Linear Momentum
Momentum is mass times velocity: $\vec p = m\vec v$. Velocity has direction, so momentum does too. A minus sign on $p$ means direction along your chosen axis, not a smaller number and not a stopped object. To find the momentum of a two-object system, add the momenta as vectors with signs, not as speeds. Most traps in this topic come from forgetting that $\vec p$ is a vector.
Most traps cluster around the vector nature of $\vec p$. Some students drop the direction and give momentum as a single number. Others read a negative $p$ as smaller magnitude or as a stopped object, when it just means direction along an axis. Another trap: ranking momenta by mass alone or by speed alone, when you need both. The lesson walks all of these.
The work
3 ways in · any order
Lesson
Linear Momentum
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Start with $\vec p = m\vec v$ for a single object, then build to a two-object system. Covers what direction means on a momentum vector, why a heavy slow object can carry the same momentum as a light fast one, and how to add momenta when their directions disagree. Ends with a ten-scenario applet on one- and two-object setups.
Diagnostic
10-item topic check
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Ten questions on the vector character of momentum, how a sign tells direction (not size), why both mass and speed matter for $|\vec p|$, and how system momentum adds as vectors. Each distractor maps to a specific misconception so the targeted-practice round can drill exactly what you missed.
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.