AAMC 9 Periodic motion Q25 and Q26

[flodhi1]

---

Members do not see this ad.

Okay so I'm a little pissed that I was one point away from actually getting my highest PS score and feel like I kind of have a reasoning for one of my two mistakes.

AAMC 9
25) Long story short it says conservation of energy which yeah made perfect sense but I picked conservation of momentum as my answer for the oscillations persisting after collision BECAUSE I looked at the whole thing with respect to water molecules and looked at the bonds as well a "fluid". I know A is fine but I slightly disagree with why their explanation of taking out conservation of momentum. I assumed that since the water molecules must have been colliding there had to be inelastic collisions due to the fact that no external force was acting I assumed or answered in this case that momentum had to have been conserved. I know sounds like a BS answer but that was my thought process.
(B) Out of scope. The idea of conservation of momentum refers to collisions and is not applicable to periodic
motion.

26) Man I am just so lost with this one. I made the thought that okay stage 1 is stressed from equilibrium , stage 2 is equilibrium and stage 3 is another stress from equilibrium (Maximum potential energy). Therefore stage 2 which is at equilibrium would have MAXIMUM Kinetic energy / 0 potential energy and thus also maximum velocity. However my answer was OBVIOUSLY wrong and I still don't really understand the question or their answer.

So if anyone could help me out exam is next week I really have to nail this Bull.

 

[Catburr]

For question 26, it's just asking what the velocity is at different points across the surface. So if you're at stage II after I has just occurred, like it says in the question, what stage are you headed towards? III, so the velocities you pick are the ones that take you from II to III. Up on the right side, down on the left side.

 

[golgiapparatus88]

25. The passage doesn't really mention collisions between water molecules on a molecular level. Sure, water molecules are probably colliding with each other, but you would need a huge amount of collisions from a specific direction (either up or down) to change the entire direction of the water. As a probably bad example, you know that "what goes up, must come down." When you throw a ball up in the air, the MAJOR reason it falls to earth is due to gravitational attraction (even though you could assume that air resistance causes some change in momentum). Now back to the question. Just like a spring, the water will move through equilibrium to a maximum restoring force and so on. It reaches that maxima because of its internal energy. The restoring force is great enough to change its direction and shift it the other way.

26. It's pretty much asking what direction the water will move in picture 1. Since the water isn't going to continue to move up, it's velocity vector points down. On the other side, since it's below equilibrium, it will want to move up.

It's late and I worked a 12 hour shift at the hospital so I apologize if none of that made any sense

 

[flodhi1]

Thanks!