rotational motion: when to use a=v^2/r vs. a=w^2r

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ibeatupnerds

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not sure if "rotational motion" is even be tested on the mcat....


I don't understand when to use the equation a=v^2/r versus a=(w^2)(r). w is angular velocity. If anyone has done the TBR course it isn't really clear. For example number 22 gives you an example (gives you radius and acceleration and tells you to find velocity) and you use the first equation to find the acceleration. Than number 23 (both are in the same passage), mentions if someone walks toward the center of rotation (for example towards the center of the merry-go-round, how does the acceleration change? If you use the first equation since radius decreases the acceleration increase. But they use the second equations now and using the second the acceleration would decrease. So using the second equation with number 23 is right.

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not sure if "rotational motion" is even be tested on the mcat....


I don't understand when to use the equation a=v^2/r versus a=(w^2)(r). w is angular velocity. If anyone has done the TBR course it isn't really clear. For example number 22 gives you an example (gives you radius and acceleration and tells you to find velocity) and you use the first equation to find the acceleration. Than number 23 (both are in the same passage), mentions if someone walks toward the center of rotation (for example towards the center of the merry-go-round, how does the acceleration change? If you use the first equation since radius decreases the acceleration increase. But they use the second equations now and using the second the acceleration would decrease. So using the second equation with number 23 is right.



Examkrackers said you won't be asked anything regarding omega, alpha, and radians (acceleration/r, velocity/r, displacement/r)
unless specifically given the eq in the passage.

Im sure if they gave you that alpha=radius(acceleration) it would be easy to see how a decreased radius gives you a smaller tangential acceleration
 
There are some MCAT prep materials like the Barron's book or the execrable REI book that present information outside of the boundaries of the AAMC MCAT outline. This is established publishing imprints using their brand strength.

Even in the least carefully edited MCAT materials there can be useful values. Conversely, the most carefully edited MCAT materials, such as at TBR, will have a reason for the inclusion of a topical area of science though it may not strictly agree with the AAMC syllabus. There are reasonably sound biology question sets in the Barrons book. Barrons would almost be worth purchasing. In the nineties my students were asked to read Barrons from cover to cover in two weeks and then throw it away. I learned to say no you don't need to spend a day memorizing all of the cranial and spinal nerves. The danger of time wasters in a pressed-for-time MCAT review process is real for students.
The ghost of Benjamin Franklin would say that MCAT review should be seized by students, teachers, and society because of the social need. The AAMC is doing its best with the MCAT prep syllabus from a utilitarian perspective, trying to encourage knowledge base structuring for all students, using the impetus of the selection process so say here is what you should review before coming to medical school. However, the same utilitarianism gives them a chance not to be onerous, so they try to say here is where we are giving you a break. Don't study electrophilic addition of alkenes. I think they decided this because they want to give students a break, and themselves a break, from advanced students missing questions because their Organic prof was adamant that they always specify isolation from any peroxide source in the Markovnikov decision tree, and then there must be another day studying rearrangements, the halogenation intermediates and all of those steric considerations. I think they just decided to give students a break. After all the MCAT must also reflect the molecular biology revolution these days.

In my opinion I think the AAMC syllabus of the nineties was better because it had rotation, the subject of your question, alkene reactions, and electrophilic aromatic substitution. My feeling is that omission of those topics has been a mistake. While giving students time to learn molecular biology revolution, it casts a normative message at the expense of ultimately understanding biochemistry. What gets left out of MCAT review is important because MCAT review is the time for students to structure their knowledge base before medical school. Regarding your question about rotation, I also liked it better when rotation was on the AAMC syllabus because if you can't distinguish translational and rotational mechanics you are cutting the stem of a flower that will keep blooming throughout science, for example, the difference is why a diatomic gas like hydrogen has a higher molar heat capacity than helium.

With regard to Berkeley Review from my own correspondence with one of the founders I know if you have serious questions about curriculum in their materials they are very open as teachers and will almost certainly give you a substantive reply.

The main point of this TL DR comment is that as a member of, for want of any credential, the 1000 question fifty teaching cycle club is that some things in an MCAT Prep curriculum or work, whether Kaplan, TBR, Examkrackers, or any others, may not correspond to a 12 week digestion, which is the prevailing SDN opinion, nor the MCAT review syllabus published by AAMC. I want to voice my personal opinion that it is impossible, when AAMC takes something off their syllabus, to reconcile the difficulties that ensue in one's mind as a teacher who uses content for teaching purposes.

I respect Berkeley review and defend their right to include rotation as an important topic in a review course covering general science for premedical students despite what is on the AAMC syllabus. There is no conflict. It is not disrespecting AAMC. They understand time is precious and they are being less onerous. They don't want you to have to work harder than they did, but you have to know a lot more molecular biology than they did, so something has to go. Put alkenes, electrophilic aromatic substitution, and any kind of intense quantitative problem solving using the radian derived quantities of rotation in the 'thank you AAMC for making this easier, though I still need to be deeply familiar with these ideas' folder.

The way to the highest score is to leave off MCAT score as a goal and pursue mastery. I've been reading Bhagavad Gita though I think Krishna just threw me off of the chariot.

My opinion is that the greatest danger in lost potential for self and society is for the people who are so smart that they reflect social acceptance of themselves as mastery; prepare for the MCAT in a month and make a 38. I confess. I was this person. When I took the MCAT in 1994 I had a BA in English from Stanford. I took no practice tests nor had no biochemistry nor physics or chemistry for seven yars I looked at the pictures in my biology textbook and read Linus Pauling's General Chemistry and went in. I learned what I had missed by tutoring students in MCAT prep. I was recovering from a power tool injury teaching one-on-one. With my BA in English from Stanford I was a good all around tutor. A person can miss the opportunity to walk around the mountain of general science and make mental progress in understanding how science fits together by not taking the time and looking at MCAT preparation in the light of mastery. It's my opinion that professionalism and careerism are at odds, at least in the worst case when the score is the end-all-and-be-all. I think this prioritization is a fallacy among students that leads to misinterpreting the relationship of the AAMC syllabus to the relationship of various MCAT prep curricula to the most important learning goals. I think as a general disposition with the Berkeley Review materials you will benefit by trusting them, and let yourself off timewise with the non-AAMC topics by the exercise of looking at how an individual topic reflects 'for the rest' rather than 'in itself'.

Students who may be not so awesome with a number two pencil will get further than the smartest through MCAT preparation, but sometimes the smartest will have a knack for understanding how things fit together and become teachers. I feel strongly that this description applies to the folks who put together the course and materials at Berkeley Review. Try to instill a trust in them as teachers. Alhough the Berkeley Review materials exist in the same Wild West of MCAT Review which produces work like Barrons or the execrable REI book, there is much teacherly responsibility in the coursework. There may be editorial inertia or insufficient orientation with the printed materials. Somethings may be more or less represented. However, in their work the prevailing intention is to make a representamen of general scientific knowledge and transmit it. ExamKrackers has this same bug. Brett Ferdinand too I think. When you look at the work there is a lot of thought and teaching behind what is included. Sometimes what you see doesn't conform to normative influences outside of specialized prerogatives of individual MCAT teachers reflecting their experience working closely with students reviewing general science, so some works may have material declared extraneous by AAMC. For my own part I even wish students would study plant biology in MCAT review and learn the animal phyla but I am ridiculous and TL DR.
 
Last edited:
There are some MCAT prep materials like the Barron's book or the execrable REI book that present information outside of the boundaries of the AAMC MCAT outline. This is established publishing imprints using their brand strength.

Even in the least carefully edited MCAT materials there can be useful values. Conversely, the most carefully edited MCAT materials, such as at TBR, will have a reason for the inclusion of a topical area of science though it may not strictly agree with the AAMC syllabus. There are reasonably sound biology question sets in the Barrons book. Barrons would almost be worth purchasing. In the nineties my students were asked to read Barrons from cover to cover in two weeks and then throw it away. I learned to say no you don't need to spend a day memorizing all of the cranial and spinal nerves. The danger of time wasters in a pressed-for-time MCAT review process is real for students.
The ghost of Benjamin Franklin would say that MCAT review should be seized by students, teachers, and society because of the social need. The AAMC is doing its best with the MCAT prep syllabus from a utilitarian perspective, trying to encourage knowledge base structuring for all students, using the impetus of the selection process so say here is what you should review before coming to medical school. However, the same utilitarianism gives them a chance not to be onerous, so they try to say here is where we are giving you a break. Don't study electrophilic addition of alkenes. I think they decided this because they want to give students a break, and themselves a break, from advanced students missing questions because their Organic prof was adamant that they always specify isolation from any peroxide source in the Markovnikov decision tree, and then there must be another day studying rearrangements, the halogenation intermediates and all of those steric considerations. I think they just decided to give students a break. After all the MCAT must also reflect the molecular biology revolution these days.

In my opinion I think the AAMC syllabus of the nineties was better because it had rotation, the subject of your question, alkene reactions, and electrophilic aromatic substitution. My feeling is that omission of those topics has been a mistake. While giving students time to learn molecular biology revolution, it casts a normative message at the expense of ultimately understanding biochemistry. What gets left out of MCAT review is important because MCAT review is the time for students to structure their knowledge base before medical school. Regarding your question about rotation, I also liked it better when rotation was on the AAMC syllabus because if you can't distinguish translational and rotational mechanics you are cutting the stem of a flower that will keep blooming throughout science, for example, the difference is why a diatomic gas like hydrogen has a higher molar heat capacity than helium.

With regard to Berkeley Review from my own correspondence with one of the founders I know if you have serious questions about curriculum in their materials they are very open as teachers and will almost certainly give you a substantive reply.

The main point of this TL DR comment is that as a member of, for want of any credential, the 1000 question fifty teaching cycle club is that some things in an MCAT Prep curriculum or work, whether Kaplan, TBR, Examkrackers, or any others, may not correspond to a 12 week digestion, which is the prevailing SDN opinion, nor the MCAT review syllabus published by AAMC. I want to voice my personal opinion that it is impossible, when AAMC takes something off their syllabus, to reconcile the difficulties that ensue in one's mind as a teacher who uses content for teaching purposes.

I respect Berkeley review and defend their right to include rotation as an important topic in a review course covering general science for premedical students despite what is on the AAMC syllabus. There is no conflict. It is not disrespecting AAMC. They understand time is precious and they are being less onerous. They don't want you to have to work harder than they did, but you have to know a lot more molecular biology than they did, so something has to go. Put alkenes, electrophilic aromatic substitution, and any kind of intense quantitative problem solving using the radian derived quantities of rotation in the 'thank you AAMC for making this easier, though I still need to be deeply familiar with these ideas' folder.

The way to the highest score is to leave off MCAT score as a goal and pursue mastery. I've been reading Bhagavad Gita though I think Krishna just threw me off of the chariot.

My opinion is that the greatest danger in lost potential for self and society is for the people who are so smart that they reflect social acceptance of themselves as mastery; prepare for the MCAT in a month and make a 38. I confess. I was this person. When I took the MCAT in 1994 I had a BA in English from Stanford. I took no practice tests nor had no biochemistry nor physics or chemistry for seven yars I looked at the pictures in my biology textbook and read Linus Pauling's General Chemistry and went in. I learned what I had missed by tutoring students in MCAT prep. I was recovering from a power tool injury teaching one-on-one. With my BA in English from Stanford I was a good all around tutor. A person can miss the opportunity to walk around the mountain of general science and make mental progress in understanding how science fits together by not taking the time and looking at MCAT preparation in the light of mastery. It's my opinion that professionalism and careerism are at odds, at least in the worst case when the score is the end-all-and-be-all. I think this prioritization is a fallacy among students that leads to misinterpreting the relationship of the AAMC syllabus to the relationship of various MCAT prep curricula to the most important learning goals. I think as a general disposition with the Berkeley Review materials you will benefit by trusting them, and let yourself off timewise with the non-AAMC topics by the exercise of looking at how an individual topic reflects 'for the rest' rather than 'in itself'.

Students who may be not so awesome with a number two pencil will get further than the smartest through MCAT preparation, but sometimes the smartest will have a knack for understanding how things fit together and become teachers. I feel strongly that this description applies to the folks who put together the course and materials at Berkeley Review. Try to instill a trust in them as teachers. Alhough the Berkeley Review materials exist in the same Wild West of MCAT Review which produces work like Barrons or the execrable REI book, there is much teacherly responsibility in the coursework. There may be editorial inertia or insufficient orientation with the printed materials. Somethings may be more or less represented. However, in their work the prevailing intention is to make a representamen of general scientific knowledge and transmit it. ExamKrackers has this same bug. Brett Ferdinand too I think. When you look at the work there is a lot of thought and teaching behind what is included. Sometimes what you see doesn't conform to normative influences outside of specialized prerogatives of individual MCAT teachers reflecting their experience working closely with students reviewing general science, so some works may have material declared extraneous by AAMC. For my own part I even wish students would study plant biology in MCAT review and learn the animal phyla but I am ridiculous and TL DR.

Dude, I notice all your posts are very long....how do you have time to write all that? Just saying.....
 
Members don't see this ad :)
Dude, I notice all your posts are very long....how do you have time to write all that? Just saying.....


I'm a fast writer with a generally clear first sentence, so the posts can be long without impinging on me. Writing can also be a process of critically assessing and improving ideas in itself, so there may be too much 'figuring things out' making it TLDR.

If I feel like there's something of interest in it for the audience here I'll go on. Some questions in MCAT preparation come up again and again. In this area the discussions seem to be circular through of the complex torsion of different positive and negative aspects, making decision-making difficult, such as the goal of learning to understand science as a an overall project, the need for effecive, efficient MCAT prep, the authority of the AAMC syllabus, and how to review curricula and materials fit in when AAMC drops an item from the main list.

I see a lot of questions on behalf of students trying to modulate the attention on non-MCAT material. I think students are way too much 'in itself' with scientific topics for preparing for today's MCAT anyway, so there are points to justify the role of topics in the structure of science. These are really important question for authors, students, and teachers, so I thought I would share my impression of how Berkeley Review might approach these issues given that I've had some conversations, background, experience etc.

I was trying to keep an inside rule I have for SDN to impose an MCAT essay time on a single post and write in thesis, antithesis synthesis style in the one above. I even cut out some stuff about Kaplan's approach. I know a long post is kind of a drag.
 
I'm the lead author for the new, completely redone Barron's MCAT (October 2011).

I just wanted to point out that the new Barron's book is completely different. The old book was, let's say, passable. The new one has been redone by scratch by a new team of authors and educators.

I won't ramble on about it here. Search my posts for details. Just wanted to give a heads up about the new book, which is especially strong in the much needed area of strategy for timing, testing, VR, and scientific problem solving.


Great job advertising your product without even attempting to address the question in the original post.

This right here shows the quality and level of commitment you get when you purchase barron's products. 👍

OP:

v= tangential velocity.
omega = angular velocity.

The angular velocity (omega) is perpendicular to the tangential velocity. So, since in problem 23, the person is walking toward the axis of rotation, they are walking perpendicular to the direction of v, which if you draw the circle (merry go round) you will draw as a tangent at any point that you choose.

Here is what omega looks like.

http://en.wikipedia.org/wiki/File:Angular_velocity.svg
 
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