Brain teaser - structural and composites engineers need not apply

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Posted: 1/7/2006 8:38:28 PM EDT

You have a beam overhead and a 1000 pound weight you must hang from the beam. In order to do so, you have a chain and a rubber bungee, EACH with a breaking strength of 600 pounds. Your job is to figure out how, using only the bungee and the chain, to hang the weight from the beam. You are not allowed to double up the chain or bungee.

Posted: 1/7/2006 8:39:22 PM EDT

[#1]

hang it with the chain

Posted: 1/7/2006 8:41:24 PM EDT

[#2]

Quoted:
hang it with the chain

I fixed the typo

Posted: 1/7/2006 8:41:55 PM EDT

[#3]

both have safety factor of 6. did I miss a zero somewhere?

nevermind, it got fixed while I typed. magic.

Posted: 1/7/2006 8:44:02 PM EDT

[#4]

I cant just set the weight on top of the beam?

Posted: 1/7/2006 8:44:07 PM EDT

[#5]

hang yourself with the chain.

I hate word problems, and I do this stuff for a living.

Posted: 1/7/2006 8:46:26 PM EDT

[#6]

use 1 chain, 1 bungee

600 + 600 =1200 capacity, load = 1000, good to go

you said we can't double up the chain OR the bungee

Posted: 1/7/2006 8:47:52 PM EDT

[#7]

Quoted:
use 1 chain, 1 bungee

600 + 600 =1200 capacity, load = 1000, good to go

you said we can't double up the chain OR the bungee

I need more detail. If you just hook them both up it won't work - the chain wil break.

Posted: 1/7/2006 8:48:34 PM EDT

[#8]

i do whatever OSHA tells me to do.

Posted: 1/7/2006 8:48:42 PM EDT

[#9]

Quoted:
Quoted:
use 1 chain, 1 bungee

600 + 600 =1200 capacity, load = 1000, good to go

you said we can't double up the chain OR the bungee

I need more detail. If you just hook them both up it won't work - the chain wil break.

attach them in parallel so they both share the load

Posted: 1/7/2006 8:50:23 PM EDT

[#10]

Quoted:
Quoted:
Quoted:
use 1 chain, 1 bungee

600 + 600 =1200 capacity, load = 1000, good to go

you said we can't double up the chain OR the bungee

I need more detail. If you just hook them both up it won't work - the chain wil break.

attach them in parallel so they both share the load

But how? The chain won't stretch, so it will just snap while the bungee is limp.

Posted: 1/7/2006 8:50:52 PM EDT

[#11]

Quoted:
attach them in parallel so they both share the load

Exactly. Split the load between them.

Posted: 1/7/2006 8:51:58 PM EDT

[#12]

Quoted:
Quoted:
Quoted:
Quoted:
use 1 chain, 1 bungee

600 + 600 =1200 capacity, load = 1000, good to go

you said we can't double up the chain OR the bungee

I need more detail. If you just hook them both up it won't work - the chain wil break.

attach them in parallel so they both share the load

But how? The chain won't stretch, so it will just snap while the bungee is limp.

allow the bungee to stretch just far enough that it removes it's stretchability but does not break - then use the chain.

Posted: 1/7/2006 8:52:08 PM EDT

[#13]

Quoted:
Quoted:
attach them in parallel so they both share the load

Exactly. Split the load between them.

See above.

Posted: 1/7/2006 8:52:14 PM EDT

[#14]

Quoted:
Quoted:
Quoted:
use 1 chain, 1 bungee

600 + 600 =1200 capacity, load = 1000, good to go

you said we can't double up the chain OR the bungee

I need more detail. If you just hook them both up it won't work - the chain wil break.

attach them in parallel so they both share the load

It don't work that way either. The chain will have it's stretch and failure moments in a shorter span that the bungee. The bungee will have to be preloaded to about 75% so that the chain wont fail first.

Posted: 1/7/2006 8:53:17 PM EDT

[#15]

oh for crissake. attach the bungie, let it stretch to maximum, just below break point. it carries just under 600 lbs. then attach the chain (edit: in parallel) to carry the remainder, chain wont stretch, it carries its own load, just over 400 lbs.

EDIT: David_g17 beat me to it.

Posted: 1/7/2006 8:53:59 PM EDT

[#16]

Use the bungee and the chain forming a V. They will equalize at 500lbs of direct downward weight each.

Posted: 1/7/2006 8:54:08 PM EDT

[#17]

attach the weight with friggin Gorilla Glue geeze some people dont know anything you can hang an elephant over the grand canyon with that chit

Posted: 1/7/2006 8:54:16 PM EDT

[#18]

cut the weighted object in half.

attach one half to the bungee - and the other to the chain.

Posted: 1/7/2006 8:54:49 PM EDT

[#19]

I say go to the hardware store and buy a bigger chain.

Posted: 1/7/2006 8:55:04 PM EDT

[#20]

Quoted:
oh for crissake. attach the bungie, let it stretch to maximum, just below break point. it carries just under 600 lbs. then attach the chain (edit: in parallel) to carry the remainder, chain wont stretch, it carries its own load, just over 400 lbs.

And if the chain is shorter than the bungee?

I still say you should just set the weight on top of the beam.

Posted: 1/7/2006 8:55:07 PM EDT

[#21]

Quoted:
allow the bungee to stretch just far enough that it removes it's stretchability but does not break - then use the chain.

Yep. I see a few others got it too. I thought it would take longer, but the hive mind is impressive.

Posted: 1/7/2006 8:58:41 PM EDT

[#22]

Quoted:
Quoted:
allow the bungee to stretch just far enough that it removes it's stretchability but does not break - then use the chain.

Yep. I see a few others got it too. I thought it would take longer, but the hive mind is impressive.

oh, i imagine some mechanical engineer is still typing up a 5,000 word post consisting of a formal proof...

Posted: 1/7/2006 8:58:47 PM EDT

[#23]

Quoted:
Use the bungee and the chain forming a V. They will equalize at 500lbs of direct downward weight each.

While this isn't the "engineer" answer, it is in fact simple genius, and would work.

While some of us would be measuring the stress on the bungee, making it say 500 pounds, then connecting the chain (which would only strain slightly) making the total load share say 501 on the bungee and 499 on the chain, Stealth would have already solved the problem.

I don't know Stealth's background or what he does for a living, but this is why farm boys with a BS from a no-name college often beat out engineers who have never touched a wrench with a friggin master's degree from a top school.

Posted: 1/7/2006 8:59:35 PM EDT

[#24]

While you guys scratch your heads over the problem, this practical old farmer is going to either weld the weight to the beam or go and get a heavier chain.

Posted: 1/7/2006 9:00:13 PM EDT

[#25]

Where is this taking place?

Posted: 1/7/2006 9:03:58 PM EDT

[#26]

.

Posted: 1/7/2006 9:05:45 PM EDT

[#27]

Quoted:
Where is this taking place?

what if its the moon or something

where it only weights a faction of what it does on earth

Posted: 1/7/2006 9:06:19 PM EDT

[#28]

Or I could cut the chain in half and have two chains.

Posted: 1/7/2006 9:07:45 PM EDT

[#29]

Quoted:
Quoted:
Where is this taking place?

what if its the moon or something

where it only weights a faction of what it does on earth

Exactly

Posted: 1/7/2006 9:10:09 PM EDT

[#30]

Quoted:
Or I could cut the chain in half and have two chains.

+1. the more i think about it, the more i don't want 1,000 lbs hanging from a "bungee" over my head.

Posted: 1/7/2006 9:11:46 PM EDT

[#31]

Quoted:
Or I could cut the chain in half and have two chains.

You guys ruin everything

Posted: 1/7/2006 9:13:03 PM EDT

[#32]

Quoted:
Quoted:
Use the bungee and the chain forming a V. They will equalize at 500lbs of direct downward weight each.

While this isn't the "engineer" answer, it is in fact simple genius, and would work.

While some of us would be measuring the stress on the bungee, making it say 500 pounds, then connecting the chain (which would only strain slightly) making the total load share say 501 on the bungee and 499 on the chain, Stealth would have already solved the problem.

I don't know Stealth's background or what he does for a living, but this is why farm boys with a BS from a no-name college often beat out engineers who have never touched a wrench with a friggin master's degree from a top school.

I don't believe this works. The vertical components would be 500 each but the tension will be sqrt(500^2 + horizontal component ^2). This depends on the angle, but if you had them at a 45 degree angle, that'd be 500*sqrt(2) ~ 700 lbs force.

Crunch.

Then there's the fact that the bungee will slip, so the V will not be symmetrical and one will bear more weight.

B.A. in math. Any engineers want to check this?

Posted: 1/7/2006 9:13:16 PM EDT

[#33]

Or simply make the chain into an inverted V, attaching the chain at two opposite points of the weight and one point on the beam.

Posted: 1/7/2006 9:13:26 PM EDT

[#34]

Quoted:
Quoted:
Where is this taking place?

what if its the moon or something

where it only weights a faction of what it does on earth

irrelevant. the ratio of strength to weight remains the same based on the problem given (re-read it).

Posted: 1/7/2006 9:13:39 PM EDT

[#35]

Quoted:
Or I could cut the chain in half and have two chains.

Now THAT's the farmer boy solution.

Posted: 1/7/2006 9:15:05 PM EDT

[#36]

Quoted:
Quoted:
Or I could cut the chain in half and have two chains.

Now THAT's the farmer boy solution.

the farmer boys I know would use duct tape.

Posted: 1/7/2006 9:15:33 PM EDT

[#37]

Quoted:
Quoted:
Quoted:
Use the bungee and the chain forming a V. They will equalize at 500lbs of direct downward weight each.

While this isn't the "engineer" answer, it is in fact simple genius, and would work.

While some of us would be measuring the stress on the bungee, making it say 500 pounds, then connecting the chain (which would only strain slightly) making the total load share say 501 on the bungee and 499 on the chain, Stealth would have already solved the problem.

I don't know Stealth's background or what he does for a living, but this is why farm boys with a BS from a no-name college often beat out engineers who have never touched a wrench with a friggin master's degree from a top school.

I don't believe this works. The vertical components would be 500 each but the tension will be sqrt(500^2 + horizontal component ^2). This depends on the angle, but if you had them at a 45 degree angle, that'd be 500*sqrt(2) ~ 700 lbs force.

Crunch.

Then there's the fact that the bungee will slip, so the V will not be symmetrical and one will bear more weight.

B.A. in math. Any engineers want to check this?

It works. It's the same principle by which a composite structure works. You need to look at the strain (stretching, or deformation) applied to each material of differing modulus. It's not just a vector problem with perfect materials.

Edited to add: I thought you were taking about my solution and not Stealth's. I thought Stealth's would work, but I guess not.

Posted: 1/7/2006 9:18:05 PM EDT

[#38]

Quoted:
Or simply make the chain into an inverted V, attaching the chain at two opposite points of the weight and one point on the beam.

Same problem as the V.

Think of it this way. You're hanging from a pole by a rope which is looped over the top and with one end in either hand (like a towel pull-up). When you're hanging straight, the tension on the rope is half your weight. But to pull it into a V, you have to pull apart the ends, exerting more tension on it. The vertical component stays the same but you introduce a horizontal component to pull it into the V.

Posted: 1/7/2006 9:19:29 PM EDT

[#39]

only one way to settle this... beam o' truth!

Posted: 1/7/2006 9:21:47 PM EDT

[#40]

Quoted:
It works. It's the same principle by which a composite structure works. You need to look at the strain (stretching, or deformation) applied to each material of differing modulus. It's not just a vector problem with perfect materials.

Composite structure? It'd work if you hooked them to the same tie point but kept the bungee and chain vertical. As soon as you introduce horizontal separation the tension goes up.

Posted: 1/7/2006 9:21:49 PM EDT

[#41]

Quoted:
only one way to settle this... beam o' truth!

Ok, your job is to find a rubber bungee that can support 600 pounds. If you find one, please post a pic of the beast.

Posted: 1/7/2006 9:22:21 PM EDT

[#42]

Quoted:
Quoted:
It works. It's the same principle by which a composite structure works. You need to look at the strain (stretching, or deformation) applied to each material of differing modulus. It's not just a vector problem with perfect materials.

Composite structure? It'd work if you hooked them to the same tie point but kept the bungee and chain vertical. As soon as you introduce horizontal separation the tension goes up.

You are correct. I edited my original reply.

Posted: 1/7/2006 9:27:38 PM EDT

[#43]

Quoted:

But how? The chain won't stretch, so it will just snap while the bungee is limp.

Don't attach the chain and bungee at the same point of the beam. Allow for bungee stretch, attach so that neither is under tension greater than breaking point.

Jim

Posted: 1/7/2006 9:29:35 PM EDT

[#44]

Can you not just attach the bungee and the chain together into a loop around the beam and attach the weight to the middle of the chain or bungee?

That way half of the bungee and half of the chain are supporting 500 pounds each.

Posted: 1/7/2006 9:31:48 PM EDT

[#45]

Quoted:
Quoted:
Quoted:
Where is this taking place?

what if its the moon or something

where it only weights a faction of what it does on earth

irrelevant. the ratio of strength to weight remains the same based on the problem given (re-read it).

I guess your right. damn Imperial system of measurements.

Posted: 1/7/2006 9:34:41 PM EDT

[#46]

No one listens. Just put the damned weight on top of the beam and be done with it.

Otherwise, I am going to the trunk of my car and getting a tow strap and a come-along.

Posted: 1/7/2006 9:35:21 PM EDT

[#47]

Quoted:
Can you not just attach the bungee and the chain together into a loop around the beam and attach the weight to the middle of the chain or bungee?

That way half of the bungee and half of the chain are supporting 500 pounds each.

That's the solution being bandied about. Descriptions have been a bit vague but that's it.

Posted: 1/7/2006 9:36:25 PM EDT

[#48]

Quoted:
No one listens. Just put the damned weight on top of the beam and be done with it.

Otherwise, I am going to the trunk of my car and getting a tow strap and a come-along.

You have to hang it from the beam not put it on top.