ConceptMechanics
← All conceptsMomentum & impulse
p=mv, impulse J=F·t=Δp (per-force vs net), conservation of momentum, perfectly inelastic and recoil/explosion collisions
11 ways this goes wrong. Each one is a named misconception the questions are built to catch.
- 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.→
