# Homework Help: Building this graph of simple harmonic motion

1. Nov 21, 2009

### rambo5330

1. The problem statement, all variables and given/known data
Hey I'm building a graph of period squared vs. mass for simple harmonic motion
finding the slope and getting K for a spring.

since T^2 = ((4*pi^2)/k)m
the slope is (4*pi^2)/k

so I get my slope from my graph and it has an uncertainty of +- 0.2

to get K we divide (4*pi^2) by the M +-0.2

just wondering what the rule is when your dividing a contstant by a number with uncertainty what the uncertainty is in the answer.?

2. Relevant equations

3. The attempt at a solution
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Nov 21, 2009

### cavalier

Re: Uncertainties

Convert your uncertainty into percent relative uncertainty. Divide by the constant keeping the percent the same. Generally you would root mean squared error, but since only one of your values has uncertainty, the uncertainty remains the same after taking the RMSE. Convert back into absolute uncertainty.

3. Nov 21, 2009

### rambo5330

Re: Uncertainties

so lets say my slope is 4.7 +-.2

you say I
.2/4.7 = .0426 .0426/ (4*pi^2) =.01079

(4*pi^2)/ 4.7 = 8.40

.01079 * 8.40 = 8.40 +- .09
???

4. Nov 21, 2009

### cavalier

Re: Uncertainties

You shouldn't divide your uncertainty by the constant.

8.40 $$\pm$$ 4.3%=8.40$$\pm$$.4

You should follow your sig fig rules when calculating uncertainty, but you can keep a few extra digits before getting to your final uncertainty to avoid mistakes in rounding.

5. Nov 21, 2009

### rambo5330

Re: Uncertainties

Hey thanks a lot makes perfect sense, went over my rules again. was mis-interpretting it...
you cleared it up thanks