.Area & Uncertainty: Circle Radius 3.9 x 10^4 cm

[sadeysnow]

Homework Statement

What is the area, and its approximate uncertainty, of a circle of radius 3.9 104 cm? (Hint: Assume an uncertainty in the radius of 0.1 104 cm. Use the correct number of significant figures in your answers.)

Homework Equations

I really don't know

The Attempt at a Solution

There's been a lot and scattered on paper...so I don't have one

 

[rootX]

what's the area of a circle?

 

[sadeysnow]

3.14r^2

and I did that and it wasn't the right answer...

 

[rootX]

is that 14 in "14r^2"

it should be pi.r^2

 

[Dick]

I can't read either one of you. What is a radius "3.9 104 cm" and why should "3.14r^2" be taken to have "14" in it? Is my browser rendering things wrong? Oh, yeah and since when are we using '.' for multiplication as in "pi.r^2"? You may be confusing each other.

 

[rootX]

oops sorry..
I thought 3 is a number (like those question numbers)..

edit: I need to get a sleep :zzz:

 

[mgb_phys]

Error propoagation is to do with how much effect errors have in the calculation.
Simple example speed, if I know the distance to 10% and the time to 10% then the biggest error I could have in speed is 20% ( try some numbers if you aren't sure why)
Whre two numbers are multiplied the percentage errors add.

In area if I know the length to 10% and the width to 10% the worst error I could have is 20%.