Sunday, October 10, 2010

Re-Cap of Physics I


Hey everyone!

First things first:  I forgot to tell you what constants you need to know for Test Day.  Here are the big seven:
acceleration due to gravity:  g = 9.8 m/s2 although you can always use 10
speed of light in a vacuum:  c = 3.0 x 108 m/s
density of water:  rho = 1.0 g/cm3
specific heat of water:  c = 1.0 cal/g K
Avogadro's number:  N = 6.02 x 1023 mol-1
Faraday's constant:  F = 96,485 C/mol e- although you can always use 100,000
atmospheric pressure:  Patm = 101,325 Pa although you can always use 100,000 = 760 mmHg = 1 atm
And a couple quick reminders:
-If you haven't yet sent me your Student Questionnaire, please do so!  If you haven't taken your Diagnostic, likewise!
-The blog is mchv10702.blogspot.com; username is mchv10702 and password is letsgo45T.  I eagerly await your questions!

After every classroom session you will receive a comprehensive email that will include the following:

  • A reminder of the preview assignments (i.e. our fancy word for homework) required for the upcoming session.
  • A discussion of some key points of the previous session.
  • A discussion of the review assignments associated with the session we just completed.
  • Helpful hints.
  • Links to stuff that may or may not be related to the MCAT/medicine that I happened to find interesting for some bizarre reason.

Now, I’m aware the depth with which we covered the material during Physics I may have been overwhelming, but do not become disheartened.  It’s going to take a few revolutions for everything to come together.  The key is that you engage in these reps NOW.  Some students adopt the “well, I don’t have time for this so I’ll just come to the classes and take notes and then figure it all out later.”  This is not an efficient way to learn, because three months from now you will not remember what we talked about this past week.  The time to review our previous lesson is right now!  Even if the Physics I made complete sense to you, open up your Lesson Book and go over it all once again.  Strive to reach the point where you can look at a blank Lesson Book page without any of your notes and be able to recall and verbalize all of the key physics concepts.  Yes, it takes time.  However, once you truly understand the core science concepts you will not forget it, because you will understand how it all works.  And once you have reached that point, you are ready to play.  You will be ready to see the connections between physics and biology.  That’s when it becomes fun, and that’s when you’ll start to realize that getting ready for the MCAT will help you succeed in medical school and thus help you become a better physician. 

So, to “come ready to play” for General Chemistry I, here’s the required homework:
  • General Chemistry Review Notes Chapters 1-3 and 7
    • These chapters review Atomic and Electronic Structure, Bonding, and Gases.  Similar to the reading for Physics I, a thorough understanding and familiarity with these concepts is required to facilitate an understanding of higher-yield topics that we will discuss in Unit II.  The most challenging aspect will probably relate to understanding the behaviors of gases under non-ideal conditions, but we will engage in a review of that topic in class.

For those of you who haven’t seen general chemistry in a while, I recommend the following:
  • General Chemistry Foundation Review Unit 1
  • Online Workshop & Quiz (OWQ): Quantum Numbers and Electronic Configuration
    • This OWQ does a very nice job of presenting all of the information you need to know about quantum numbers and electronic configuration.  We will open our discussion of general chemistry with a brief review of these concepts.

To reinforce what we have covered in Physics 1 try the following:
  • Physics Subject Tests 1, 2, & 3
  • Translational Motion Topical Test 1
  • Force, Motion, Gravitation and Equilibrium Topical Test 1
  • Work, Energy, & Momentum Topical Test 1

All of these items are available on your KapTest syllabus.

Helpful Hints:
  • For Translational Motion Test 1, Passage I, Question 5, use critical thinking rather than math to solve the ratio.
  • For Trans. Motion Test 1, Passage II, use critical thinking to minimize the amount of math; especially for Question 12 – check out units!
  • The Force, Motion, Gravitation and Equilibrium Test 1, Passage I, is a very difficult passage and question set.  It may require multiple go-throughs in order to gain complete understanding.

Also, before progressing to the associated topical tests for Physics I, ensure that you know and understand these Physics I concepts:
  • Kinematic formulas
    • The big ones are in the middle of page 21 of the Review Notes.
    • Be comfortable applying these concepts to projectile motion (ex: Question 7 from the Lesson Book on page 44).
  • Newtonian Mechanics
    • Understand how to draw a free-body diagram and use Newton’s 2nd law to solve physics problems.  In class we used this approach to solve:
      • why an object in free-fall will reach an acceleration of zero (force of air resistance opposes force of gravity)
      • why a scale will register a person’s weight to be greater if the person and scale are in an elevator accelerating upwards (Fnet = N – Fg)
    • Be able to distinguish between forces arising from Newton’s 3rd Law and the normal force (recall the “book on the desk”).
    • Kinetic vs. static friction
      • Be able to interpret the graph on page 39; recall that Fstatic friction = Fapplied as long as Fapplied < Fs max
    • Inclined Plane
      • Be able to resolve the force due to gravity into its component vectors.
      • Be able to explain what will occur to Fapplied, Fs, Fs max as the angle of inclination changes.
      • Be able to derive algebraic expressions for the net force (or acceleration) of the block.
    • Centripetal Motion = Uniform Circular Motion
      • Know the basic definition: object moves at constant speed, net force points to center of circle, Fc = mv2/r = Fnet
      • Net force may be result of one force (gravity, magnetism), or it may be the result of two or more forces (see “roller coaster” passage in Force, Motion, Gravitation, and Equilibrium Test 1).
      • Be able to derive new expressions for velocity of object by equating Fc expression to Fnet (ex: mv2/r = GMm/r2 à v2 = GM/r)
      • We will revisit this concept in the context of angular frequency when we discuss simple harmonic motion in our Physics II session.
  • Rotational Motion
    • Know the torque formula, the +/- convention, its units, and recall that it is a vector quantity.
  • Work & Energy
    • When we discuss thermodynamics in Physics II, we will distinguish between work done on a system versus work done by a system.  The formula W = Fdcosθ is referring to the work done ON a system by an external force applied to the system over some displacement d
    • The Work-Energy Theorem states that the net work performed on an object is equal to the change in its energy. 
    • Mechanical Energy = K + U (any Jayhawks fans?)
    • Conservation of Mechanical Energy: MEi = MEf
      • Many equations can be derived from this relationship, thus it is important to know when mechanical energy is conserved.
      • Conservative forces = gravity, electrostatic, elastic.
      • Non-conservative forces = dissipative (air resistance, kinetic friction, etc.).
    • We also took the time to review the concepts above in equation form:
      • Wnet = ΔE
      • Wconservative = ΔKE = - ΔPE (i.e. no change in mechanical energy)
        • In other words, if there is conservation of energy any change in KE is reflected in an opposite change in PE.  For example, an object in free-fall gains KE as it loses PE.
      • Wnonconservative = ΔKE + ΔPE (i.e. change in mechanical energy)
        • The derivation is below, but the key point to understand is that the work performed on a system by a non-conservative force can result in a change in the object’s mechanical energy.
        • Derivation of expression for the work done by a non-conservative force.
    • Power
      • Know the definition and formula, as well as its units
  • Momentum
    • Beyond the basic formula mv, understand the differences between the three different types of collisions.  The critical thinking exercise question on page 48 nicely contrasts the key differences, so if you have a good understanding of that question you should be good to go for momentum questions. 
    • The impulse formula can come in handy on some questions; it usually comes into play for objects that experience a significant change in momentum in a very short time period.  The example we reviewed in class is a head-on collision, but the clown blasted out of a cannon is another example where we can use that formula (see topical test discussion below).

Once you have a firm grasp of these fundamental concepts it is time to take it to the next level and challenge yourself with the Physics 1 topical tests: 
  • Translational Motion Test 1
    • Passage II is a challenging experimental passage that features a clown shot out of a cannon.  The sixth question is particularly challenging; when you don’t know how to get started use the answer choices as a clue to deduce the concept behind the question.
  • Force, Motion, Gravitation and Equilibrium Test 1
    • Passage I is the infamous roller coaster passage.  The key is to derive an expression for the normal force.
  • Work, Energy and Momentum Test 1
    • The best question from this set is the discrete question that illustrates a block sliding down a frictionless surface and subsequently colliding with a stationary block in a totally inelastic collision.  The reason this question is good is that it requires an application of the conservation of momentum and the conservation of energy.  The MCAT is unlikely to contain many questions as involved as this one, but the purpose of the Topical Tests is to help deepen your understanding of the concepts so don’t be put off by this challenging problem.  Embrace it!

Please remember that these science topical tests exist to serve as learning tools, NOT testing tools!  Do not get upset or fed up when you miss a question – read through the explanations of all the questions to ensure you have learned the material!

Saturday, October 9, 2010

Volatile

What was the MCAT's definition of something that is volatile?
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Volatile means low boiling point, or easily vaporizable.

Thursday, October 7, 2010

Make-Ups

I was wondering what you think the best way to make up a class is (at Kaplan or online) and how I would go about doing this.
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In general, I strongly encourage students to make-up classes in person rather than online whenever possible.  Here are how you can find make-up sessions:
-Sign in on Kaptest.com
-Right below the "GO" button for your syllabus, you'll see:  "Missed a session?  Find a make-up"
-You can input which session you need to make up, a ZIP code to search around, and a time period
-Keep in mind that you need not do the sessions in order, so you can make up Gen Chem I two months from now if need be


If you are going to make-up the class online, instead, go to your Kaptest.com syllabus and click on the "My ToolBox" tab.  Then click on "Lessons-On-Demand" and watch the session you missed.  This will allow you to see all the material, but you can't really ask questions.  That's why I tell people to make them up in person. Also keep in mine that while you can watch as many sessions as you'd like as Lessons-On-Demand, only two of them can count as make-up sessions.


Also, for attendance's sake -- make sure that the teacher writes down your name for the make-up session.  If you do an online make-up, please let me know so I can mark the attendance for you (it will not automatically do this for you).


-Alex

Monday, October 4, 2010

Ammonia alkylation

During the direct alkylation of ammonia, why do we need to add a base to get rid of the Br from CH3NH3Br?  Doesn't the Br come off by itself since N cannot be attached to 5 things?
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Here we need a base because we actually have an ionic salt of CH3NH3+ and Br-.  The base here deprotonates our NH3 group in the compound, thus making a neutral CH3NH2.  The Br- is no longer attracted, and thus comes off the molecule too.  The base is needed, though, to cause that deprotonation in the first place.

Online Make-Up Classes

Hey Alex!

I hope you're doing well and aced those med-school exams you had a couple weeks back. I'm still trying to figure out exactly what I'm going to do this week, because I have an exam on Friday.

Just to clarify - I wanted to figure out how I'm going to fit in my make-up sessions. There's a class next Saturday that I can go to to make up the Gen Chem session. But for Physics, I believe the closest session is on a Wednesday. I can make-up that session online, right? It's just that there's a maximum number of online classes you can use as your make-up?

Thanks a bunch.. I really don't want to miss the sessions with you this week, but we'll see how much pedigrees and gene linkage kick my butt in the next couple of days.
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Two sessions can be made up online, by watching the Lessons-On-Demand.  You can access these on your Kaptest.com syllabus, by going to the "My ToolBox" tab and clicking on "Watch Lessons-On-Demand".
 
The attendance for these will not automatically be recorded, so please send me an email or let me know in class when you do use an online make-up.  You'll see that these sessions go through the same material we covered in class, although you lose the ability to ask questions during the session.  Please feel free to post questions right to this blog and I can answer them there if you need clarification on anything in those sessions.
 
You can watch as many Lessons-On-Demand as you want -- they act as a great review for sessions that you feel you need extra help on.  But remember -- only two of those Lessons-On-Demand can count as make-ups.

Using the Flashcards

Hey Alex,

Hope your exam on Friday went well!  I have some questions regarding the flashcards.  How would you suggest handling unfamiliar or review-worthy topics/equations?  Would it be best to read up on them now, even if they don't appear until the later chapters in the review notes, or to try and focus on the concepts in the order they are presented in the review?  Also, would you recommend concentrating on one subject at a time or rotating through them as we do in the schedule of class sessions?  I'm just not sure how to best incorporate the flashcards with the review notes.

Thank you for your help! See you tomorrow.
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For the flashcards, I would definitely encourage you to start working on them even if we haven't gotten to those topics in class yet.  Make sure that you complete the Preview and Review work between sessions, but if you still have some free time -- absolutely go ahead and start looking at the material later in the course.
 
To answer the order of subjects, I would say that in most cases, the best model would be to use what we're doing in class -- focus on the subjects by mixing up the sciences.  This will help you find connections between material and will also prepare you for the fact that all the topics are mixed up on Test Day within a section.  It will help you continually "change gears" during your actual MCAT.
 
That being said, if you feel that one particular area is notably weaker for you, you should spend some extra time with it, especially early in the game.  This way, you can spend most of your prep fine-tuning your knowledge and strategies, rather than doing very basic content review.
 
I usually encourage students to just always have the flashcards on them.  They're a great resource to use when you're riding the bus, waiting for something, etc.  You'll see that I still -- to this day -- tend to have flashcards for some subject or other on me.

Sunday, October 3, 2010

Buffers

Hi Alex,

For Question 12, I could not figure out exactly what the question was asking.  I think I'm having a problem understanding the concept of buffers.  This statement confuses me:  "A certain buffer solution is 3 M in HF, and 2 M in NaF."  So is the mixture of HF and F- the buffer??  Or is HF/F- the solution being buffered?   And if so, how can the equation Ka=HxF/HF be used?  Wouldn't you have to use the concentrations of the buffer mixture?

Sorry if this is confusing!  Thank you!
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The statement is saying that the buffer is composed of 3M HF and 2 M NaF.  Since NaF is a salt, and will completely dissociate, then the buffer is 3M HF and 2 M F-.

The equation here that you'd want to use is the Henderson-Hasselbalch equation.  Remember that this is the one that relates pH and the pKa of an acid (you can also use the Ka directly, but I think it's more math-heavy).  So, Henderson-Hasselbalch says pH = pKa + log [CB]/[A].  Here that would be
pH = -log(7 x 10^-4) + log [2/3].  The log of a difference is the division of logs:
log (2/3)/(7x10^-4) = log (2/21 x 10^4) = log (2/21) + 4.
log (2/21) is around log (1/10), which is -1.  Thus, the pH = 4-1 or 3.  The actual answer is 2.98, but this is plenty close.