wvmarle:
Look for accelerometers. They're quite cheap and can be attached to the moving part, measuring its instantaneous acceleration.Now as I don't really understand the scope of your project (both the physical layout as what you're trying to do and measure with it) I have no idea whether those would be suitable for you.
vffgaston:
There are many threads about FFT, although almost all of them in the audio field.Is yours a project for civil structures?
Regards.
I was thinking about making something like this: https://powerkube.net/
The only difference I want to implement is that my box would hang on an moving punchbag, so do you need the weight of the punchbag to make correct calculations?