@misc{indiciae1001a15d149b,
  title = {Equivalent spring-mass system for normal modes},
  author = {Bamford, R. M. and Gayman, W. H. and Wada, B. K.},
  year = {1971},
  url = {https://ntrs.nasa.gov/citations/19710009805},
  note = {Source identifier: 19710009805}
}