andy jackson Posted October 14, 2005 Share Posted October 14, 2005 I was wondering yesterday evening what would happen if the home made projector cradle were to fail and it were to come crashing down on top of an unsuspecting audience member? Then this made me scour the bits of the grid I could see looking for safety chains, then I started to add up what was actually up there, 30 odd prels 5 kilos each, two dozen parnels with scrollers, 8 kilos each? hazer or two 20 kilos?, pair of projectors, mirror ball, etc etc etc... Now off the top of my head - someone please correct me if I'm completely wrong Gravity = 9.4m/s2 (squared) an object falling 10 meters from a grid will be traveling at 98 meters a second when it collides with a person/the ground. Or 49 mps from my 5mtr grid. And Force = mass x gravty, so say a mac 250 weighing 21 kilos falls at the speed of gravity its mass/weight would reach 201.5 Kilos when it hits someone. Thats a lot, And so what Im really asking is has anyone had to use a safety chain/bond in anger, and what was yours / the audiences reaction. Andy Link to comment Share on other sites More sharing options...
Solstace Posted October 14, 2005 Share Posted October 14, 2005 Hang on! This isn't the version of the "Have you ever..." game I'm used to!! Can't say I've ever seen a situation arise where a safety bond has been "used" in that way, other than forgetting about it during a de-rig and wondering what the pesky lantern was getting snagged on... Your speed calculation is right to a point - the object will pick up 9.4 m/s per second, but only until it gets to terminal velocity, where the air resistance prevents further acceleration. At a guess, I'd think the 94 m/s does seem "just a tad" too quick - I believe that would work out at 338.4 Km/h! Though having never dropped a MAC 250, nor having worked out it's drag coefficient, I wouldn't know! Also, the Force equation you give would I believe be kinda correct to describe the force acting on the object - but since the object is now moving, it's the velocity that counts, not the force acting to get the object to that velocity in the first place. Have a look at http://hyperphysics.phy-astr.gsu.edu/hbase/flobi.html From there, I get a rough idea that 14m/s sounds more reasonable, And assuming that the human head taking the impact might allow the Mac250 to "give" by 1cm when absorbing the impact, we're looking at an impact force of 205800 Newtons!! That's about 21 of our Earth tons!!! I'd still suggest that I don't wanna be standing under it if it should fall! Before anyone gets worried about thin safety bonds/chains taking that huge tension - don't forget that the less distance the object falls, the less time it has to pick up velocity and therefore the less energy would need to be transferred to bring the object to a halt. Link to comment Share on other sites More sharing options...
J Pearce Posted October 14, 2005 Share Posted October 14, 2005 Gravity is 9.81m/s2, not 9.4. Also a better measure ofthe impact would be impulse, force applied=weightxtime applied for. Weight in this instance is massxacceleration. Link to comment Share on other sites More sharing options...
Brian Posted October 14, 2005 Share Posted October 14, 2005 see here. Link to comment Share on other sites More sharing options...
BlueShift Posted October 14, 2005 Share Posted October 14, 2005 Im afraid andy that your physics is wonky :nerd: Fisrtly, gravity is 9.8 ms^-2 Secondly, to know how fast its gonna speed up to, you need to figure out how long it is gonna be falling before it hits the ground with that acceleration. The terminal velocity comment is valid, but irrelevant in this case, as its never gonna get close to reaching that speed over a 5m drop :( Also, air resistance will be negligable aswell because it isnt falling very far, nor very fast so the forces of friction arent gonna be huge. To figure out what speed it will be going you need to use the following equation S = ut + 1/2at^2 (s = distance, u = initial speed, a = acceleration, t = time) this will let you figure out the time, which you can then plug into v=u+at to get final speed on impact.Once you know how fast it is travelling then you can figure out its momentum which will then allow you to figure out the impact force. So, taking the 21kg mac 250 on your 5m rig as an example: 5 = 0 + 1/2(9.8 * t^2)5 = 4.7t^24.7t^2 - 5 = 0solving this quadratically gives us two values for t: -1.03 and 1.03obviously the relevant value is 1.03 since the mac will hit the ground after it begins to fall, not before ;) so thereforev = 0 + 9.8*1.03 = 10.094 ms^-1 Knowing its speed at impact we can figure out its momentum: p = mv = 21*10.094 = 211.974 kgms-1 since force equals the change in momentum and the initial momentum was 0 (coz it was stationary) F = (delta p)/(delta t) = (211.974-0)/(1.03) = 205.8 N therefore its gonna hit the poor punter with 205.8 N of force. I think :) however, the easiest way to figure out the speed is to use kinetic and potential energy.... Following brysons link, you can calculate the energy in joules applied since Ek = p^2/2m = 1/2mv^2 therefore E = 211.974^2/42 = 1069J - thats 10 times as much energy is needed to fracture the human skull on impact. So the guy/girl would most likely be dead. Link to comment Share on other sites More sharing options...
erroneousblack Posted October 14, 2005 Share Posted October 14, 2005 I think the real equation to consider in all this, is how much the agrieved punter would sue your ass for if you didn't rig it properly! Link to comment Share on other sites More sharing options...
BlueShift Posted October 14, 2005 Share Posted October 14, 2005 Well, he/she'd be dead so I think someone else would be doing the suing.... that rhymed :) Link to comment Share on other sites More sharing options...
andy jackson Posted October 14, 2005 Author Share Posted October 14, 2005 I was rather hoping for stories regarding things falling and stopping in the nick of time rather than people scrutnising the hazy remains of A-Level physics from years gone by. But I think when you read the figures it makes you think. Also zip ties for safety chains, although Someone somewhere could possibly reason with themselves and use them, I however could never! Andy Link to comment Share on other sites More sharing options...
J Pearce Posted October 14, 2005 Share Posted October 14, 2005 My A level physics is far from days gone by, I'd only just got out of a physics lesson on standing waves when I posted that! Link to comment Share on other sites More sharing options...
landy Posted October 14, 2005 Share Posted October 14, 2005 In a venue with very tight gaps between bars and voluptuous wipes I have seen many a barn door try and top it self. on a different occasion when bars were flying in and out the g clamp bent when a fly bar hit it and the lamp fell onto the bond. So they do get used. Link to comment Share on other sites More sharing options...
Just Some Bloke Posted October 14, 2005 Share Posted October 14, 2005 I was one asked to fly a video camera so the band could record their performance. I did so, obviously adding a safety chain at the same time (it was the days before bonds). Band were told not to fiddle with it but, apparently, while I went for a meal break, they got the ladder out and fiddled. When they came to de-rig the camera it inadvertently fell off its cradle after the bolt had been removed. Turns out that when they fiddled with it, it meant removing the safety chain which they then failed to replace (unbeknown to me). Thus, when the camera fell there was no secondary suspension and it fell to the floor breaking into several pieces. Luckily it was their camera and they learned an important lesson that day! Link to comment Share on other sites More sharing options...
peternewman Posted October 15, 2005 Share Posted October 15, 2005 A friend had a safety chain catch a lantern a long time ago at his school, it was next to a tab track and as the tabs moved back and forth each night they unscrewed it until the bolt fell out and the chain caught it. PN Link to comment Share on other sites More sharing options...
pete LD Posted October 15, 2005 Share Posted October 15, 2005 safety bonds are meant to take 10x there rated load so a 10kg bond wont just snap when your 10kg lamp falls off the bar! there are far to many barn doors etc out there that don't get secondary attachments. is there laws on this or have lantern accessories slipped through un noticed?pete Link to comment Share on other sites More sharing options...
James Posted October 15, 2005 Share Posted October 15, 2005 And so what Im really asking is has anyone had to use a safety chain/bond in anger, and what was yours / the audiences reaction. Andy Andy I can think of two instances where I have seen safety chains used in anger for their intended job. INSTANCE 1 Mirror ball rigged from a motor. The motor didn't have a clamp so was gaffer taped to a clamp. Prelude 16/30 focused on it all night Heat melted adhesive and caused motor to drop. Safety caught motor + ballEffect switched off for rest of evening. INSTANCE 2 Moving disco effect. Bolt was attached without sprung washer or nylock buts. Motion of effect caused bolt to come undone. I was standing under effect when nut and bolt hit dance floor 2' from where I was standingSafety caught effect. I have also on ocation used safetys as primary means of suspension with a secondary in place. James Link to comment Share on other sites More sharing options...
Andrew C Posted October 15, 2005 Share Posted October 15, 2005 In 30 years, Am & Pro, I have never seen a chain catch anything. I have been involved in an instace where something (Bose 802) fell, but that was during the de-rig when both fittings were removed simultaniously. Link to comment Share on other sites More sharing options...
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