Re: Poms view of the Gilly debate

On 18 May, 18:06, a...@xxxxxxxxxxxxxxxx (Dr A. N. Walker) wrote:
In article <1179399514.208156.54...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Gavin Cawley <g...@xxxxxxxxxxxxx> wrote:

I can't be bothered to deal with much of the petty exageration and
sophistry below,

Ignoring the emotive language, that's exactly what you did
in another posting;

Yes, there is another post explaining why (and apologising for my
error). The above is not in emotive language, for once I used quite
exact language.

but it makes more sense to respond to this one.

I don't blame you, it saves the bother of snipping the sophistic
traffic flow example in the hope that no one noticed.

so I'll go directly to the key issues (that you have
largely snipped):
Does your ideal model suggest that a hard rigid bat (represented by
the brick wall) is better than a soft, springy bat (represented by the
rubber wall), as the ball will rebound further from a brick wall?

No.

In that case why did you not correct my misaprehension the first time
(a couple of posts ago) rather than encourage me to continue to labour
under it?

Not unless your "soft, springy bat" is *so* soft, and
so *un*springy [ie, designed to dissipate impact] that the ball
really does rebound significantly less.

Why do you think I asked "what kind of rubber"? For some kinds of
rubber it would bounce well, in others the kinetic energy would be
disappated as heat energy and vibration, and so would not bounce so
well. Given that it was a relevant question, why did you unhelpfully
reply "Sorry, those are unbounded questions, you'll have to ask
elsewhere. [Didn't they teach you not to argue with the examiner?]"

Balsa wood might be a
problem,

Balsa wood would be a problem not so much because it is soft, but
because it isn't springy, so it wouldn't store any energy, just
disapate it.

but willow is plenty hard and springy enough to bounce
things off.

Yep, as I pointed out several posts ago.

So is reasonably hard rubber, or near equivalents,
such as a vinyl floor.

Ah, so the kind of rubber does matter then? Pity you chose to be
flippant rather than answering my question.

Do you agree that this prediction is at odds with reality, in that
given the choice between a hard and a soft bat, the best professional
batsmen (who don't have to pay for their bats) normally choose a soft
one?

N/A, since there is no such prediction.

So why did you not correct my misaprehension the first time (a couple
of posts back)?

When I said "If a hard brick wall is better than a soft springy rubber
one, why is the blade of a cricket bat made out of soft, springy
Willow, rather than hard Oak (note the main function of the handle is
to absorb the vibrations)?"

you replied "That's an interesting question." and a bit later "I guess
that oak is heavy and perhaps brittle, and may well not have the right
sort of grain."

Both of which strongly suggest that a hard bat ought to be better
according to the ideal model. It isn't an interesting question if is
based on a misaprehension that you could easily have pointed out. Why
suggest that oak is not used because it is brittle or does not have
"the right sort of grain" (whatever that might mean), rather than
because it is hard?

If I didn't know better I'd say you had realised you had messed up and
were back-pedalling.

Do you agree that if a hard bat were a good thing, there would be
changes in bat design that could take advantage of this (within the
current laws), but no such innovation is present in the modern game?

N/A. But note that there is plenty going on in bat design.
Modern materials can be designed with all sorts of "interesting"
properties.

Yep, even corking is legal, but not carbon fibre any more.

Do you agree that your model is not able to answer the question as to
whether stored energy has any part to play, simply becuase it has no
mechanism to incorporate stored energy

Firstly, see response to RH -- impact is "all about" turning
KE into "stored" energy and back to KE [plus sound, heat, etc]. But
the gross effects can be "integrated" into conservation laws and
restitution laws [until we get to extremes and to special materials,
not relevant here].

Yes, its funny that now you have started to talk in terms of energy it
is far easier to understand what you are saying, perhaps you now see
my point about communication?

The problem is that a squash ball on the handle would dissapate so
much of the energy (being too soft and not very springy) that the
above approximation isn't reasonable any more. As a result your ideal
model can't answer the question. Drop a squash ball onto a hard
floor, how high does it bounce?

(or the contribution of the
batsman)?

The contribution of the batsman is to control the velocity
of the bat at impact.

I take it that you have never played then. Your bottom hand should
punch through the ball at the moment (sic) of impact, i.e. force is
applied to rapidly accelerate the bat through the ball using the top
hand as a pivot. The batsman doesn't just control the velocity, he
substantially creates it.

If you have a soft material that holds the ball onto the bat face for
a split second that force acts on the ball for longer and so increases
the "impulse" (which I understand is force times time applied). Which
was the point I have been trying to make and you have been ignoring.

Galileo then tells us that there is an
equivalent impact in which the bat is [momentarily] stationary
[eg, if you observe the action from a suitable train, or for that
matter if the action takes place *on* a suitable train and is seen
from the ground].

But is this valid for a bat that is accelerating through the ball due
to an "external" force?

More usefully for the detailed mechanics, there
is also an impact in which the bat and ball mutually bounce off
each other, which is much easier to understand [and then you can
add a constant velocity back in to the results].

except that it isn't necesarily constant.

Can we discuss a mathematical model of sufficient complexity to
properly model the deformation of the ball, blade and handle and to
account for factors relating to the forces exerted by the batsman etc?

We can, if you like.

Go on then. It would be of more interest.

But you have not given us any reason
to suppose that the deformation of [in particular] the ball and
blade is more "interesting" in cricket than it is in snooker, so
that we can't just treat it as a restitution problem.

because in snooker the cue ball does not accelerate through the impact
(unless it is a push shot), in cricket the bat is being forced through
the impact. Think about the way a trampoline works, the elasticity of
the surface helps to store the energy resulting from muscular force
exerted by the trampolinist and then convert it back into kinetic
energy. The impact in cricket is somewhere between snooker and a
trampoline, I am not sure where on the spectrum, but my intuition is
that it is more than enough to invalidate the simple model.

With or
without that extra complexity, it's still going to need some real
mechanics, not just arm-wavery. That is, of course, if you *want*
a proper model.

I don't believe a realistic model is possible on this forum, however
if you want to write up such a model mathematically, I would be
genuinely interested in reading about it.

If the answer is "no", can you see the value of honest "arm waving"
discussions that don't pretend to have the backing of sound scientific
analysis?

N/A. But I quite like arm-waving discussions anyway -- until
we start worrying about the "stored energy" of a squash ball travelling
down the blade ....

If you hadn't noticed *I* was arguing that the squash ball would have
no benefit in terms of power, it is too soft and would only dissapate
the energy rather than returning it. The softness of the blade is
accompanied by a more "rapid" springyness (sorry, don't know the
technical term), which is why it does increase power.

If we have a wall of infinite mass, that is completely hard and stiff
(i.e. it cannot be deformed in any way), won't all of the momentum
remain with the ball (having nowhere else to go)? In which case the
ball would rebound to its original height (assuming an ideal ball).
Now say we have an infinitely massive rubber wall. Now some of the
energy will remain with the wall in the form of vibration, in which
case the ball won't rebound as far? If I've made an error, I am
genuinely happy to corrected.




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Relevant Pages

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