Thursday, Sept 11, 2008 - Block 3

Students measured the mass, and dimensions of an aluminum block and calculated the density. Examined table of density measurements.

1. When making measurements with a digital device (electronic balance, stopwatch) the reading uncertainty is the smallest division.
2. When making measurements with a ruler, you can read to 1/2 the smallest scale division, but you might also take the uncertainty to be the smallest scale division since there is uncertainty at both ends.
3. When making measurements with a ruler, be sure to look straight down to avoid parallax errors
4. Precision vs Accuracy
Precise but not accurate - systematic errors - consistent reading errors, problems in calibration, or problems in measuring instrument (measuring with metal ruler on very hot day, not zeroing measuring device)
Accurate but not precise - random errors - uncertainty can be improved by repeated measurements (time it takes an object to fall, landing position for fired projectile)

In the case of random errors, you can calculate the uncertainty by taking the sum of the absolute values of (avg - measurement) and dividing by the number of measurements.

Uncertainties in calculated values
1. When adding or subtracting measurements, you add the absolute uncertainties to get the uncertainty in the result (example - perimeter of rectangle)
2. When multiplying, dividing or raising to power...
A. Min-Max method
avg = (min + max)/2
unc = (max - min)/2
B. Method of relative uncertainties
unc = quantity * (sum of relative uncertainties)
if you have exponents, the absolute value of the exponent is the coefficient for the relative uncertainty.

Handed out worksheet on uncertainties. Do as many as you need to to understand the methods.

Explained Sun Lab
This is an IB lab that will be graded on Data Collection and Processing, and Conclusions.
1. Data Table - table neatly made with labels, units, uncertainties
Short explanation on why you chose the uncertainties the way you did.
2. Calculation Section:
Calculate values and uncertainties using
A. Min/Max method and
B. Method of Relative Uncertainties. Clearly explain all steps in calculations (state what it is you are calculating, show equations, plug in numbers with units, get result)
3. Conclusions
A. Clearly state result with uncertainty, explain significance of result, compare to accepted value
B. Evaluate procedure - how good was it? sources of error or uncertainty?
C. Ways in which method could be improved? or other ways to get the result.

Did Sun Lab - write-up due Tuesday