Strategy question - TBR.

[MedPR]

---

Members do not see this ad.

So one of TBR's strategies (one of my favorites, too) for those formula identification problems is to look at each variable and try to decide what would happen if you increased/decreased the variable.

For instance, if it wanted you to identify the correct equation for acceleration and your two choices were a=F/m and a=m/F, you could simply think about whether or not you would expect increasing force to increase or decrease acceleration. Since an increase in force increases acceleration, you know the answer is a=F/m (pretend you didn't have F=ma memorized!)

So on to my question. Is this strategy really a viable one for the MCAT? 100% of the formula identification problems I've seen in TBR can be solved (rather easily, I might add) using this method. I've gotten very comfortable with it and it is pretty much the only method I use to solve such problems, but I don't want to get too comfortable with it if it is not as useful on the real MCAT.

 

[milski]

That plus plugging in units are my major methods of verifying formulas that I don't remember well. It may be a bit less helpful if you're trying to decide between ab/r vs ab/r^2 but in general is a very viable method.

 

[MedPR]

That plus plugging in units are my major methods of verifying formulas that I don't remember well. It may be a bit less helpful if you're trying to decide between ab/r vs ab/r^2 but in general is a very viable method.

I've tried this method, but it doesn't work well for me. I'm too slow with the units most times. That plus I don't remember some of the more complicated units like Watts or amps or any of that stuff.

I know that 1N = 1kgm/s^2 and 1J = 1Nm, but beyond that I probably have no clue.

 

[milski]

I've tried this method, but it doesn't work well for me. I'm too slow with the units most times. That plus I don't remember some of the more complicated units like Watts or amps or any of that stuff.

I know that 1N = 1kgm/s^2 and 1J = 1Nm, but beyond that I probably have no clue.

They are complementary - use whatever is convenient and remember at the time. MT Headed's method of taking things to extreme is useful as well - what happens to the acceleration if we make the mass infinite or zero?

 

[MedPR]

They are complementary - use whatever is convenient and remember at the time. MT Headed's method of taking things to extreme is useful as well - what happens to the acceleration if we make the mass infinite or zero?

Yea I use that one as well from time to time! My first try is always the extremes of increasing/decreasing a variable.

 

[chiddler]

furthermore, why would TBR suggest bad strategy!

it's useful if the answers are distinct enough. like milski said, if it's x/y vs sqrt(x/y), then won't work very well.

 

[MedPR]

furthermore, why would TBR suggest bad strategy!

it's useful if the answers are distinct enough. like milski said, if it's x/y vs sqrt(x/y), then won't work very well.

I guess I was asking to try and get an idea of just how good of a strategy it is. I mean, we have all these mnemonics and tricks to remember stuff, but often times it isn't enough just to recall the facts. For instance, TBR taught me that diverging mirrors/lenses always create small, upright, virtual images that are located inside of the focal length (SUV). This trick might help me rule out two answers on the MCAT, but if the other two are small, upright, virtual, I will have to do a little more work.

I should've been more clear in the OP.

So far, the TBR strategies have worked for to rule out every single answer but 1. I'm sure it will sometimes be that way on the MCAT, but not all the time, right?

 

[chiddler]

I've always wondered. When you see questions of this type, what do you do first?

My first instinct is to try to solve it the old fashioned way, which worries me that it might be too slow.

But if I do the strategy written or dimensional analysis it does not always work.

 

[MedPR]

I've always wondered. When you see questions of this type, what do you do first?

My first instinct is to try to solve it the old fashioned way, which worries me that it might be too slow.

But if I do the strategy written or dimensional analysis it does not always work.

My first strategy is to pick the variable that I know most about. Then consider what increasing/decreasing that variable would do and from there decide if it should be in the numerator or the denominator. Doing that 1 variable at a time is pretty quick and can usually rule out multiple answers.

 

[chiddler]

Here's a question with which the strategy is difficult:

A particle with a positive charge q and mass m moving with speed v undergoes uniform circular motion in a mag field B. If radius is r, which expression gives the particle's orbit period; in other words, the time required for the particle to complete one revolution?

A. 2π/qvB
B. 2πm/qB
C. qvB/2πm
D. qB/2πm

answer

 

[milski]

Here's a question with which the strategy is difficult:

A particle with a positive charge q and mass m moving with speed v undergoes uniform circular motion in a mag field B. If radius is r, which expression gives the particle's orbit period; in other words, the time required for the particle to complete one revolution?

A. 2π/qvB
B. 2πm/qB
C. qvB/2πm
D. qB/2πm

answer

I don't know, I guess it depends on what you are sure about. I certainly don't remember the formula for the period. The first thing that I considered was the charge. Since the speed is constant, higher charge will generate a large force which will correspond to higher acceleration leading to smaller circle which in turn leads to knowing that q has to be in the denominator. So it's A or B. Next question - does it depend on the mass? The force will not depend on the mass but the acceleration will, so higher mass will make for lower acceleration and larger period. Thus it's B.

In retrospective, considering only the mass should have been enough. The thinking about B should be about the same as Q.

If they wanted to make it interesting, they should have had 2πm/qvB as answer A. The presence or not of v seems a bit less trivial to deduce.

 

[chiddler]

I don't know, I guess it depends on what you are sure about. I certainly don't remember the formula for the period. The first thing that I considered was the charge. Since the speed is constant, higher charge will generate a large force which will correspond to higher acceleration leading to smaller circle which in turn leads to knowing that q has to be in the denominator. So it's A or B. Next question - does it depend on the mass? The force will not depend on the mass but the acceleration will, so higher mass will make for lower acceleration and larger period. Thus it's B.

In retrospective, considering only the mass should have been enough. The thinking about B should be about the same as Q.

If they wanted to make it interesting, they should have had 2πm/qvB as answer A. The presence or not of v seems a bit less trivial to deduce.

damn.

 

[MedPR]

Here's a question with which the strategy is difficult:

A particle with a positive charge q and mass m moving with speed v undergoes uniform circular motion in a mag field B. If radius is r, which expression gives the particle's orbit period; in other words, the time required for the particle to complete one revolution?

A. 2π/qvB
B. 2πm/qB
C. qvB/2πm
D. qB/2πm

answer

I'm guessing that means B is the answer? I guessed B because I remembered F=qvB, so increasing B and q increase the force = increase acceleration = decrease period. So q and B should be in the denominator. Since acceleration is constant (uniform circular motion) v shouldn't even be in the equation. In addition, increasing mass should increase the period, so it should be in the numerator.

 

[chiddler]

yeah

well..

screw you guys.

i got it wrong.

 

[milski]

yeah

well..

screw you guys.

i got it wrong.

Oh well, after my miserable ochem midterm, I did need something to cheer me up. 😛

 

[chiddler]

Oh well, after my miserable ochem midterm, I did need something to cheer me up. 😛

aw 🙁

don't worry ochem gets even the best of us.

 

[milski]

aw 🙁

don't worry ochem gets even the best of us.

Thanks. It'll be ok, I'm just whining before I have to sit and study more.

 

[MrNeuro]

yeah

well..

screw you guys.

i got it wrong.

LOOOOLLL SAME

medpr and milski got this stuff down