E=mc² : Why Mass Is Stored Energy

How can a charged battery, a compressed spring, or a bound nucleus have a different mass—even when no matter has been added or removed? In this video, QUESTIONKIND explores mass–energy equivalence - from energy stored in everyday systems and the deuteron’s mass defect to what E_0=mc^2 actually means, why system boundaries matter, and why moving objects and light require momentum in the fuller relationship. CHAPTERS 00:00 Can Stored Energy Change Mass? 00:58 Where Did the Mass Go? 02:08 What E_0=mc^2 Actually Means 03:06 The Deuteron and the Mass Defect 04:08 Why c^2 Changes the Scale 05:03 Measuring an Extremely Small Mass Difference 06:00 Binding Energy Inside a Nucleus 07:30 How a Microgram Becomes About 90 MJ 08:10 System Energy and System Mass 08:58 Rest Energy, Moving Objects and Light 10:18 Three Pairs to Remember 11:19 Einstein’s 1905 Insight 11:50 The Conversation-Ready Explanation 12:40 From Inertia to Spacetime Geometry PRODUCTION DISCLOSURE This is an original QUESTIONKIND production created under human editorial control. AI tools assisted with narration, selected illustrative visuals, thumbnail development, and production workflow. AI-generated imagery is not presented as archival evidence or authentic footage. All assets were used under applicable commercial-use terms.