Here I derive the formula used for logarithmic decrement. This is way to experimentally determine the values of your damping ratio. To do it:
1) Find the maximum amplitudes of your first peak and second peak
2) Find delta (from definition)
3) Plug value into final formula to find damping ratio (zeta)
4) If you're asked to find the damping constant you'll need to use the formula
c = 2 zeta * sqrt(km). Note that you'll need to know the spring constant for this final optional step. Also if your value of zeta is greater than 1 then you've made a mistake!

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