F = ma Was NEVER Newton's Second Law

Newton's second law is not F = ma. Everyone can recite that equation, and Isaac Newton never wrote it. Not in the Principia, not anywhere. What Newton actually wrote is that force is the rate of change of momentum. The difference is not pedantry. The version we teach quietly assumes the mass never changes, and when you break that assumption F = ma does not just get less accurate, it stops being able to express the problem at all. That is why it cannot describe a rocket, a falling chain, a cart in the rain, or anything moving near the speed of light. Newton's own version handles all four without modification. This is where F = ma actually came from, the exact point it breaks, and why the equation on the first page of your physics textbook is a special case wearing a law's name. #Physics #Newton #ClassicalMechanics #ScienceExplained