Practice momentum, impulse, collisions, and conservation principles. Quiz one topic at a time or take the whole unit.
The idea each topic turns on, and the mistake students make most often.
Key ideaMomentum is mass times velocity, p = mv. It is a vector, so direction matters.
Common mistakeTreating momentum as a scalar, for example adding the momenta of two objects moving in opposite directions instead of subtracting them.
Key ideaImpulse equals the change in momentum: J = FavgΔt = Δp, which is the area under a force–time graph.
Common mistakeForgetting that the velocity changes sign when an object bounces back. A bounce gives a larger momentum change than simply stopping.
Key ideaIf the net external force on a system is zero, its total momentum stays the same, even though individual objects’ momenta change.
Common mistakeChoosing a system that has a significant external force, or conserving speed instead of momentum.
Key ideaMomentum is conserved in every collision with no outside force. Kinetic energy is also conserved only in elastic collisions, and the most kinetic energy is lost when the objects stick together.
Common mistakeAssuming kinetic energy is conserved in a collision that isn’t stated to be elastic.
Key ideaThe center of mass is the mass-weighted average position. For two objects, it lies closer to the heavier one.
Common mistakePlacing it at the geometric midpoint without weighting by mass.
Key ideaA quantity is conserved for a system only when its condition holds: momentum when there is no net external force, mechanical energy when no nonconservative forces do work, and angular momentum when there is no net external torque.
Common mistakeApplying a conservation law to a system without checking its condition, for example conserving mechanical energy when friction acts.
This is one of the 28 questions in the Unit 4 quiz.
Which equation gives an object's momentum?
Answer: B. p = mv
Momentum equals mass times velocity: p = mv. Since velocity is a vector, momentum has the same direction as the object's motion.
AP-style FRQs for this unit. Students score themselves point by point against a rubric and see a full solution.
