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Concept · Mechanics
Momentum & impulse
p=mv, impulse J=F·t=Δp (per-force vs net), conservation of momentum, perfectly inelastic and recoil/explosion collisions
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Named traps
11 ways this goes wrong.
Each is a named misconception the questions are built to catch. Open one for the move that tempts, and the fix.
- Collision solved by energy onlyRelies on energy conservation alone to find post-collision velocities without applying momentum conservation.→
- Force equals momentumSets force equal to momentum rather than to the rate of change of momentum.→
- Impulse as velocity changeSets impulse equal to the change in velocity rather than the change in momentum (mass omitted).→
- Impulse ignores timeEquates impulse with force alone, omitting the time factor (J=F·t).→
- KE conserved in inelastic collisionAssumes kinetic energy is conserved in an inelastic (especially perfectly inelastic) collision.→
- Momentum conservation misappliedApplies momentum conservation when a significant external impulse acts, or denies it for an isolated system.→
- Momentum treated as scalarAdds momenta of oppositely-moving bodies as positive magnitudes, ignoring vector direction/sign.→
- No positive direction chosenSolves a collision without fixing a positive direction, so opposing velocities are not given opposite signs.→
- Per-force vs net impulse confusedEquates the impulse of a single force to the total momentum change instead of using the net force's impulse.→
- Recoil signs ignoredIn a recoil/explosion sets the two momenta equal in sign so total momentum is nonzero instead of zero.→
- Stuck bodies keep separate vTreats bodies that stick together after collision as having different final velocities.→
Key formulas
No formula sheet is matched to this concept yet. Work it from first principles, and watch the traps.
Shaky underneath?
If the concept underneath still feels loose, patch it here before coming back to CSCA questions.
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