1. Dec 25, 2007

### Dokugogagoji

Hi! I'm Simone, I come from Italy and I'm a newbie..glad to meet you...and ehm...I have already a question!

My question is related about the example of human pyramid.

There are 5 people, one at top of the other..like:
o1
o2
o3
o4
o5

o5 push o4 up...o4 push o3 up and so on..
the perceiped force on the shoulders of o5 is only o4's weight or the
sum of o1,o2,o3,o4 ones?
if it is the last response, so must be o5 the strongest one and can
they sit without difference of force on o5? thanks!

2. Dec 25, 2007

### mathman

Weight on o5 is total of the others. I don't understand your second question, other than presuming o5 is probably the strongest, but not necessarily.

3. Dec 25, 2007

### Dokugogagoji

Because I guessed as following:
http://www.pctunerup.com/up/results/_200712/20071225231005_Untitled.gif

(the image is ugly, i know )

and |F21|=|P1| |F32|=|P2| and so on..

finally, the total weight on o5 should be (and the module of F54) P4 (and in the floor P5), shouldn't it?

instead, on sitting people..they don't make any "pulling action" so |F21|=0 |F32|=0 and so on... so, the total wieght on o5 (and module of |F54|)should be P1+P2+P3+P4 (and in the floor P1+P2+P3+P4+P5), shouldn't it? thanks for reply

4. Dec 25, 2007

### dst

Yes, O5 must be the strongest because the percieved weight will be the sum of all those above.

Otherwise, you could have O5 carrying a kitten which is carrying a shipping crate. I like kittens. Preferably retaining all three of their dimensions.

5. Dec 26, 2007

### belliott4488

This is the part that I don't understand; can you try asking it again? What do you mean by "on sitting people"? Why do you say that any of these forces equal zero?

6. Dec 26, 2007

### Valayar

This seems to be a problem related to Newton's Third Law.

You cannot apply this law the way you like it. Of course you can add vectors the way you want (using vector field's rules), but forces need an application point (I'm not sure it's the right expression, English is my second langage)

This is similar to the following problem : if you're pushing a box to make it slide on the floor, Newton's Third Law tells you that there will be another force pushing in the opposite direction with an equal intensity. Then, how can the box move ?

To answer this question, you need to know where the forces are being applied.

As a matter of fact, the way you interpret your problem could lead to the conclusion that all objects should float in the air.

7. Dec 26, 2007

### mda

As stated above, o5's legs and/or arms will have to exert a force equal to the weight of five people. I've just had a look a proper pyramid:

Code (Text):

o
o o
o o o

And it turns out that for a few layers the force exerted by those at the base edges tends towards 2, and the base centre exerts a bit less than 'n', where 'n' is the number of layers. So the strongest people should go in the middle, and not too many layers.

Last edited: Dec 26, 2007