Mirror ball drop

[mackerr]

It will be safe to do as long as there is noone on stage and every part of the system is rated at double the shock load generated by instant stop of the mirror ball.

(snip)

<{POST_SNAPBACK}>

Here in the US most rigging equipment, and the attachment of this mirrorball would qualify as rigging equipment, are safety tested to at least 5 to 7 times their rated load capacity. This is generally for a static load. I would expect that in the UK, where safety seems to be taken more seriously than over here, the requirements would be at least as much. Unless you are a trained certified rigging specialist, don't make recommendations about load ratings of dynamic loads. If you are a trained certified rigging specialist I am surprised by your recommendation.

 

All that aside, I have never heard of a rated mirror ball.

 

Mac

[bruce]

Let's put some numbers in.....

 

Air resistance is fairly irrelevant, unless your mirror ball is actually a balloon with little lightweight mirrors on it....

 

Suppose the mirror ball weighs 4Kg.

 

And suppose it drops 5m before the rope "catches".

 

 

So it's velocity at this point is sqrt(2as) = 10 m/s. Note that this is independent of mass. We're taking "g" to be 10ms^-2 rather than 9.81, to keep the numbers round.

 

At this point, the rope "catches". Now, it won't stop immediately - the rope will deform (stretch) and the ball will decelerate over a finite distance. That distance will depend on the kinetic energy of the ball (which depends on the mass and speed) and the elasticity of the rope. Climbing ropes are deliberately designed with a high elasticity (ie they are a bit stretchy) to minimise the shock in this situation.

 

I don't know how much the rope would stretch in this case, but bearing in mind that it's a relatively low mass and velocity (compared to a human falling off a mountain) I'd guess perhaps 2 cm.

 

So using the same formula as before, we get a deceleration of 2500ms^-2. Which is about 250 "G"s.

 

So the force exerted during this deceleration phase =mass x accel = 10,000 Newtons.

 

Obviously this force would be less using a more stretchy rope (bungee cord) and higher using something like a wire line.

 

So what's that in real money? Well, a force of 10,000 N is equivalent to just over 1.1 tons.

 

Will the fixing loop on a standard mirror ball support 1.1 tons reliably? Somehow I doubt it.

 

(edited to include calculations...)

Bruce.

[mackerr]

We know what WYSIWYG stands for - it's one of the best-known software packages in the lighting industry. What I don't understand is what it has to to with your question.

 

Still decided not to bother with punctuation, I see .....  :P

<{POST_SNAPBACK}>

Long before it was a well known software package it was the term to describe the first bit mapped computer displays, as used in the Xerox STAR and Apple Macintosh computers in the early 1980's. They were the first electronic displays to show text in the actual typeface, size, and format that it would be printed at.

 

And yeah, that punctuation thing is still a problem.

 

Mac :)

[V.A.]

We can call the final velocity equal to the deceleration, simply because we are stopping it instantly.

 

That, my good sir, is nonsense. Dimensionally wrong as well....

 

Bruce.

<{POST_SNAPBACK}>

Exactly.

 

If it stops instantly, you have infinite deceleration, but the final velocity is 0, if I remember my mechanics right (it's been a while!).

 

This is all theory anyway - as I said before, I'm not convinced that you can apply Newtonian Mechanics to a real world problem.

[bruce]

This is all theory anyway - as I said before, I'm not convinced that you can apply Newtonian Mechanics to a real world problem.

<{POST_SNAPBACK}>

 

'course you can.... old Isaac was a bit of a genius.

 

I have a friend who is a physics lecturer at University. He says that you could probably get a pretty good class of physics degree if you knew nothing but Newton's laws, and how to apply them properly.

 

But you'd have to know them very very well..... :)

 

B.

[Jivemaster]

Are you planning to drop the mirror ball, ball and motor or the ball motor and a board?

If this is a college project do you have a friendly maths dept that would do the calcs for you and using calculus will give you the load profile at different parts of the movement. Really it should be no calcs - no drop. If you dont know whats happening you certainly dont know that it is safe.

 

We can offer you all the distant advice but only you are on site with all the bits to hand for weighing etc.

 

A more typical safety factor would be 8 in this country so find the max load then rate the system a 8 or more times that. Remember thats the peak dynamic load! Adding some elasticity to the suspension will reduce the peak dynamic load greatly.

[peternewman]

We know what WYSIWYG stands for - it's one of the best-known software packages in the lighting industry. What I don't understand is what it has to to with your question.

<{POST_SNAPBACK}>

Long before it was a well known software package it was the term to describe the first bit mapped computer displays, as used in the Xerox STAR and Apple Macintosh computers in the early 1980's. They were the first electronic displays to show text in the actual typeface, size, and format that it would be printed at.

<{POST_SNAPBACK}>

WYSIWYG has also been used for years since, and in fact still is as a feature of software, as Mackerr says, that shows something as it will looks when printed/rendered in a web browser etc. For example Microsoft Word can show you a WYSIWSG version of the page without you printing it out.

 

[minirant] This particular choice of name also causes lots of issues when searching the web for the lighting package, as of course you come up with lots of links to programmes which show you a WYSIWYG representation of something such as DTP and Web software. [/minirant]

 

PN

[Deathride]

thanks to all your feedback the director has now cut the mirror ball drop. The reason being it was hard for the actors to do it aswell as the rest of the scene. With that I have had a steep learning curve like not to say wysiwyg in a topic I knew it was lighting software but I would have thought this not being lighting that people would have thought what else it might be. I have personaly used wyiwyg and liked :) .

 

A HUGE thankyou to everyone who has given feedback and puting up with my poor gramma and spelling

 

Thanks again

Deathride

[j_b]

I pleased to hear that it has been cut - it just sounded a bit too dodgy.

 

Your questions are always welcome, and as for spelling and grammar, nobody is perfect, so long as the gist is there, but those couple of full stops just then made the world of difference to me reading it - thank you for that, much appreciated

 

I hope you have more questions in future, and something that we can help with and lead to a successful conclusion. This board maybe be full of people who jump in and say "no - don't do that!" but they will offer excellent advice too, to help make a show look better where appropriate.

[Bryson]

To be fair, when people jump in and say 'don't do that!" it's for a damn good reason, made from years of hard-won and painful experience. And for the record, no-one ever says "don't do this at all", they usually say: get someone who knows what they're doing to do it for you, at great expense! :)

[Ben Langfeld]

Also would like to reemphasise that Climbing rope MUST be replaced after being subjected to a shock load.

<{POST_SNAPBACK}>

 

I'd like to contest that point. I am a regular climber and have taken several falls on the ropes I use. Most dynamic ropes that are used for a lead climb are rated to anywhere between 4 and 20 UIAA falls and that means they should hold the weight of an average adult male that number of times before they need to be replaced. Following that logic through, a 20Kg disco ball should be dropable roughly 4 times that. By my maths, that means that this rope should hold the disco ball 40 times before it has to be discarded.

[Darkfold]

I was banking on him using the cheap stuff, after seeing the rest of the idea. The rope used at my school (and hence what I am used to) says it must be replaced after each fall, although this may be the manufacturer covering themselves.

[Jivemaster]

And now you had got so far.............!

 

Try to privately learn from this, all the questions to ba answered to show that what you want is possible and safe.

 

Theatre involves a lot of "suspended disbelief" Sometimes it should look scary but never should it be scary.

[Ben Langfeld]

I was banking on him using the cheap stuff, after seeing the rest of the idea. The rope used at my school (and hence what I am used to) says it must be replaced after each fall, although this may be the manufacturer covering themselves.

<{POST_SNAPBACK}>

 

I think you'll find that isn't actually climbing rope That link is the cheapest rope designed for climbing that I could find. Anything cheaper is called string to a climber :unsure:

[V.A.]

Bruce - Isaac Newton's work was indeed magnificent, and laid the path for much of what we know today, however his work did not include the effect of air resistance and terminal velocity, nor did it consider the motion of anything other than a particle with a central centre of mass and negligible size.

 

For these reasons, I don't believe that it is entirely valid to use Newtonian Mechanics in a real world context - it is fine as a guideline, but for the kind of accuracy required here is clearly not appropriate.