Your uncle works at the railroad freight yard, and he has asked you to watch the yard hands couple 20,000-kg boxcars together. As he leaves to get a sandwich, he tells you, "Whatever you do, don't let

Your uncle works at the railroad freight yard, and he has asked you to watch the yard hands couple 20,000-kg boxcars together. As he leaves to get a sandwich, he tells you, "Whatever you do, don't let those cars bang together too hard. Each coupling will take only 6,000 J on a hit before it shatters." You ask how hard is too hard, but your uncle has already ducked out. How fast can one moving car bump into a stationary car without the coupling shattering?