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'[OT] The physics of stuff getting hit'
2008\07\23@191122 by

What kind of units do you use in physics to describe how hard a smack
something got, or how hard a smack something can sustain?

I mean let's say you're selling tripple-glazed windows, and you're
might say that they can sustain a smack as high as 400 neckles. I'm just
wondering what a "neckle" would be? Would it be force? Or pressure? Or
impulse?

Let's say there's a golf ball whose mass is 50 grams, and it's moving
through the air at 200 metres per second, and let's say it hits one of
these windows.

Speaking more from human intuition rather than a knowledge of physics,
it seems to me that you've to take into account the following in order
to determine just "how hard a smack" the window got:
* The mass of the golf ball
* The speed of the golf ball
* The hardness of the golf ball
* The surface area of impact

Well the momentum of the golf ball is 200 x .05   =   10 N s

Let's say that the golf ball's made of titanium. Titanium has a hardness
of 6 on the Mohr's scale.

I don't know how you'd calculate the impact surface area seeing as how
the golf ball is a sphere... hmm... but anyway, is there any sort of
unit that gives you an idea of how hard a smack something got? It's be
cool to be able to say "the helicopter crashed into the building with
1700 neckles".

A bigger concern is safety if it does break for whatever reason, and
building codes generally have some wording that helps in that area and
should be followed as a minimum. Here in the USA all door glass and
other glass that is at a location where it could have humans near is
required to be tempered. Currently I am replacing a skylight insulated
glass. The outer pane of glass is tempered so if it breaks it ends up as
small relatively harmless pieces, the inner pane is laminated safety
glass like a car windshield, that will contain and broken pieces of
itself or the other pane.

Tomás Ó hÉilidhe wrote:
{Quote hidden}

>
> I don't know how you'd calculate the impact surface area seeing as how
> the golf ball is a sphere... hmm... but anyway, is there any sort of
> unit that gives you an idea of how hard a smack something got? It's be
> cool to be able to say "the helicopter crashed into the building with
> 1700 neckles".

I don't think it is specified that way. It is more common to specify the
test method, e.g. when making large CRTs for TV sets, Philips had a
specification that they had to survive a 1 inch steel ball bearing dropped
from 10 feet onto the centre of the tube face, after the air was evacuated
from the tube. Now I don't know what this works out to be in terms of impact
force, except that it will be reasonably high. The test was done in an
enclosed chamber to contain any flying glass from ones that failed, and the
production was 100% tested.

FWIW:
Last I heard aircraft certification required windshields be tested to not
break with a four pound bird strike at cruise speed.

That may not be true because the rest of the story claimed that a company
suddenly could not pass the test. It turned out that the Quality Control
folks reduced their expenses by switching to frozen chickens for the test.
Returning to fresh chickens resolved the problem.

John Ferrell    W8CCW

"All that is necessary for the triumph of evil is for good men to do
nothing." -- Edmund Burke
http://DixieNC.US

{Original Message removed}

John Ferrell wrote:

{Quote hidden}

Some of this may have started from the chicken cannon that was at Uplands
Airport in Ottawa in the 60's. I worked at the airport one summer when
they were doing research on bird strikes on military jets and were 4 shots
short of their complete set. Someone was sent out for more ammunition and
the came back with frozen not fresh.

I saw the results. The frozen was more like a point source mass. It cracked
the outer layers of forward looking armoured cockpit windshield on a
military jet.

The chicken cannon was basically an over grown spud gun.

w..

Tomás Ó hÉilidhe wrote:

{Quote hidden}

A titanium golf ball being hit with a club exhibits momentum conservation
where the total momentum of the system remains constant. Momentum
is the product of velocity (speed with a vector) multiplied by mass.

Before the hit the ball has no momentum and the club has it all. After the
hit the total of the club and balls momentum remains the same as
clubs before the hit.

The answer to your question is neckles is the product of mass and
velocity.

w..

Walter Banks wrote:
> The answer to your question is neckles is the product of mass and
> velocity.

I don't think so.

I rather get hit with a 5 kg pillow travelling at 40 mph, than a 100
gram golf ball moving at 30 mph, even though the former has far more
momentum.

I think you've to take into account:
* Momentum
* Hardness
* Surface are of impact

On Thu, 24 Jul 2008, John Ferrell wrote:
> FWIW:
> Last I heard aircraft certification required windshields be tested to not
> break with a four pound bird strike at cruise speed.
>
> That may not be true because the rest of the story claimed that a company
> suddenly could not pass the test. It turned out that the Quality Control
> folks reduced their expenses by switching to frozen chickens for the test.
> Returning to fresh chickens resolved the problem.

Mythbusters did an episode on that.  They screwed up the test in the end,
but frozen birds indeed do far more damage at least at any speed a plane
would be going.  If it gets fast enough, say the speed of a meteor
intersecting our orbit it suddenly stops mattering what it's made of.
Dust, pebbles or a solid chunk all will give you an equally bad day. :-)

--
Ian Smith
http://www.ian.org
I think hardness and momentum give you surface area (since the hardness will
imply how much the object deforms, which will give you the area of impact).
Of course you have to consider the hardness of both objects, too.
- Marcel

On Fri, Jul 25, 2008 at 8:56 AM, Tomás Ó hÉilidhe <toelavabit.com> wrote:

{Quote hidden}

>
> I don't think so.
>
> I rather get hit with a 5 kg pillow travelling at 40 mph,
> than a 100 gram golf ball moving at 30 mph, even though the
> former has far more momentum.
>
> I think you've to take into account:
>     * Momentum
>     * Hardness
>     * Surface are of impact

NASA used to think like that.  Maybe they still do.

Tony

> > Last I heard aircraft certification required windshields be
> tested to
> > not break with a four pound bird strike at cruise speed.
> >
> > That may not be true because the rest of the story claimed that a
> > company suddenly could not pass the test. It turned out that the
> > Quality Control folks reduced their expenses by switching
> to frozen chickens for the test.
> > Returning to fresh chickens resolved the problem.
>
>
> Mythbusters did an episode on that.  They screwed up the test
> in the end, but frozen birds indeed do far more damage at
> least at any speed a plane would be going.  If it gets fast
> enough, say the speed of a meteor intersecting our orbit it
> suddenly stops mattering what it's made of.
> Dust, pebbles or a solid chunk all will give you an equally

They did that one twice.  After the first test they were eventually informed
that the windscreens they were using weren't rated for bird strikes.

Tony

Quoting Marcel Birthelmer <marcelcarrietech.com>:

> I think hardness and momentum give you surface area (since the hardness will
> imply how much the object deforms, which will give you the area of impact).
> Of course you have to consider the hardness of both objects, too.
> - Marcel

A 0.145 kg baseball traveling at 100km per hour (what a talented
12-year-old boy can do) has a momentum of about 4kg*m*s-1

A 20-grain 0.22 bullet (0.0012959782 kg) traveling at 1000fps (300 meters
per second) has a momentum of about 0.4kg*m*s-1

kinetic energy might be a useful calculation.

{Quote hidden}

>> -

Spehro Pefhany wrote:
> A 0.145 kg baseball traveling at 100km per hour (what a talented
> 12-year-old boy can do) has a momentum of about 4kg*m*s-1
>
> A 20-grain 0.22 bullet (0.0012959782 kg) traveling at 1000fps (300 meters
> per second) has a momentum of about 0.4kg*m*s-1
>
> kinetic energy might be a useful calculation.

Let's say a car is exactly 1 Megagramme and it hits a wall at 100 metres
per second.

Now second time around, if the car hits the wall at 300 metres per
second, will it hit it with exactly three times as much of a "smack"? If
so, then that means "smack" would be proportional to speed, rather than
proportional to the square of speed, so that would rule out the
measurement of kinetic energy.

But then again maybe the car will hit the wall with nine times as much
smack... ?

What do physicists use to describe "smack", something like "impact
force"? I vaguely recall learning a little bit about "impulse" when I
was in school, I'm not sure whether it applies here.

Tomás Ó hÉilidhe wrote:

> What do physicists use to describe "smack", something like "impact
> force"? I vaguely recall learning a little bit about "impulse" when I
> was in school, I'm not sure whether it applies here.

It depends, that's why there are both measurements. And because the details
of the smack can vary substantially, there is no universal unit for smack.
For example rather tiny details of the form of both colliding bodies can
make big differences in how it smacks. Elasticity, limits of it and
(usually non-linear) sensibility to elastic deformation have a major
influence on the permanent effects of a smack.

Since this is not really universal, I don't think there is much about it in
physics textbooks. The basics are, and they are that momentum needs to be
conserved, energy needs to be conserved, properties of elastic and
modeling a specific case of smack and its impact -- but because of the
multitude of factors involved, there is no universal measure of smack, for
one because the measure of impact depends on what you want.

So I think you'll find more about smack in technology than in physics, and
there it's usually specified as a specific case of smack (like a specific
model of a bullet into glass of a specific size in a specific frame under a
specific angle, varying only the speed of the bullet) and specific damages
measured in specific criteria (like whether the glass breaks or not, given
a specific definition of "breaks").

Gerhard
> Let's say a car is exactly 1 Megagramme and it hits a wall at 100 metres
> per second.

I do not know, if a car at a speed of 360km/h hits a concrete wall, there is
no protection at all - even a F1 would be smashed into tiny little pieces,
even though a F1 is less than 1 Tonne (something like 600kg?). An F1 has a
very good safety measurements nowadays, but even then when they smack into
the rubber wall at 200km/h the safety of the driver is not guaranteed - they
may survive with minor injuries but it is very hard to predict what would
happened. And that is not a concrete wall... As far as I know usually the
impact is calculated and tested for a much lower speed.

Tamas

On Fri, Jul 25, 2008 at 8:59 PM, Tomás Ó hÉilidhe <toelavabit.com> wrote:

{Quote hidden}

>
> Last I heard aircraft certification required windshields be tested to not
> break with a four pound bird strike at cruise speed.

Are you sure this is tested in cruise speed? I thought it was for landing
speed only - at 6km altitude it is quite unlikely that a bird is flying but
I may wrong on this? Or you are talking about sport planes that are flying
at low altitudes?

Tamas

On Thu, Jul 24, 2008 at 11:15 PM, John Ferrell <johnferrellearthlink.net>wrote:

{Quote hidden}

> {Original Message removed}
It very much depends on exactly what you're trying to state with your
measurement.  If, for instance, you wanted to say that your tempered
glass, properly mounted, could withstand certain impacts you'd follow
what most of the industry does and simply state that it can withstand
the impact of a 1" diameter stainless steel ball from a height of 1
meter (or something similar depending of how good your glass really
is).

If you're talking about electronic components you'd usually mention
that your product is capable of handling an acceleration, for instance
it might work after being "hit" with an acceleration of 5,000 G's over
a period of not more than 5 milliseconds, and 50G during operation
indefinitely.

The nice thing about G measurements is it removes the mass and point
of impact from the equation - you get to figure those out based on the
environment.  This is useful if you're attaching your device to a car,
for instance.  In fact ECU modules experience greater impact dropping
from a worker's hands and hitting a concrete floor than they do
installed in the car and involved in a crash.

There is no single unit that encompasses all of the factors you
mention that I'm aware of, and if it exists it would really have a
very narrow application and be prone to manipulation, much like the
EPA mileage numbers.

For instance, you couldn't make a number (neckles) that accounts for
the particular deformation that occurs in both the glass and the ball
that is linearly related between a steel ball and a copper ball.  The
copper ball, being less dense, would be larger and have a larger
surface area for a given mass.  Or trade that out for a lead ball
which is more dense, and smaller, but deforms more readily.  A single
number couldn't account for all three types of contact.

It's hard to compare apples to apples without a common measuring
system, and most industries have developed standard test setups for
their products, but you'll be hard pressed to find out if an
electronic component can take the same blow as tempered glass merely
by looking at the data sheets and converting both to this same magic
neckles number.