# Homework Help: Leaning Tower of Pisa

1. Aug 29, 2012

### unscientific

1. The problem statement, all variables and given/known data

The problem is as attached.

3. The attempt at a solution

I tried doing double integration of r from 0 to 9.8 and θ from 0 to 2pi. But here im assuming all forces from the soil act along the tower (no friction) and managed to solve C1.

But im not sure how to solve C2?

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Last edited: Aug 29, 2012
2. Aug 29, 2012

### Simon Bridge

It helps if you show us how you have been trying.
Can you not find the max and min locations without knowing C2?

3. Aug 29, 2012

### voko

What are the equilibrium conditions?

4. Aug 29, 2012

### unscientific

1) Vector Sum of forces about any direction = 0

2)no resultant moment about any point

5. Aug 29, 2012

### voko

That should give you two somewhat different integrals, hence you should be able to determine the two constants from them.

6. Aug 29, 2012

### unscientific

Yes, but the cos θ becomes sin θ and then from 0 to 2∏ it cancels out to be zero..

7. Aug 29, 2012

### unscientific

Here's what I got:

Let infinitesimal moment be dM.

∫ dM = ∫ r dF = ∫ rP dA = ∫∫ r2P dr dθ...

Then when ∫cosθ becomes sin θ the limits going from 0 to 2∏ it becomes 0, so C2 cannot be found..

8. Aug 29, 2012

### voko

Observe how the second term of pressure behaves. Its moment about O must be non-zero.

9. Aug 29, 2012

### abel_ghita

what about..
you know that Pmax is right underneeth the center of gravity which is r=2.67 and θ=0
and also Pmax/min is the second derivative of function p..which i suppose is
$\partial$^2p/$\partial$r^2 * dr+ $\partial$^p/$\partial$θ^2 * dθ
and your only variable is C2

10. Aug 29, 2012

### unscientific

And with that, you only get r = 0... when u equate each = 0 ..

11. Aug 29, 2012

### voko

Like I said, this cannot be zero. The specific mistake you are making is that of ignoring the vector nature of r and F. What you denote as dM is a vector product of r and dF. Work it out.

12. Aug 29, 2012

### unscientific

yes, dM = r x F

but I assumed all F acts upwards, so vectors r and F are perpendicular so it simply becomes r*F...

13. Aug 30, 2012

### voko

Draw the resultant moment in a few different places and you will see different directions. You can't add them together as scalars.

14. Aug 30, 2012

### unscientific

That is true...I realized that only for the same angle they are pointing in the same direction

So how do we find the net moment? It's like trying to sum up all the vectors in different directions..

Last edited: Aug 30, 2012
15. Aug 30, 2012

### voko

Expand the vectors involved in terms of the coordinate unit vectors i, j, k. Do the vector product. What do you get?

16. Aug 30, 2012

### unscientific

d$\overline{M}$
= $\overline{r}$ x d$\overline{F}$
= ($\overline{r}$ x $\overline{p}$) dA
= (r cosθ $\widehat{i}$ + r sinθ $\widehat{j}$) x (C1 + C2 r0.625 cosθ)$\widehat{k}$
= -( C1 cosθ + C2 r1.625 cos2θ) $\widehat{j}$ + (C1 r sinθ + C2 r1.625 sinθcosθ ) $\widehat{i}$

Along j

$\int$ d$\overline{M}$
= - $\int$$^{9.8}_{0}$ dr $\int$$^{2∏}_{0}$ dθ [C1 r cosθ + C2 r1.625 cos2θ]
= -478.9 C2

Along j
Everything cancels out to be zero!

Last edited: Aug 30, 2012
17. Aug 30, 2012

### voko

Do the vector product :)

18. Aug 30, 2012

### voko

I am sure you can integrate that now.

19. Aug 30, 2012

### unscientific

Already did! (Page 1 please) Sorry for the long edit....was trying to figure out how latex works

20. Aug 30, 2012

### voko

So, have you solved the entire problem?

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