Elastic and Inelastic Collisions
Two carts collide on a frictionless track. With no outside push on the pair, total momentum is the same before and after. Total kinetic energy is not. Elastic collisions keep all of it. Perfectly inelastic ones lose the most they can; the carts stick together afterward, sharing one velocity. What sets the difference isn't the track, it's what the carts are made of.
Three traps. First, mixing up which law goes with which collision type: momentum is conserved in every collision, but kinetic energy only in elastic ones. Second, reading "perfectly inelastic" as "stops"; the carts do share one velocity afterward, but it's only zero if the system started with zero net momentum. Third, applying both laws to an inelastic collision and getting a contradiction, because energy actually left the kinetic ledger.
The work
3 ways in · any order
Lesson
Elastic and Inelastic Collisions
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A five-section read on telling collision types apart, with a worked perfectly-inelastic example and three pitfall cards. Finishes with a ten-scenario applet that drills the topic-specific misconceptions and the cross-cutting ones from the rest of Unit 4.
Diagnostic
10-item topic check
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Ten multiple-choice items, mixed quantitative, factor-of-change, and symbolic. About fifteen minutes. Each wrong answer flags a specific misconception so the targeted-practice round below knows what to drill.
Targeted Practice
Drill a single misconception
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Pick one of the failure modes you've missed and grind it on its own. The round is adaptive: two correct in a row clears the misconception and you move on.