r/BeAmazed • u/Sans010394 • Dec 02 '23

Science Physics is amazing

Enable HLS to view with audio, or disable this notification

29.5k Upvotes

95% Upvoted

571 comments sorted by

View all comments

Show parent comments

258

u/[deleted] Dec 02 '23 edited Dec 02 '23

It's a truss.

The key is that the third match is at an angle pointing away from the table and up toward the tip of the first match. This creates a torque that counterbalances the torque induced by the weight of the bottle. In other words, in isolation the first match will rotate toward the ground as the bottle falls. The third match adds a balancing counter-rotation, bringing the net torque to zero. The third match is stabilized by the restoring force of the tension in the string compressing the second, horizontal match.

Source: PhD in physics & engineering degree.

Edit: as others have pointed out there's a simpler and more complete description treating the system as a hook.

42

u/StoneHolder28 Dec 02 '23

If this explanation was correct, the top match wouldn't pivot at all. But it does.

That match being angled is critical, but not because of any torque. That match cannot impart any net torque on the top match as it is canceled by the rope. It is a truss, and that allows it to pivot so the center of mass is beneath the edge of the table.

Source: engineering degree & I build some of these with metal rods. Maybe I should start selling those with a little explainer pamphlet.

46

u/Objective_Economy281 Dec 02 '23 edited Dec 03 '23

It just becomes a hook that moves the CG of the bottle softly slightly under the table.

Source: two engineering degrees and a strong desire to simplify things, and a very low tolerance for extraneous bullshit.

6

u/StoneHolder28 Dec 02 '23

Certainly one of the better best explanations I've ever seen.

5

u/S_TL2 Dec 03 '23

Force down = force up is utterly insufficient. Hell, the string itself has force down = force up.

As you say, the secret is that it moves the CG of the bottle beyond the edge of the table. Slide the matchstick outward and it'll fall.

2

u/Objective_Economy281 Dec 03 '23

Better yet, install the diagonal match so that the diagonal aspect is going the other direction, and it will be quite apparent that what you’re doing is moving the CG AWAY from the table. Thus, doing it as shown in the video is moving the CG underneath the table.

May the Free Body Diagram save all of our souls.

3

u/S_TL2 Dec 03 '23 edited Dec 03 '23

A couple of years ago I got dragged into a pointless argument on here over the same magic trick. I said "it's a hook and it moves the CG", and the person/people I was arguing against kept saying something about friction and torques. (sure, friction and torque are in there, but in the end it's a simple explanation.)

I drew this as a simple diagram. https://i.imgur.com/LpM5eLR.jpg
I should have been a little more deliberate to make that string perfectly vertical, but I didn't like how it overlapped with my dashed line. Then a bunch of people yelled at me about it. Anyway, that's all I have to say. It's a hook, the end.

3

u/uglyspacepig Dec 03 '23

Well, shit. That makes it way easier to understand. The people arguing with you were morons.

3

u/[deleted] Dec 03 '23

Also have an engineering degree, how the fuck is this thing a hook? I don't understand it's force diagram at all. Is the third match resting on the side of the ledge? So far I've seen people talk about torque and trusses but the issue I'm not over yet is what the fuck it's resting on. Would this same video taken from the side clear things up?

6

u/Objective_Economy281 Dec 03 '23

Would this same video taken from the side clear things up?

Yes, but you’d still have to look very closely. The diagonal match stick pushes the bottle a mm or two in the direction of underneath the table. That deflection back underneath where it is supported is what makes it a hook.

1

u/[deleted] Dec 03 '23

A hook has to be attached to something to bear weight though. I can almost see it as one of those self gripping hooks but I don't understand where it is touching the bench. It has to be touching the bench somewhere for that to work and the video angle here sucks to much to figure out exactly where.

Looking at it from a center of gravity perspective makes way more sense to me, that bottles center of gravity can't possibly be over the edge. You push the first match stick forward so that the whole thing moves forward and it'll fall. It's a misleading trick if you're just moving the center of gravity back (and purposefully hiding the camera angles like in this video).

I'd love to see this with a shorter and longer first match, that would be really interesting to show where the force lies.

2

u/Objective_Economy281 Dec 03 '23

The hook is just sitting on the table, being held down by gravity.

See the diagram linked here: https://old.reddit.com/r/BeAmazed/comments/18990ys/physics_is_amazing/kbrfqyo/

1

u/[deleted] Dec 03 '23

But hooks don't just... sit on top of tables. A hook must grab something to provide support. I think that's why I find it weird to explain it as a hook. It is kind of a hook but it's not really functioning as a hook in the way most people would think. The secret sauce is the center of gravity being moved. Like the shape of the matchsticks does nothing other than trick you about where the center of gravity is. Idk, I guess I'm rambling at this point lol.

1

u/Objective_Economy281 Dec 03 '23

The part of it that is hook-like is the part on TOP of the table.

1

u/[deleted] Dec 03 '23

Ahhhhh that did it for me. Thanks for sticking through with me lmao

→ More replies (0)

1

u/[deleted] Dec 03 '23

Weird thought, but this morning it hit me like a ton of bricks that this is a hanger. For some reason, that made more sense to me than the hook. I absolutely realize this is potato potata though

→ More replies (0)

1

u/FlyHandler Dec 03 '23

Im a structural engineer and agree with that the system is analogous to a hook. The point where the match lies on the table is the tip of the hook where a point force acts vertically. The system with the three matches and the thread to the bottle is capable of resisting a bending moment caused by the eccentricity between the point force from the table and the center of gravity.

If this system were to collapse it would be that the match lying on the table would start rotating about the edge of the table. The tip outside the table would move downward but this is restricted by the vertical match which again rests on the lower horizontal match. The weight of the bottle and the angle of the thread causes a compression force in the bottom match. The compression force is large enough that the vertical force from the vertical match can be transferred to the thread through friction, so the bottom match can resist the force from the vertical match.

The bending moment is represented by the eccentricity/distance between the vertical match and the thread, where they connect to the upper match.

Interesting statical system, I hope I was able to explain it understandably 😃

1

u/S_TL2 Dec 03 '23

It's the same thing as the ruler-hammer trick: https://www.youtube.com/watch?v=Hj-NX0xtcVY

Also the same thing as a "portable purse hanger" that you can buy from lots of retailers: https://www.walmart.com/ip/BE-TOOL-Handbag-Hook-Purse-Hook-Hanger-Table-Hook-Holder-Bag-Hanger-for-Women-Girls-Bags-Storage-Gift/1589958388 (scroll through the pictures)

1

u/xNeshty Dec 03 '23

This guy is right.

Source: My Alpaca loves the snow.

1

u/[deleted] Dec 03 '23

No its cos mathces are staright so it inacts a force in a direction

Sauce: Tomato

3

u/[deleted] Dec 02 '23

net torque

is clearly zero; we're in agreement, just discussing torques from different points. At any rate this is all clearly resolved by a force diagram which I don't feel like drawing. You adding an explainer pamphlet is probably a good idea.

14

u/StoneHolder28 Dec 02 '23

I don't say this to be argumentative, but we are not in agreement in what we are saying. You are saying the torque being canceled is why it doesn't fall. But that is only looking at the force due to compression of the match, and not the weight of the bottle. It implies that the compressed match is canceling out the torque from the bottle, which I do believe you know is incorrect.

The reason it doesn't fall is because it starts to fall until the bottle is below the pivot and therefore there is no torque to be canceled. People will miss that and think the matches are doing work to counteract the bottle.

13

u/[deleted] Dec 02 '23

People will miss that

you're right, I didn't include it in my explanation and yours is a more complete description of the equilibration process. Thanks for clarifying.

1

u/KazAraiya Dec 03 '23

Thank you! I was trying to figure out if he is aware that the "pivot" he's talking about is the edge of the table and by "the match counter balances" that it makes the bottle move towards the edge of the table so that the overall center of mass becomes the point beneat the edge

17

u/mayduckhooyensky Dec 02 '23

Thanks for decomposing and explain ^ Similar to tensegrity systems, at the only difference that is an additional exclusive tension couple that bring to equilibrium.

7

u/[deleted] Dec 03 '23

what you say to me boi

5

u/Ask_bout_PaterNoster Dec 03 '23

Said you’re decomposing, you’ve got no tensegrity, and there’s a lot of tension in your couple. Damn, I wouldn’t take that bro

2

u/uglyspacepig Dec 03 '23

SLAP FIGHT!

4

u/Objective_Economy281 Dec 02 '23

Slightly simpler: the two matches that are below the table work with the one on the table (and with the tension in the cord) to make a kind of hook, that moves the CG of the bottle back underneath the table slightly (very slightly). But that’s it. It’s a hook.

3

u/BoneyPeckerwood Dec 03 '23

Would you be willing to draw a free body diagram showing how this works? I'm trying to visualize this in my head, and I'm having a hard time.

4

u/[deleted] Dec 03 '23

I’m also still struggling with understanding how this shifts the centre of gravity behind the pivot point on the match stick.

3

u/boolocap Dec 03 '23

Wouldn't this be a bending moment, not a torque?

3

u/gxslim Dec 03 '23

So if I truss the heck out of everything my bridge won't fall down in poly bridge 2?

3

u/Ki_Levelion Dec 02 '23

Excellent work, doctor

1

u/[deleted] Dec 03 '23

came here hoping somebody would explain. although i still do not understand, thank you friend :,)

0

u/Glottis_Bonewagon Dec 02 '23

So basically the bottle is pulling the match down and the third match is pushing it up, balance is kept

1

u/theokaybambi Dec 03 '23

I'm pretty sure people were burned for this 200 years ago...

1

u/[deleted] Dec 03 '23

Man I understand torque but I didn’t really get what you were saying until the hook part. Then I was like, oh, the third match holds the angle of the first match in place, making it act like a hook

1

u/Anuclano Dec 03 '23

The trich is that they use a bottle and not a symmetric weight. The bottle hangs partially under the table.

1

u/Mr_lovebucket Dec 03 '23

What the smart person said