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Is this real?
3

Is this real?

RE: Is this real?

Certainly not in the way they are trying to make you think it is. If anything the rope could be caught on the bench and supported at that point, but I claim trickery!

RE: Is this real?

Yes it’s real.. the distance between the top of the match and the string is the “backspan”. You can see how it would work with a free body diagram. I think if the vertical match was not inclined then it collapses.

RE: Is this real?

Right at the very start thet is something going on with the match being slotted into position and a dark bit at the end. The match is being held at the end for sure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Is this real?

It looks real and entirely plausible to me. For the top match to tilt down it must push the vertical match down and the cross match must be pushed under the table. This cross match is straight above the center of mass. So we have a hook, made of matches and string, that is held together by friction. It is not magic or trickery.

RE: Is this real?

I agree with MIStructE_IRE.

The centre of gravity of the load is behind the top edge of the bench. The cruciform matches provide a mechanism to apply a moment to the top horizontal match. Why wouldn't it work?

Edit: As can be clearly seen in 3DDaves' link.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Is this real?

3D's case is different that the OP's. The former relying on the tilting vertical to create leverage, while the latter is not. However, by place the string tightly against the bench might have that effect too. I think it has to do with friction.

RE: Is this real?

I think it could be the camera angle that makes them look different, also 3D's is probably more exaggerated. I was also on team friction to start with, but I am being swayed.

RE: Is this real?

Quote (retired13)

3D's case is different that the OP's. The former relying on the tilting vertical to create leverage, while the latter is not. However, by place the string tightly against the bench might have that effect too.

It's more obvious in 3DDave's link, but the "vertical" match in the original link clearly is inclined. It seems to me they both work by exactly the same mechanism.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

RE: Is this real?

Yeah, you all could be right, as my eye sight wasn't that good any more. It is a well designed trick.

RE: Is this real?

Quote (Enhineyero (Structural)..Found this video on the internet showing a cantilevered system without a counterweight. I don't think it is real!)


If you know this guy, pls ask him to repeat the same experiment with polished matches and normal thin rope on glass top bench... He is using rubber thread which has high friction with match surface , the diagonal match somehow pushing the C.G. of suspended wt thru bench and the friction developing between match and bench surface, rubber thread and horizontal match justifying that C.G. does not have ecc. to OT the horizontal match..

RE: Is this real?

Agreed, there’s friction and all sorts happening here. However that’s all legitimate as far as statics goes. There’s nothing here to suggest it isn’t real though.

RE: Is this real?

Would anybody agree that the below is accurate? Apologies for deleting posts for some reason I'm not being allowed to add images to an edited post, so I've had to make new ones.

“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”

RE: Is this real?


MegaStructures ,


I have copied your sketch and make some correction with paint -brush ..here is my opinion..

RE: Is this real?

LittleInch in your example there is an OSHA compliant spotter resisting the moment applied to the board via the ladders. Very sound physics principles applied by these experts!

“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”

RE: Is this real?

All through truss action and shear friction. I admire the guts of these guys, especially the one on top, been so close to the wall standing on a steep ladder and working.

RE: Is this real?

(OP)
Now that I look at it closely. The camera angle is a bit deceiving...the couple working to stabilise the stick works for this particular situation. Now our next challenge...convincing architects that hanging a house on the edge of a cliff using this technique does not work!

RE: Is this real?

MegaStructures has it correct; once the CG shifts to inside the edge of the table, the system is stable. In the first video, you can see in the last few seconds that the left cord is bent inwards under the board, so the CG has shifted to just under the edge of the board. Note also, that the bottom of original match is ever so slightly lifted from the board; the camera angle deceives, but you can see that the shadow of the matchstick is slight taller near its bottom.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Is this real?

Well, I can't find it anywhere but I am sure this was discussed in the past as the Devil's Hook or some such...

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

RE: Is this real?

There is no need for the horizontal friction at the top for this to be stable.

RE: Is this real?

The friction is along the string on contact surface, the match provides pivot point.

RE: Is this real?

Friction is absolutely not necessary; the Devil's Hook cited above uses no friction. If you carefully look at the belt, its profile isn't horrifically different than that of the cord in the video.

Likewise, Megastucture's figures require zero friction; stability is guaranteed by moving the CG inboard of the pivot point on the edge.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Is this real?

Quote:

Friction is absolutely not necessary;

I wouldn't be so sure. Who can write equilibrium equations for this system, say the hanging weight >= weight of string?



RE: Is this real?

That's not the same mechanism @retired13. The compression strut is required to create a force couple. Also, the load under the table is resisted vertically by contact between the match and table. In your figure you provide no vertical, or lateral, restraint, which of course creates a system that cannot be in equilibrium.

“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”

RE: Is this real?

“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”

RE: Is this real?

Mega,

Very good. How about this now?



RE: Is this real?

So what's supporting the pulley and the pin? This is a completely different mechanism than what is being discussed.

RE: Is this real?

Assume the pulley and pin have out of page supports that can take vertical load. The concept is the same.

RE: Is this real?

The concept is not remotely the same!

In the video, the match at the top is taking the load through shear. Your sketch has a fabric string that will not be able to take shear.

RE: Is this real?

You didn't think hard enough :)

RE: Is this real?

Friction and compression are all that hold joints at matches and string together. The free body diagrams are simple.

RE: Is this real?

@retired13
Your drawing bears zero resemblance to the problem described, since there is no W2 anywhere in the problem, other than the weight that held the match in place, initially. By your drawing, removal of the weight would immediately result in everything falling, and there cannot possibly enough friction to keep the string (match) from sliding off the table. You are solving some other problem of your own creation.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Is this real?

IR,

See Composite' response. I am amused that a chemical engineer knows structural tricks.

RE: Is this real?

I can't believe this thread is still going. Isn't this something we played around with as kids? Alternatively if you are still confused it isn't too hard to do it yourself.

RE: Is this real?

(OP)

Quote (human909)

I can't believe this thread is still going. Isn't this something we played around with as kids? Alternatively if you are still confused it isn't too hard to do it yourself.

Shows why many of us chose engineering. Or did it choose us?

RE: Is this real?

It isn't difficult to prove that the problem is statically stable.. if my calculations are right, zero (or nearly close to zero) horizontal reaction and overturning moment is produced at the support.

RE: Is this real?



RE: Is this real?

It's stable. It's a devils hook.

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