# Homework Help: Calculate the percentage difference

1. Jun 19, 2007

### physicsnewby

1. The problem statement, all variables and given/known data
This is related to my last post. Again, I am getting a ridiculous answer hoping someone can help. I need to calculate discrepancy between two calculated values.

2. Relevant equations

EQN 1 difference between values = Value 2 - Value 1
EQN 2 error of values = square root of (error value 1 ^2 + error value 2 ^2)
discrepancy = EQN 1/EQN2

3. The attempt at a solution
When I use EQN 1, I'm subtracting one value of spring constant k from another. The difference between my values is in the 3000 range. For EQN 2, my error value is in 2 x 10^-3.

When I divide 3000/2x10^-3 I get 1500000 ! The answer should be relatively close to 2!

Now, my previous post was regarding units for k. Could this be the problem? The error values I calculated though were based on the data given in cm, g, seconds squared, etc. Does this matter? It seems strange to me that my calculate K is say 14000.00 +/- 0.003 (for example). Small error for such a large number!

2. Jun 20, 2007

### andrevdh

Are'nt you suppose to calculate the percentage difference, $$p$$, between the two values? If the "correct" value is $$k_o$$ and the experimental value is $$k_e$$

$$p = 100 \times \frac{|k_o - k_e|}{k_o}$$

Last edited: Jun 20, 2007
3. Jun 20, 2007

### physicsnewby

I don't have a correct value to compare to. I have two experimental values I'm comparing.

4. Jun 20, 2007

### andrevdh

Well the formula will give the percentage difference (with the average) if you take half the difference between the two and use the average for the "correct value". It is not really sensible to compare measurements with each other this way though. Statistically one normally calculate the standard deviation and the average of a larger set of measurements.

Last edited: Jun 20, 2007
5. Jun 20, 2007

### physicsnewby

Thanks!! Figured it out