# What did I do wrong?

#### kashmirekat

I had a physics lab for Hooke's Law and Simple (ha!) Harmonic Motion. I am not pleased with my grade, but my professor did not indicate what I did wrong and I am banging my head against the wall trying to figure it out (btw, it's not helpful). So if any of you all can take a gander at it and tell me hey! You did this wrong! That would be great. Thank you.

First Section: Spring Elongation

weight(grams)*9.18m/s^2 | Scale reading (mm)
m1g = 114.26&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; | y1 4.7
m2g = 124.26&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; | y2 = 5.2
m3g = 134.26&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; | y3 = 5.9
m4g = 144.26&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; | y4 = 6.3
m5g = 154.26&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; | y5 = 6.7
m6g = 164.26&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; | y6 = 7.3
m7g = 174.26&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; | y7 = 7.8
m8g = 184.26&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; | y8 = 8.2

---
Okay, the first thing I did was convert the grams to kg and multiply by g (9.8m/s^2).
For these values I got:
mg1=1.12N, mg2=1.22N, mg3=1.3157N, mg4=1.414N, mg5=1.512N, mg6=1.61N, mg7=1.708N, mg8=1.806N

Then I converted mm to m....0047, .0052, .0059, .0063, etc.

It's asking for k and in parentheses next to it has slope of graph. [ie k(slope of graph)] So k=slope of graph. Slope equals rise/run, (delta y)/(delta x). So I choose two points 1 & 4.
(1.12N - 1.414N) / (.0047m - .0063m) = -0.294N / -.0016m = 183.75 N/m.

His answer is k = 20. Where did go wrong to get a value so far from the answer?

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#### Coughlan

uh I'm not math head but I would recon that you didn't label something correct on your graph.......20 what? perhaps he wants you to label everything, and I thought weight was in Newtons therefor you don't have to say weight * 9.8 m/s² because 9.8 m/s² is already included in the Newton measurement......yes? no?

#### HallsofIvy

Science Advisor
According to your table:
weight(grams)*9.18m/s^2 | Scale reading (mm)
m1g = 114.26 | y1 4.7
m2g = 124.26 | y2 = 5.2
m3g = 134.26 | y3 = 5.9
m4g = 144.26 | y4 = 6.3
m5g = 154.26 | y5 = 6.7
m6g = 164.26 | y6 = 7.3
m7g = 174.26 | y7 = 7.8
m8g = 184.26 | y8 = 8.2

"m1g" is ALREADY multiplied by 9.81 (not "9.18") m/s^2. In order to convert that to Newtons, you only need to multiply by 0.001 to convert grams to kg. If that is correct, then m1g is 0.11426 Newtons and m8g is 0.18426. For those two values, y1= 0.0047 m and y8= 0.0082 m so the "slope" of your graph is (0.18426-0.11426)/(0.0082- 0.0047)= 20.

#### kashmirekat

Coughlan, k is suppose to be 20N/m.

HallsofIvy, oh if that were only true, but those values did not include the multiplication of g. My masses were 114.26, 124.26, but were not already multiplied by g. Sorry, I should have written g next to them, but that could also be mistaken for grams (which they are in). I would convert to kg before multiplying with 9.8 anyhow, right? The m1g tells us to multiply m by g, ergo .11426kg*9.8m/s^2=1.12N.

Just in case I didn't clear it up, 9.8m/s^2 is not included in the measurement of 114.26, 124.26, etc...those are all masses in grams.

Any more suggestions?
Thank you.
Christina

#### Coughlan

Eh sorry about that......I'm fresh out of ideas.....

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