Noob possum got pwned by avo cannon

[moT]

Nice - but it doesn't take wind resistance into account. If the firing range is decent then the error on that calculation will be significant.

Having half a metre between the muzzle and the pendulum would pretty much give you the muzzle velocity - with less error. The measuring is a bit more difficult though.

i agree there is too much error doing it your way the only way i will find a fairly accurate mussle velocity is shooting the block

[moT]

I'm more concerned with the terrorist applications of these weapons. They can be made with x-ray transparent materials and smuggling on aboard a plane should be possible...

hmmm very true

[BMW_RACER]

i agree there is too much error doing it your way the only way i will find a fairly accurate mussle velocity is shooting the block

Or you could just use a chronograph.

[moT]

The simplest way of making a rough estimate would be to consider the problem from an energy conservation perspective.

The increase in potential energy of your pendulum should be equal the decrease in kinetic energy of the projectile:

dEkin + dEpot = 0

This is pretty rough since some energy will be expended deforming your projectile, localised heating, wind resistance, non-perfect wires, etc.

dEpot = (m+M)*g*dh - where m is the mass of the projectile, M the mass of the pendulum, g the gravitational constant and dh the change in height of the pendulum.

dh = l * cos(theta) - where l is the length of the wire and theta is the deflection angle.

The velocity should then be given by:

v = sqrt(2*(m+M)*g*l*cos(theta)/m)



It's important that you choose the weight of the pendulum carefully in relation to the weight of the projectile. It is also important that the entire projectile moves with pendulum. You need to establish the centre of mass of the pendulum fairly precisely - or use a very long wire.

And is mass in g or kg

[chrisso]

Are we talking your standard Avo or the Hi power Magnum Avo round?

[moT]

Are we talking your standard Avo or the Hi power Magnum Avo round?

high power unripe avo round

[Skunk]

Pretty simple things. This one is Lynx powered.

Best barrel to chamber volume is 1:1.5

[Forest]

Wouldn't it be more fun just to eat the avocado?

Mmmmmm. Avocados on toast.

[moT]

photos of current one and project bolt action cannon

[moT]

Wouldn't it be more fun just to eat the avocado?

Mmmmmm. Avocados on toast.

yer there nice but its more fun shooting them

[Mikkel]

Come one Mikkel its not like you to overlook anything.
Yes doing the parabola test has fail in it - but not due to wind resistance - due to fact that it gives you average speed.
Also shooting a avo into a bit of wood wont tell you speed if the wood spins, as the avo will deform loosing most of the energy.
If you want the best calculaiton your going to have to time between 2 points.
i.e. end of barrel to some distance that can be timed and is not too long.
If you were really cheaky you could put 2 sensors in your gun. average the speed down the barrel.

Nope - the parabola calculation should provide you with the starting velocity, not the average. You don't directly measure the time it takes before the projectile touches down again - measure how far horizontally the projectile travels inside the timespan it takes for the projectile to reach the apex and return to the ground.
However you expect the ballistic profile to be a perfect parabola and it is not, due to wind resistance. Actually BMW_RACER has the essence of the parabola experiement sorted in the below quote - but that approach is not viable when using explosive shells or if you lack a precise stopwatch.

The reason I did not suggest the timing approach to measuring the muzzle velocity was quite simply that it is much more technical to do precisely. You need an electronic data logger and two sensors to respectively start and stop the logger.

Having a two point pivot on the pendulum using two wires could be sorted out in a way that would prevent spinning. The avocado will always deform, unless it's a crappy cannon - and there will be an energy loss there. It is imperative that all the avocado fragments are collected and swings with the pendulum, then the impact characteristics (bouncy or squashy) shouldn't matter.

If you have access to the right gear measuring muzzle velocity is both simple and precise.

Fire it directly up, measure how long it takes to hit the ground, apply maths ?!

[Mikkel]

And is mass in g or kg

Mate, as long as you're consistent you can use pounds for all I care...

If you want the velocity in cable lengths per forthnight - just make sure you convert the gravitational constant to cable lengths per fortnight squared and that the length of the wire is measured in cable lengths.

The SI unit for mass is kg - 1 g is equal to 1*10^-3 kg.

[moT]

Mate, as long as you're consistent you can use pounds for all I care...

If you want the velocity in cable lengths per forthnight - just make sure you convert the gravitational constant to cable lengths per fortnight squared and that the length of the wire is measured in cable lengths.

The SI unit for mass is kg - 1 g is equal to 1*10^-3 kg.

you sort of lost me there and is the gravitational consistant 9.8mps(squared)

[Mikkel]

you sort of lost me there and is the gravitational consistant 9.8mps(squared)

The gravitational constant is ~9.8 m/s^2 yes - varies slightly with your position on the planet... But it's in the range of 9.78-9.82 m/s^2. I think it's 9.81 m/s^2 here - but do the maths and you'll see it won't make much difference.

[moT]

The gravitational constant is ~9.8 m/s^2 yes - varies slightly with your position on the planet... But it's in the range of 9.78-9.82 m/s^2. I think it's 9.81 m/s^2 here - but do the maths and you'll see it won't make much difference.

and by consistancy you mean like everything is done in grams and centimeteres