Define momentum as mass times velocity, compute momentum, and explain why momentum makes some collisions harder to stop than others.
Momentum measures how hard it is to stop something moving: p = m x v, in kilogram-meters-per-second (kg*m/s). A loaded truck at a modest speed can have far more momentum than a bicycle flying much faster, because momentum depends on both mass and speed.
Momentum matters most in collisions. The total momentum of everything involved stays the same before and after -- conservation of momentum. If a moving ball hits a still one and stops dead, the still ball speeds away carrying the momentum the first ball lost.
Mastery looks like: They compute momentum from mass and velocity, and explain why a more massive or faster object has more momentum and is harder to stop.
Common stumbles: Treating momentum as the same thing as speed and ignoring mass; forgetting momentum can transfer between objects during a collision.