here do or should believe the article. I'm just making the point that the view in the article is not the one taught in schools/universities and apparently hasn't been approved by the scientific community (passed peer review).