# Thread: Deep ocean waves, help! ode system

1. ## Deep ocean waves, help! ode system

Please I need help with this one question 3 (b) (see the att).
Thanks

2. Originally Posted by simfonija
Please I need help with this one question 3 (b) (see the att).
Thanks
Ah! So you need numbers for a, b, and c.

This is a dimensional analysis problem. Remember that the units on both sides of an equation must be the same. So all we need to do is count the number of units on each side and equate them.

$\displaystyle u = k \lambda^a g^b \rho^c$

Where
$\displaystyle u \to L/T$
$\displaystyle k \to 1$ (Unitless)
$\displaystyle \lambda \to L$
$\displaystyle g \to L/T^2$
$\displaystyle \rho \to M/L^3$
where L is distance, T is time, and M is mass.

This problem is strange. Technically I would say it can't be done, but I suppose there is an answer, however misleading the question is. Let me explain:

Matching the units on both sides we get that:
L: $\displaystyle 1 = a + b - 3c$
T: $\displaystyle -1 = -2b$
M: $\displaystyle 0 = c$

The solution is:
c = 0
b = 1/2
a = 1/2

So the relationship between the variables is:
$\displaystyle u = k \lambda^{1/2} g^{1/2} = k \sqrt{\lambda g}$

My problem is that the question specifically stated that the wave speed depends on the density, which it doesn't according to the answer. I don't like questions that do this!

-Dan

3. Thank you sooo much