# Percentage Error Question

1. May 21, 2010

### Procrastinate

There is a 0.8% error in the surface area of a sphere. Find the resulting % error in the volume of the sphere.

I am having trouble solving this and I was wondering if someone could give me a hint?

I currently have reached a dead end with:

2 delta r
-------- = %SA
r

However, I don't have any known r values to work with...

2. May 21, 2010

### Cyosis

$$S= 4 \pi r^2 , \; V=\frac{4}{3} \pi r^2$$. Solve r in terms of S and plug it into the formula for the volume.

3. May 21, 2010

### Staff: Mentor

You will also need differentials dr, dS, and dV, since you want dS/S. Recall that dS = dS/dr * dr.

4. May 22, 2010

### Procrastinate

My final answer was 2.4%; was that right?

5. May 22, 2010

### HallsofIvy

There is an engineer's rule of thumb, derived from the differential of Mark44, that "if measurements add, their errors add, if measurements multiply, their percentage errors add. Since here, you have a percentage error of .008 in r, and $V= (4/3)\pi r^3$ involves multiplying r three times, yes, the percentage error in V is 3(.008)= 0.024 or 2.4%.

Last edited by a moderator: May 22, 2010
6. May 22, 2010

### Redbelly98

Staff Emeritus
Recall that 0.8% is the error in S, not r. So 2.4% is incorrect.

Procrastinate, try finding the % error in r first, what do you get for that?