What are the conservation laws, and when do they apply?
- In theory, momentum is always conserved, regardless of what happens. On MCAT problems, momentum is conserved if the system, i.e., all the objects that are mentioned, includes everything that exerts force on anything else. The most common trouble here is the Earth — if gravity causes objects to fall, but the Earth is not mentioned in the problem, then AAMC would claim, confusingly, that momentum is not conserved.
Momentum and its conservation is usually mentioned in collision problems.
- Everything listed above for momentum is also true of angular momentum: it’s always conserved in theory, but might not be according to AAMC if something is missing from the system. Angular momentum appears infrequently on the MCAT.
- Energy is not conserved, except in special cases: it is conserved during perfectly elastic collisions (you know that it’s perfectly elastic because they tell you so), and when the only forces involved in a problem are "conservative."
Conservative forces on the MCAT are: gravity, electrostatic, springs; also, any physical pushing or pulling that doesn’t involve dynamic friction. Dynamic friction, electrical resistance, radioactive decay, and any collision that is not perfectly elastic, are not conservative. If in doubt, it’s not.
- Mass is conserved on the MCAT, except possibly in radioactive decay. If anyone has seen an exception to this principle in AAMC materials or tests, please advise.
- Net charge is conserved, always.
- The number of baryons (for us, protons and neutrons) is conserved, always. On the MCAT, we can just add them because we don’t encounter antiprotons.
