Your uncle works at the railroad freight yard, and he has asked you to watch the yard hands couple \(20,000-\mathrm{kg}\) boxcars together. As he leaves to get a sandwich, he tells you, "Whatever you do, don't let those cars bang together too hard. The couplings will take only \(10,000 \mathrm{~J}\) on a hit before they shatter." You ask how hard is too hard, but your uncle has already ducked out. How fast can one moving car bump into a stationary (but free-tomove) car without the coupling shattering?