# Calculating the speed of P-waves.

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:

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Doc Al
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?)

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 HallsofIvy
Homework Helper
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!).