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Head to https://squarespace.com/physicsexplained to save 10% off your first purchase of a website or domain using code PHYSICSEXPLAINED You may have heard that Einstein’s theory of relativity says objects become more massive the faster they move. This idea appears in textbooks, popular science explanations, and all over the internet. But many modern physicists consider this explanation misleading, and even Einstein later discouraged using the concept of “relativistic mass”. In this video, we unpack what relativity actually says. By looking at force, momentum, energy, and the famous energy–momentum relation, we’ll see why modern physics keeps mass fixed while energy and momentum increase with motion, and why this is important for particle physics. 🤝 Support the channel Patreon → https://www.patreon.com/physicsexplained 🔗 Follow me Instagram → https://www.instagram.com/physics_explained_ig X (Twitter) → https://twitter.com/PhysicsExplain1 References Adler, C. G. (1987). Does mass really depend on velocity, dad? American Journal of Physics, 55(8), 739–743. https://www.fisica.uniud.it/URDF/laurea/idifo1/materiali/g5/AdlerDoes_mass_really_depend_on_velocity_dad.pdf Hecht, E. (2009). Einstein Never Approved of Relativistic Mass. The Physics Teacher, 47(6), 336–341. https://pubs.aip.org/aapt/pte/article/47/6/336/275760/Einstein-Never-Approved-of-Relativistic-Mass Okun, L. B. (1989). The Concept of Mass. Physics Today, 42(6), 31–36. https://faculty.washington.edu/seattle/physics544/Eismc2/1989%20Okun%20Physics%20Today.pdf Okun, L. B. (2006). The Concept of Mass in the Einstein Year. https://arxiv.org/abs/hep-ph/0602037 Okun, L. B. (2008). The Theory of Relativity and the Pythagorean Theorem. https://arxiv.org/abs/0809.2379 Baez, J. (Physics FAQ). Does mass depend on velocity? https://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html Baez, J. (Physics FAQ). What is the mass of a photon? https://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html Petkov, V. (2009). On the Relativistic Mass. https://vesselinpetkov.com/V.Petkov_On_Relativistic_Mass.pdf Sandin, T. R. (1991). In Defense of Relativistic Mass. American Journal of Physics, 59(11), 1032–1036. https://pubs.aip.org/aapt/ajp/article-abstract/59/11/1032/1039242/In-defense-of-relativistic-mass Jammer, M. (2000). Concepts of Mass in Contemporary Physics and Philosophy. Princeton University Press. Goldstein, H., Poole, C., & Safko, J. Classical Mechanics. Addison-Wesley.