10–15% of the AP exam. 7 key topics. 4 common errors analyzed.
Unit 4 (Linear Momentum) is part of AP Physics 1 (Algebra-Based). Concept-first: understand the physical reasoning before memorizing equations — the four named FRQ types reward explanation over plug-and-chug.
These are the most frequently penalized mistakes on AP exam FRQs and MCQs. Review each before your next practice session.
Treating momentum conservation and energy conservation as interchangeable — momentum is always conserved in a closed system, but kinetic energy is only conserved in elastic collisions; applying energy conservation to a perfectly inelastic collision yields incorrect results
Ignoring the vector nature of momentum in one-dimensional collision problems: assigning the same positive sign to objects moving in opposite directions, leading to incorrect momentum sums
Confusing impulse with force — impulse equals the change in momentum (J = FΔt = Δp), not the force itself; on F-t graphs, students must integrate (find area) rather than read a point value
Failing to recognize explosion scenarios as momentum conservation problems — when a system at rest breaks apart, the total momentum remains zero, so the fragments must have equal and opposite momenta
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