Covers: Impulse, collisions and center of mass. 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 ideaThe impulse ∫F dt equals Δp, which is the area under the force–time graph.
Common mistakeForgetting that the velocity reverses in a rebound. The momentum change is m(vi + vf) in magnitude, larger than for stopping (double only if it rebounds at the same speed).
Key ideaWith no net external force, the total momentum is constant. Conserve each component separately in 2D.
Common mistakeConserving momentum through an event where a large external force acts, such as a pivot or the ground.
Key ideaMomentum is always conserved in a collision. Kinetic energy is conserved only if the collision is elastic, and perfectly inelastic means the objects stick together.
Common mistakeAssuming kinetic energy is conserved without being told the collision is elastic.
Key ideaThe center of mass moves as if all the external forces acted on the total mass: Fext = M·acm. Internal forces, like those in an explosion, can’t change its motion.
Common mistakeThinking an explosion shifts the path of the center of mass.
This is one of the 23 questions in the Unit 4 quiz.
Two cars collide on a road with friction. Why can you treat their total momentum as conserved during the collision?
Compare the size of the collision forces with the friction force, and think about how long the impact lasts. How big is friction’s impulse in that time?
Answer: A. Collision forces are much larger than friction during the brief impact
During the brief collision, the cars push on each other with very large forces. The external friction force delivers a tiny impulse in that short time, so it barely changes the total momentum.
AP-style FRQs for this unit. Students score themselves point by point against a rubric and see a full solution.
