OOAK Table Tennis Forum

Can you quantify this somehow? I can't see the significance:

The ball is ~1/100 the mass of the racket. As a matter of conservation of momentum the velocity change in the racket per unit time is insignificant compared to the ball. This mean for every X mm the ball is moving the core of the racket (the blade) is moving X/100 mm. This is why you don't see the racket move much in the video until well after the ball's left. IOW, the ball and racket are not equal participants in this exchange and IMO much confusion comes from the assumption they are.



I don't understand what this means.

I didn't come here to "contribute substantive" in this particular thread; I came here because I was told someone had attacked me, i.e. you, just as you did before at the mytabletennis forum. (And you were banned from that forum for a time for those attacks). As noted in my previous note, you also made up something about me here that was blatantly false (the "chewbacca" thing"), and when confronted with this falsehood and the fact that you had lied about me, you ignored it and instead went after me again above. Perhaps you can figure out from this why I have no interest in discussing table tennis issues with you, whoever you are. These personal attacks really detract from anything you may have to contribute and derail the topic from substantive discussion. (On a related note, there are reasons for some to use an alias when posting online, but you often use it to hide behind while ridiculing and ripping into others. That's a pretty cowardly thing.)
-Larry Hodges

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I obviously don't really know but you seem to take any mention of your name as a personal attack no matter what the post is about. If you are shouting all the time there is a good chance nobody will listen to you. Besides that if you are so hostile and defensive all the time you create the appearance there is at least some truth in the points people make.

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It wouldn't take much, really, no need for a video. Just use a couple of C-clamps to clamp a blade by its handle to the edge of a table. Mount a camera right on the plane of the blade, from the side. Then proceed to put weighty objects on the blade, near the tip. Start with a bottle of aspirins, maybe, and if you're brave end with a liter bottle of water. Take photographs (or a video, if you must) as you go. Any flex in the blade >1mm should be plainly visible by putting a straightedge on the picture, and if what you were saying earlier is correct, it should bend like a banana over its entire length. And it shouldn't take more than 2-3 ounces of pressure if what you said about "finger pressure" is true.

Iskandar

1)
Start with a steel blade thick enough to make it inflexible and immovable in a ping-pong* context.
Mount a normal rubber.
Fire a ball at it straight on at a ping-pong* relevant high speed.
The ball will compress the rubber down to some thickness (just call this thickness "x", ok?),
and then the rubber will push back on the ball.
The ball will leave the rubber after some time (just call this duration of time "t" please).

2)
Now construct a blade out of paper (which is made of wood),
thin enough that we can all agree that it will flex during impact,
but thick enough to survive the impact in a ping-pong* relevant way.
Mount the same rubber and fire the ball as before.
Now the ball will compress the rubber but the paper moves back under the pressure.
The rubber won't be compressed to a thickness of "x" because the paper is moving away.
It won't be compressed as tightly.
So when the rubber pushes back on the ball it will be with less force.
This impact in #2 is of lower force (and probably of longer duration) than the one in #1.

3)
A normal ping-pong* blade is somewhere between those in #1 and #2.
So the rubber will not be compressed all the way down to "x", but to a little thicker than that.
Since "x" is some tenths of millimeters, then a flex of just some tenths of millimeters will make a difference (just as a tiny change in topsheet or sponge make a difference both in feel and in how the ball flies).

4)
How many tenths of millimeters the wood of a normal blade distorts in this way could be hard to measure, but I've explained how a flex of just tenths of millimeters can make a ping-pong* meaningful difference; I've explained why arguments and observations involving the whole blade and it's entire mass are overlooking the relevant mechanism.

5)
I explained in a previous post how it is possible for the (heavy) wood to be moved by the (light) ball in such a short time: It need not move the entire blade but just a small region under the ball. Do you need more explanation? Did you see the black and white "vibration mode" maps posted earlier? The white and black show high and low regions. The ball need only displace the wood in one of those regions; the point of the drumstick need not move the entire drumhead at once, and the ball need not move the entire blade but just one of those regions.

First, a TT blade is many orders of magnitude closer to steel than a sheet of paper. Try bouncing a ball on all three suspended in air to get a grasp of how many. On the other hand, the same trial with different speed blades make it unimpeachably clear that thicker/harder ones rebound the ball at faster upward speed (or more accurately, dampens the ball's rebound less).

> I explained in a previous post how it is possible for the (heavy) wood to be moved by the (light) ball in such a short time: It need not move the entire blade but just a small region under the ball. Do you need more explanation? Did you see the black and white "vibration mode" maps posted earlier? The white and black show high and low regions. The ball need only displace the wood in one of those regions; the point of the drumstick need not move the entire drumhead at once, and the ball need not move the entire blade but just one of those regions.

If this were a significant effect the ball would bounce visibly differently depending on where it struck according to those patterns. Again, you can easily do the bounce test to see if this were the case.

Really, what's the end game here? Ejecting the ball from the racket in a downward direction on a loop is not a helpful thing. It runs counter to the argument that flexing blades are good for looping.

1)

a. The point was that a fraction of non-zero is still non-zero. [This dismisses the possiblility that it is zero. It is not zero.]
b. I then explained why a flex of only tenths of millimeters is ping-pong* relevant: it is some meaningful fraction of the thickness of the compressed rubber, affecting the force with which it repels the ball back out. So only a tiny "give" under the ball makes a ping-pong* relevant difference. [This dismisses arguments based on videos of whole-blade behavior because the flex need not be that big.]
c. I then argued that just as the distortion caused by a drumstick is (initially) just one small region of the drumhead, the ball need not distort the entire blade during impact. [This dismisses the argument that the mass of (a large fraction of) the whole blade could not be moved in the time available.]

2)

a. A drumstick can strike anywhere and displace a small portion of the drumhead; it need not be aimed at one of those patterns. (After asking if anyone needed any more explanation, I referred to the images only to show that some small spots can be low while others high.)
b. But the ball does bounce differently on hard hits depending on where it is struck, and consensus is that those differences are larger on a more flexible blade.

3)

The only mechanism I described above is one that would reduce the speed of rebound in that direction. If increasing it in that direction is not helpful, then reducing it in that direction might be helpful.

It seems like you're arguing for something that has nothing to do with the "flex" that some folks believe they feel. Vibrations on a blade which happen in numerous patterns & frequencies is not the bendy "flex" that foam et al speak of so it's probably best you use a new term to avoid conflation.

In any case:

> I then explained why a flex of only tenths of millimeters is ping-pong* relevant: it is some meaningful fraction of the thickness of the compressed rubber, affecting the force with which it repels the ball back out. I then argued that just as the distortion caused by a drumstick is (initially) just one small region of the drumhead, the ball need not distort the entire blade during impact

These vibrations aren't localized but move the blade as a whole (propagated at the speed of sound) or else the wood itself is damaged (each part of the blade only curves very gradually). To see what a localized effect on the wood looks like, push a small solid object into it to see what happens when it doesn't move as a whole.

Physical materials don't really act as naive intuition might lead us to believe. Otherwise we wouldn't need to study physics.


> But the ball does bounce differently on hard hits depending on where it is struck, and consensus is that those differences are larger on a more flexible blade.

The consensus of many TT players is that the blade flex rebounds the ball or that dwelltime is long enough to guide the shot somehow. This "consensus" doesn't seem to be very useful.

I've explained how flex can decrease the speed of the rebound.
Flex that is *backward* does that. I think we already knew that.

The more difficult and interesting question is whether or not *whatever flexed backward* can spring back *forward* (in the time available) and increase that speed.

As soon as we acknowledge that *something* flexed backward (and thus, that "blades don't flex" is false), I could continue to the interesting part!

I think what Agenthex is getting at is that your definition of "flex" isn't the same as what Foam defines as "flex". You're talking more about blade vibration, not "flex".

Iskandar

To iskandar taib:
Blades don't have mass. I should start a thread with that title. Later, I could explain that when I said "mass" I meant something else. What would you think of that?

If "blades don't flex" is intended to mean: "the blade (or part of it) can't both bend backward AND spring back forward in the available time", then we're part of the way to answering whether or not that is true. As soon as someone acknowledges that *something* bends back ["flexes", in the common tongue], then I'll try to explain the springing back, if any.

I would say it's silly, because it's easily disproven. (Tie a rubber band to it, stretch the rubber band to produce a given force and let the blade go, it will accelerate at a non-infinite rate.)



In order to have a meaningful discussion, we need to define what we're discussing. This was done in the first post. What we mean by "flex" was that the blade bends (as Foam put it some time back, like a banana) when you apply a force to it (he says finger pressure is enough). I'm not saying that what you are describing doesn't happen, I'm sure it does (especially for my balsa/birch blades, where I'm sure there's some elasticity in the face skin that makes the blades, at least at the table, really "fast"), but it's an altogether different discussion than the one we were having. No problem in having another discussion, though, even in the same thread, but as agenthex says, we'd better be clear about what we're talking about, and a term other than "flex" would be desirable, since we've already "claimed" it. Years ago, on Usenet, I called this the "drumskin effect".

Iskandar

You missed the point. Doesn't matter.


Thank you. It was argued that the whole blade can't bend and then spring back in the available time. I showed that it is not necessary that the whole blade bend, just a region under the ball.

Perhaps what Zhaoyang is trying to picture is the initial wave propagation through the blade, not even the vibration. I believe is more of a longitudinal not transverse wave. The following links explain this:

http://www.acs.psu.edu/drussell/demos/w ... otion.html
http://physics.tutorvista.com/waves/lon ... waves.html

Longitudinal waves travel in the structure internally.

> Blades don't have mass.

Do you own a scale that resides in gravity? Maybe try that on a racket.

That's interesting, thanks.
But no, right now (so far), I'm just pointing to the initial disturbance. As a drumstick can hit anywhere on the drumhead and displace (initially) just a small region. I'll be surprised if you don't understand what I'm saying.


It was just a little joke. Sorry.

Edit:
It was argued that the whole blade can't bend and then spring back in the available time. I showed that it is not necessary that the whole blade bend, just a small region under the ball.
As soon as someone acknowledges that *something* bends back ["flexes", in the common tongue], then I'll try to explain the springing back (forward), if any.