Point of Impact ( in kg)

It depends on how quickly the animal comes to a stop. Let's say it takes one-tenth of a second, 0.1 s.

The deceleration is ΔV/ Δt, which here would be [(50 Kph) x (1000 m/K) ÷ (3600 seconds/hr)] / (0.1 s) = 138.9 m/s². That's the A in F=mA.
If you think the deceleration time is different than 0.1 seconds, plug in whatever value you think is correct and re-calculate your new value of A.

The force would be F=mA = (25Kg) x (138.9 m/s²) = 3472.5 Newtons.

Or, expressed in terms of 'g', where g = 9.8 m/s², (25 Kg) x (14.2 g) = 354 "Kilograms" (where 'Kilograms' here means weight.)

The impact would then be 354 Kg/(22 x 10)cm² = 1.61 "Kilograms" per square cm.

[Unless, of course, the gazelle jumps over the wall...]

OK. You think 1.61 kg per sq. cm?

That will work.

The "solid barricade" scenario is just that ... a scenario. I have no other way to describe this except to put it in really simple terms. Of course the animal and the net will move a great amount on impact.... I just wanted a base number, in a WORSE CASE... thank you.

Yes, jumping can be a problem which is why I am raising the net another 3 feet.however they say these critters sometime slide under the net . I don't know much about it except what i am told. I usually design for FISH! ha ha !

• Netting stretches; ideal barricades don't.
• Animals deform on impact whereas the in the assumed case, they don't.

One way to look at it is to examine the energy. The kinetic energy of the animal has to be converted into stretch energy at the net at the moment the animal has come to rest. So the "stretchiness" of the stuff forming the net has to be evaluated in terms of the strength of the individual strands and the number and configuration of them, assuming the animal is not shredded or otherwise deformed when collecting the net. It is probably better to do a series of tests rather than to plough through endless network analysis calculations, computer-driven or otherwise.

• Road vehicle makers do extensive testing on their vehicles rather than endless calculations to establish the survivability on impact of the animals they contain; there is a lesson there somewhere.

I will have no time for any testing and there are no gazelles in Louisiana...ha ha!

This is just a crude way of finding out the worse case...which would be a solid wall.

Thank you for the comments.

There must be some give in the net, if it were a brick wall, the animal would die! A net cannot be a brick wall.

The kinetic energy must be used up by doing work, in this case, in deflecting the net. The greater the deflection, the smaller the force needs to be. The rate of resistance is governed by how tightly the net is strung to a stiff frame. It it were an immovable object, the animal would have to do all the crumpling to absorb the energy. The more deflection you allow, the less strong the net needs to be.

I'm sure others will provide some numbers soon.

Exactly. Once again, I know the net will give ...a LOT. I know the animal will NOT be hitting a solid barricade.

I just wanted an absolute worse case scenario to use as a base and a 50 lb. gazelle moving at 40 mph or so hitting a solid fence or wall IS a worse case ( probably never to happen in the real world).

Yes they do jump but on this desert plain, I was told they actually try to 'slide' under the net.

there will be some modifications done to the set up also....strong winds sometimes knock the net down before the animals can hit it.

thank you as always...........

Some razor wire along the bottom should keep 'em from sliding under.

Calculate the kinetic energy of the animal : 1/2 * mass * velocity squared (E=(1/2)mv^2).

This kinetic energy is absorbed in some distance D by a net or solid barricade which exerts a force F. The energy absorbed is D*F. (Technically, you would have to integrate if the force is not constant). You can calculate the pressure by dividing the force, F, by the area in contact A. (P=F/A).

You can see that a net, which allows a larger stopping distance, produces less force than a brick wall, which has very little stopping distance.

Thank you. I understand the concept of a net absorbing impact. I've kept 1200 lb. sharks captive while they were tagged and released. In this case I just wanted to see an impossible, worse case scenario...an agile, movable critter hitting an immovable force. That way I have an absolute impossible and improbable number to use as a base , so when I suggest to the customer a downsized twine size, I have something to show as an 'impossible' base number.

I am really simple minded and simple descriptions work the best for me.

That's why testing is the answer. Rolling a dead ostrich or similar down a hill into the net would be a simple, achievable first step. <unsubscribes>

This reminds me of the old joke about testing the windshield of a commercial airliner with a chicken shot from a cannon.

Most of the time, the windshield deflected the chicken without any damage. But when they used frozen chicken, it went right through the windshield.

Elasticity is a big factor. In the first answer, that 1/10 second deceleration would not be representative of the elasticity of the impact. F=MA works but the deceleration would most likely build up exponentially or as a function of 1/r*r where r = radius of the affected area in the net. It is much more likely that a deceleration into a net would last for up to 2 seconds but it would be a function of the momentum = mass * velocity. And if the Gazelle still had his feet in contact with the ground, it is likely that it could push just as the force peaked.

In the case of a brick wall, all of the stopping distance would be within the body of the animal (ouch!).

Velocity = 14 m/s (estimated speed of Gazelle)

mass=30 kg (estimated mass of Gazelle)

Energy=(mass*velocity^2) = 5880 kg.m^2/sec^2 (Kinetic energy of running Gazelle)

Stopping distance D=.01 m (Estimated 1 cm crushing distance)

Force=Energy/D = 588,000 kg.m/sec^2 = 588000 newtons

Impact Area (est) .1 m^2 (about 1 sq ft area absorbing blow)

Pressure = Force/Area = 5880000 nt/m^2 =5889 kpa = 853 psi

If any of these assumptions do not seem reasonable, you can adjust and recalculate.

Thank you Rixter. Your numbers are quite different from the others but that is OK. It gives me a HIGH and LOW range.

This issue does not require an exact answer. Generalities are acceptable. I just want a number or numbers to make an argument that I can downsize the current .5cm diameter twine in the current 178 mm stretched mesh to a .350cm dia. mesh and make a lighter more entangling net and still have adequate tensile strength.

I just want a number or numbers to make an argument that I can downsize the current .5cm diameter twine in the current 178 mm stretched mesh to a .350cm dia. mesh and make a lighter more entangling net and still have adequate tensile strength.

The spring constant of a rope, K, is proportional to its area.

For a given amount of energy absorbed from a galloping gazelle, E=(1/2)*K*D^2, where D is the amount of stretch. The maximum force is F=K*D.

Now as an example, if you halve the diameter, area is reduced to 1/4, and K is reduced to 1/4. For the same amount of energy, E, D (the stretch) is doubled. The amount of force, F=K*D, will be half. So the maximum force is proportional to the diameter of the rope.

This analysis is good assuming that the spring constant, K, is "constant". In the real world, ropes stretch and then reach an elastic limit, where K becomes much larger. If a smaller rope reaches its elastic limit, it could exert a larger force than a larger rope.

I hope this analysis is helpful.

You forgot the 1/2 when you calculated the kinetic energy: ½ m v².

Also, your stopping distance of 1 cm yields a stopping time of only 0.0014 seconds, which, to me, seems awfully fast.

You are correct, forgot the 1/2. Thanks.

I know of only 2 ways to calculate the force:

• Calculate kinetic energy and use stopping distance (E = F x D)
• Calculate momentum (p=mv) and use stopping time (p = F x T)

The first method seems the most useful.
Measure K:You can think of the net as a kind of a spring with a spring constant K. You could determine K by stretching the net and measuring the force (F = K x D).
Calculate D:The energy absorbed by a spring displaced distance D is E = (1/2)KD^2. Knowing Energy and K, you can calculate D.
Calculate Force:The Force = K x D is maximum when the net has stretched to the maximum when the Gazelle is brought to a stop.

GA and practical, but I would suggest that a 1 cm stopping distance is on the low side.

The head will stop within a very short distance due to deformation of flesh/bone etc, but the major mass of the body will continue to compress the neck before stopping.

I'd suggest an inch (approx. 2.5 cm) as an average figure for the body- that's still not very much, but will lower the specific impulse by a factor of 2.5. Assuming you're not worried about concussion, of course!

My understanding is that the netting is soooo heavy (210/354) 520 kg. of tensile strength per strand x .50cm dia.....that the critters just slam into it and knock the poles down ( shown in the pictures at the front of this thread). The handlers then jump on them, wrap them up in the net until they get a collar on the animal.

My designs of Fence Net entail a lighter netting (210/192), 40% more netting to rope ratio ( coefficient) and a slightly larger mesh so the critter can get part of his head and one or both front legs in....the looser, lighter netting tangles him....without cutting him up.

Also taller net.

thank you for the post and the figures.

Found this netting used for deer control...

• Deer Blocker is extremely strong, long lasting, virtually invisible and easy to install.
• Strong, 380 dernier, 36 ply, knotted, virgin (un-recycled) polyethylene mesh.
• Each UV resistant strand has a 175 lb. breaking strength.
• The 4" square mesh is durable yet inconspicuous - almost invisible at normal viewing distances.
• Full 8 foot tall and 100 foot long pieces for deer exclusion.

Too thin of a netting. It will cut into the animals skin.

Also, it will wave around too much in the wind.

...no square mesh...it does not wrap up well enough for these Fence Nets....

For this job we need a nasty ol' heavy nylon.

the 210/36 makes for a good Deterrent netting...for wrapping up an animal you need a Capture netting which is heavier and will wrap up the critter.

With fish its almost all HDPE or UHDPE.

thank you for the thought though....

I think you have your answer. In your worst case scenario of all energy being dissipated by the animal through broken bones/torn tissue/etc/etc you have 2 solutions. Impulse and work energy.

Impulse was the first method given, work energy is the second.

Both deviate from worst case of having a gigantic wall, and the main variable in both methods is essentially the same, how long does it take for the animal to stop.

I realize you know the net will give, stretch, deform, whatever, but bear with me, as I think this will be a bit more useful.

If you use both in conjunction you'll have your specification for the netting. I'm assuming that you want the animal unharmed. Don't know if there are g force numbers out there for survivability of a gazelle, but if they knock heads in battle to be top dog, there is probably an estimate of the G-forces involved.

Plug that survivability acceleration into the impulse equation. This will give you force as well as the impulse duration, although the impulse doesn't really matter as we are working backwards. PSI comes from the force and whatever you want to use for area.

So now you have an acceptable force based on the mass of the animal, as well as the kinetic energy based on speed and mass. Use these down in the work energy equations to find the distance across which the force needs to act to stop the animal.

I think this would be more useful to show the customer, as it's closer to reality, and actually helps you spec the netting. If you want to show the customer what the numbers would look like if a super gazelle came around, just multiply the force by oh I dunno 4. (Double the mass and acceleration) Then recalc the psi. Calculate the impulse duration to give you and the customer a little reality check. As the impulse gets shorter the worst case gets worse.

You can easily use excel to graph the various equations. If you're comfortable with 3d plots you can place accelerations from zero to well past the worst case and put them on the Y-Axis, use the X-Axis for mass, and the Z-Axis for force or psi. You can make a carbon copy of that plot and replace force/psi with impulse values to serve as a "reality" check. On both plots you can mark the average mass for gazelles as well as average mass for males and females of different ages and also the largest recorded mass of a gazelle. That mass and beyond could be considered worst case values. Also mark the accelerations deemed survivable (I'm pretty sure the males butt heads for dominance so that info has to be out there somewhere), but this acceleration isn't really what is survivable, it's just researchers guesses on how hard they hit, so a little fudge factor is likely in order. You can make plots using the work energy equations, but those would really only be for you.

If you're interested in going down this route let me know, and I can put some graphs together for you. I'm a mech engineer that has been out of practice for a few years due to health issues, so I need some practice. And I rocked my labs at Cal Poly, so I'm able to put together some good quality stuff.

Steve

Just curious. Do you have to consider the strength of the net (not to break) and the safety of other animals running into it.

They are not going to break the netting I am going to use....the danger is in them cutting it with their hooves or the number one enemy of any net....misuse by the operators! ha ha !

Also, they do not leave these nets un-attended. This part of Asia does NOT have much of anything else in the wild that is.

why aren't you firing frozen chickens out of a compressed air gun like they do when they test windscreens for aircraft.. or there is the dead ostrich theory, which i quite like.. as you'll never get a big enough gun for that bird.

FYI.. I work in the deserts of Oman, and they JUMP over any damn thing in their way. fences included... had to share!

HA HA! yes they do and my first suggestion after seeing the pictures was to offer a higher net. We concluded business last night ( his morning) and they are going to compromise in height, netting/rope ratio, twine size and mesh size....

Thank you for the comment.

Is it a case of existing nets not being suitable and you are trying to design one that 'works', or is an improved design to make you more competitive?

If the latter then 'reverse' engineer the existing net and re-design to achieve the same effect more efficiently to make it better (lower cost, I guess).

They are using nets that work. They are just looking at nets that might work a little better. I generally stopped making Frank Buck nets years ago here in the USA due to liability issues with the misuse of the gear.

The Fence Nets and Impoundment Nets were always a challenge as you just never knew what a 40mph critter could or would do.

Cost is an issue to them but not for me. I do not make cheap $h!t for anyone....its the first step in losing your reputation .When the cheap stuff fails...and it always does.. the customer NEVER tells anyone that HE ASKED for a cheap net. He just says YOU GAVE him a cheap net.

In this case they just want to catch these little critters and not hurt them or themselves. I think what I have in mind will do just fine and the information from CR4 has helped explain that to him and his supervisors.

thank you again.

I think you want the impulse pressure on the netting. Impulse is the integral of force applied over time, and works out to the integral of momentum, simply stated. Assumption: constant force of deceleration (although this is entirely untrue, since the initial force applied to the netting is much less than as the net is elastically stretched. Even having said that, we simplify to maximum extent, and approximate what happens... even though this may not be a dependable model upon which to base the strength of the net.

J=m₁v₁-m₂v₂ and since the mass (of the antelope/goat) is constant, less any excrement taking place, and/or expletive related bleating, J=mΔv, limiting this to the normal impact, not glancing. J= 25kg x 50,000m/3600s, so the result is 347.2 kg-m/s (same as N-s). From this we arrive at impulse pressure: N-s/m² units,

J/A= 347.2/0.022, with the result of 15,781.8 N-s/m². I hope you can decide what to do based on this, however this does not state how much deformation the netting will be subjected to, may not be a useful model at all.

You should consider the netting as a spring material with an elastic limit where it irreversibly breaks. Note that the force integral of deceleration is time dependent, with most of the actual force being applied near the point of maximum extension of the netting.

James,

Thank you for the information .

we concluded business last night and he and his supervisors have agreed to let me use my plan for the new nets.

Your explanation backs up what I was trying to tell them only in my greatly simplified words.

Thank you for the time.