# Measuring the mass and linear dimensions of the block

1. Aug 15, 2010

### jinx007

The density of the material of a rectangular block is determined by measuring the mass and
linear dimensions of the block. The table shows the results obtained, together with their
uncertainties.

mass = (25.0 ± 0.1)g
length = (5.00 ± 0.01) cm
breadth = (2.00 ± 0.01) cm
height = (1.00 ± 0.01) cm

The density is calculated to be 2.50gcm–3
.

What is the uncertainty in this result?
A ± 0.01gcm–3
B ± 0.02 gcm–3
C ± 0.05 gcm–3
D ± 0.13gcm–3

According to the book the answer is C and i am having D as answer..

my work

(o.01) x 3 = 0.3

0.3 + 0.01 = 0.13

i cannot figure out where is the mistake help

2. Aug 15, 2010

### arkofnoah

you should add up the fractional uncertainties not the absolute uncertainties.

3. Aug 15, 2010

### jinx007

aww fractional error...but the question deal with uncertainty..can you start it for be i will get a better idea

4. Aug 15, 2010

### arkofnoah

?? I don't get you.

5. Aug 15, 2010

### jinx007

the question is about uncertainty..why are you working with fractional error...please start the number...work out the first part i will get a better idea of how to attempt the question

6. Aug 15, 2010

### arkofnoah

isn't fractional uncertainty (or fractional error if you choose to go by that name) a form of uncertainty? i don't get what's the issue here and what do you mean by the "first part"?

but anyway the full solution is just this:

total fractional uncertainty: 0.1/25 + 0.01/5 + 0.01/2 + 0.01/1 = 0.021
total absolute uncertainty: 0.021 x 25/(5 x 2 x 1) = 0.0525 = 0.05 gcm^-3

Last edited: Aug 15, 2010