# Homework Help: Calculating the speed of P-waves.

1. Sep 16, 2005

### stepha

Any one any good at calculating equations?????

Using the equation below,

$$V_{p}=\frac{\sqrt{\frac{x+4}{3y}}}{p}$$

P = 3.4 x10³ Kg m¯³

X = 1.6 x1011 N m¯²

Y = 6.4 x1010 N m¯²

I need to calculate the value of Vp and I need to give my answer in Km per second.

Does anyone have any advice on how to do this???

Last edited: Sep 16, 2005
2. Sep 16, 2005

### Staff: Mentor

Where did you get that equation? What are the units of the 3 and the 4 under the square root? (The 4 must have the same units as x, but what about the 3?)

3. Sep 19, 2005

### stepha

more like this

it should really read like below, but couldn't get the formula right to put the equation in prop.

x+4/3y

this should be on the top line then its divided by the p.

any idea's please any help will be usefull, cheers

4. Sep 19, 2005

### HallsofIvy

You do understand that this is just arithmetic don't you? But it's pretty complicated arithmetic!

x is 1.6 x1011 so x+ 4= 1.6 x1011+ 4.

What??? That's like 160000000000+ 4= 1600000000004!

Since you only have 2 significant figures, that's 1.6 x1011 again!

In other words, 4 is so small compared with 1.6 x1011 that adding it really doesn't make any difference. (If you had 1.6 x1011 dollars, 4 dollars wouldn't matter much!)

Since we can ignore the "4", (x+ 4)/(3y) is just x/(3y).
y= 6.4 x1010 so 3y= 18.4 x1010 and
x/(3y)= 1.6 x1011/18.4 x1010= (1.6/18.4) x1011-10. Can you do that?

Of course, taking the square root divides the power of 10 by 2 (I recommend you rewrite the number, if necessary, to make that power even!).

5. Sep 21, 2005

### stepha

Thank you for your help.

Any ideas how to calculate the Meters squared into Km per second?

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook