# The physics behind Jenga help

1. Dec 12, 2005

### rayden54

I just found out that I'm supposed to explain the physics concepts behind the game Jenga. All I've been able to find thus far is that it has something to do with inertia and "center of mass"

I've read that it was best to quickly flick the block away as opposed to gently pulling it out which makes me think it's like the tablecloth trick magicians are so fond of. As for the rest I have no clue.

Oh Yeah. I'm supposed to have this by tomorrow. Any help's appriciated.

2. Dec 12, 2005

### PhY_InTelLecT

Erm.. Not really exactly the same as what the magicians are doing..
If you were to look really carefully, magician tablecloth tricks uses glasswares that have little area of contact with the tablecloth or that the plates have large bases that increases its stability when they are made to stand on the table. And the cloth is mostly smooth..So when u actually pull the cloth, the glasswares simply slip off the cloth onto the table.. Partly also due to inertia but it oso requires quite alot of other factors such as friction..

However for jenga blocks, its not that simple.. The area of contact between the blocks are large and the surface of the blocks are rough. U can try pulling out the blocks fast but trust me.. Most of the time..it wont work. In this case, the frictional forces between the blocks will be larger than that of inertia so it wont work that well.. Well unless the surface of your jenga blocks are frictionless..

Perphaps, getting the blocks out from the top is still one of the most reliable methods. The concept behind it..is stability. An object is stable when u have a large base and a short height.. Since the base is the same wherever u take the block from, its wise to take it from the allowed-top level because taking the level u take the block from as the base, u have a shorter height and thus maintaining the stability of the tower..

Another method is that u take the middle blocks instead of the blocks at the sides.. This is to spread out the weight of the blocks above the level u are taking the block from. U will tend to maintain the stability of the tower instead of lessening it by reducing the area of the base.

Any doubt can be clarified. I'm ok with it. I'm oso not sure if my explanation is clear enuff or not..Haha:)

Last edited: Dec 12, 2005
3. Dec 12, 2005

### Homer Simpson

I'm afraid I disagree

I don't see this strategy from the official source:

"jenga, jenga, j-j-j-jenga. you take a block from the bottom and you put it on top, you take a block from the middle and you put it on top. That's how you build the tower; you just don't stop. You keep building that tower putting blocks on top. It teeters and it totters, but you don't give up; it weebles and it wobbles, but you build it on up. You take a block from the bottom and you put it on top, you take a block from the middle and you put it on top, till someone knocks it over, and that's when you stop, but you can start all over putting blocks on top."

http://www.inthe80s.com/tvcommercials/j.shtml

4. Dec 12, 2005

### rayden54

jenga

I was looking more for the physics behind the tower itself. Why it doesn't fall down and why it eventually does. Or at least I think I am. I still don't know what to say. Inertia is the thing I've found mentioned most. That's it, just the word. Nothing more.

5. Dec 13, 2005

### PhY_InTelLecT

Yes. What i meant was you do not take the blocks at the extreme bottom lvl..U take the blocks at a preferably higher lvl..

6. Dec 13, 2005

### Wishbone

"U can try pulling out the blocks fast but trust me.. Most of the time..it wont work. In this case, the frictional forces between the blocks will be larger than that of inertia so it wont work that well.."

Why would the fricitional force be larger the faster you pull? The force of friction is independent of the speed of the object. What would cause the blocks to fall down if someone pulled really fast would be the fact that you loose control, so instead of moving in only 1 dimension (outward towards the puller), it would be wobbling all over the place in 3 dimensions, and thus hitting other blocks n stuff... However, you do need to pull it fast enough that the you get past static friction, if not, you will pull the other blocks with it. And the blocks on the bottom have a larger frictional force on them, because they have a larger normal force due to the blocks that are sitting on top of them.

Last edited: Dec 13, 2005
7. Dec 15, 2005

### mybsaccownt

"The area of contact between the blocks are large and the surface of the blocks are rough."

"magician tablecloth tricks uses glasswares that have little area of contact with the tablecloth"

I may be wrong...

but I don't believe area has anything to do with friction.

If you have a smaller contact area, then the weight of the object is pushing against the surface imperfections much harder. Fewer ridges to get stuck on, but it is getting stuck harder.

With a large contact area, the weight of the object is spread out across many surface imperfections and so they don't resist very much, but since there are so many the friction is high anyway.

The formulas for calculating static and kinetic friction reinforce my belief that area has nothing to do with it.

8. Dec 15, 2005

### PhY_InTelLecT

The difference doesn't lies with the speed of the block, but the force on the object. When You pull the block out fast, the force on the block is much larger.

I don't really noe if these examples are correct..

Let me state an example.. Let's say i've got an adjustable giant clamp.. in case 1, i made a real aeroplane move in between the clamps and in case 2, i made a model aeroplane of the same size move with a speed of just 100km/h move between the clamp. After both planes passes through the clamp, we will see scratch marks on both bodies, but if we were to see both markings carefully, the marks on the model plane should be less severe..

And also, another example will be that imagine you are in a building when it actually collapses.. You were thrown down by a heavy pillar and are not able to get out. Rescuers come but yet was also unable to lift up the pillar.. They have got only one opportunity, to pull you out.. So.. would you want them to wriggle you out slowly or to drag you out very quickly?

What i'm trying to show is that the difference in speed or force actually results in difference in friction..? I really don't know about this but from the above two experiment, we can see that, the drag, the pulling back on the block goes harder when the block is pulled out fast..:)

Last edited: Dec 15, 2005
9. Dec 16, 2005

### PhY_InTelLecT

The weight of the object doesn't spread out, weight of the object will alwas act at the center of gravity of the block.. What will change with area is the pressure acting on the surface of the blocks.

10. Dec 16, 2005

### mezarashi

It *is* a game afterall. It would be easier to use physics to just explain why it falls or why it hasn't (although that is already pretty much overkill imo). Removing the 'removable' blocks is quite the obvious rule. If you remove blocks that contribute strongly to the structural integrity, then it goes. I would still say that winning the game would have much to do with composure of your movements than anything else.

11. Dec 16, 2005

### mybsaccownt

"The weight of the object doesn't spread out"

maybe my terminology isn't perfect, but you know what i mean

with a larger contact area, each ridge will stick less, but there are more of them

with a smaller contact area, the object will be pushing into a small amount of ridges, so each ridge will stick more

can you explain how you would factor surface area into an equation for force of friction...for example Fk = muk*N

?

I honestly don't know why you are saying that friction depends on surface area.

12. Dec 16, 2005

### mezarashi

I'm going to have to edit, that in the point you are making, you are right. Given a mass, the friction does not depend on the contact area of the mass.

Last edited: Dec 16, 2005