# Uncertainty/errors in meter stick measurements

1. Jun 9, 2007

### _Greg_

Hi

just got a couple of question about errors in measurements.

I have a table of results of different meter stick measurements which are accurate to 0.1 cm.
Now a meter stick reads to 0.1 cm and the uncertainty is usually 20% of the smallest reading.
so all of my readings should be +/- 0.02 cm

(1) Now im wondering, say i have 3 results, do i have to add all 3 uncertainties to give a total uncertainty of 0.06?
or do you not add the uncertainties since they are all the same, therefor a total uncertainty of 0.02?

Iv also made a graph with these values and calculated the uncertainty in that using:

G/deltaG = 2 x delta Y / y2 - y1

(2) so i have an uncertainty in the graph and i assume i have to add that to my uncertainty of measurements to get the total uncertainty?

Last edited: Jun 9, 2007
2. Jun 9, 2007

### esalihm

u never add uncertainities, it is specific to the apparatus u use and is does not change

if you want to indicate an uncertainity in your graph, draw two best fit lines istead of one, then give a range of values

3. Jun 9, 2007

### _Greg_

In fact i think it just asks for the uncertainty in my graph.

The way iv been taught to calculate the uncertainty in gradients is with the equation above where delta Y is the near vertical seperation of the points from the line.
so basically you measure the distance each point is from the line of best fit, add them up and divide it by the total number of points. then use that in the equation.
so deltaG/G is the fractional uncertainty in the gradient which can then be multiplied by 100 to give the percentage error.

have you come across this before esalihm?

btw, this gradient is 1/focal length, you may remember from my other topic yesterday.

Last edited: Jun 9, 2007
4. Jun 9, 2007

### esalihm

I have never done anything about uncertainity like that, but I really don't think adding uncertainities is right.
because as I said it is specific to ur equipment.
and I don't understand what u mean by the uncertainity in your graph do you mean the R^2 value?