Sucipto, Try Adi. “Measurement of Friction Coefficient Between Glass and Rubik’s Beam on the Sloping Field Using Rubik Lubricant”. Journal of Natural Sciences and Mathematics Research, vol. 5, no. 1, June 2019, pp. 7-9, https://journal.walisongo.ac.id/index.php/JNSMR/article/view/11023.