Help with Uncertainties in Measurements

[umer007]

I did a lab recently where I had to measure length, width and height of different sizes of beach wood with a ruler (so gave each measurement 2 decimals). I also measured each piece on an electronic balance (the reading gave 2 decimals). I calculated the volume of each block and graphed against the mass.

 

Could you plz. tell me as many sources of absolute uncertainties affecting my observations for each measurements and a brief explanation for each so I understand.

 

I have this so far: parallax, edges of block not smooth. Could someone plz. explain parallax to me and how it affects my lab.

 

Thanks in advance.

[ydoaPs]

measuring device itself

[umer007]

what uncertainty would come from the electronic balance? can you explan a bit more

[ydoaPs]

i was thinking your meterstick or whatever. the lines have thickness. there are not an infinite number of them. so your measurement could be like 8.6 +/-.1cm. that would give you about .01% uncertainty in itself

[swansont]

Could someone plz. explain parallax to me and how it affects my lab.

 

parallax is not looking straight on - i.e. the alignment of the edge vs. ruler mark is off, so it will be long or short depending on your angle of view.

[umer007]

How will temperature and humidity affect the volume (length, width or height) and mass of the wood blocks??

[Klaynos]

what uncertainty would come from the electronic balance? can you explan a bit more

 

There would be an error listed either on the device or in the instruction booklet of the device.

 

How will temperature and humidity affect the volume (length, width or height) and mass of the wood blocks??

 

Expansion and contraction would effect both what you are measureing and what you are measureing with...

[swansont]

Expansion and contraction would effect both what you are measureing and what you are measureing with...

 

But presumably not the same amount.

[Tetrahedrite]

Ok, if I was a lab teacher (which I am), I would be looking at four sources of error for each piece of wood.

 

1/ Error in height

2/ Error in length

3/ Error in width

4/ Error in mass

 

I will go through an example to show how we would arrive at an absolute error.

 

Eg for ease of calculation I will assume that the piece of wood is 10.00 x 10.00 x 10.00 cm and weighs 500.00g.

 

Now errors in measuring lengths is usually given by half the smallest division on your ruler ie probably 0.05 cms, and, the error on a balance with two decimal places is usually 0.01 grams.

 

So now what we do is work out the percentage error in each of the measurements.

 

So % error in height = 0.05/10*100 = 0.5%

 

% error in length = 0.05/10*100 = 0.5%

 

% error in width = 0.05/10*100 = 0.5%

 

% Error in mass = 0.01/500*100 = 0.002%

_______

 

Therefore, total % uncertainty = 1.502%

 

I assume you were calculating something such as density, which in this case would be 0.5000 g/cm^3.

 

Thus the absolute uncertainty is 1.502% of 0.5000 = 0.00751 g/cm^3

 

Overall answer therefore is 0.5000 +/- 0.008 g/cm^3