# physics topics too hard for me

#### tank you

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include torque and electromagnetism.
does anyone else find these 2 topic difficult?

#### Parscope

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Electromagnetism is sort of abstract, but torque isn't too bad.

Torque is just the measure of twisting strength. If you have a rod with a lever perpendicular to its axis, the force at the end of the lever, times the distance from the center of the rod is the torque.

Think about a see saw... a fat kid sits close to the fulcrum while a skinny kid sits out towards the end of the see saw. They are both in the air, balanced perfectly because the torque from the fat kid (big force x short distance from fulcrum) is the same as the torque from the skinny kid (small force x large distance from fulcrum).

Electromagnetism is a little bit broader subject, but here are some of the basics.
The same rules of physics apply to these problems as any others; the force just comes from a different source... Moments in electromagnetism are the same as the torque problems above; the charge in a field supplies the force and the distance of the charge from the center is the distance. Coulomb's law, F = k(Q1*Q2)/r^2, gives the force exerted on one charge by another based on their charge strength and distance. To determine the strength of an electric field you take Coulombs law and let one of the charges go to 0, giving you: E = k*Q/r^2. An E field value times a charge will give you the force on that charge. Field lines are always drawn from + to - and are just tools to show the strength and nature of interaction of different fields. Other than that, just think magnets; like charges repel and opposites attract.

Someone else can probably explain electromagnetism better than I can.

#### NRAI2001

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I have trouble with center of mass questions, which kinda use torque also.

#### cfdavid

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If you try changing a tire with a short shafted wrench, and then switch to a longer one, you'll no longer have a problem intuitively understanding torque!

#### Shrike

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NRAI2001 said:
I have trouble with center of mass questions, which kinda use torque also.
The trouble is probably that the way you were taught center of mass has something to do with torque.

For c.o.m.:

1. if it's a uniform object, the c.o.m. is at the center.

2. If there's more than one point mass, you need the one formula (sorry): sum of (distance times mass) divided by (sum of mass). Distance from where? You get to pick. Pick one you like.

3. If there's more than one mass and they're not point masses, use rule 1 first, and then 2; on the MCAT this is enough for the hardest problems.

4. In the very rare case where you need c.o.m. in two dimensions, do one at a time, just as described above.

5. On the MCAT the center of gravity and the center of mass are identical, so always do center of mass, because the equation is less scary. Yes, I hereby give you permission to cross out the c.o.g. equation in your book, and never to look at it again.​

The tricks, to the extent there are any, are, first, to pick a nice happy origin, ideally under one of the masses so one of the (distance times mass) terms goes away, and even better would be to have it under the mass on one end or the other, so you don't screw with negative signs (even better than that would be always to put it under the left-hand mass, so the problem always looks the same to you); second, to be sure to remember to add all the masses in the bottom of the equation, including the one at the origin that you picked; and third, to be sure you use all the masses -- it's easy to forget the mass of the meter stick or board or whatever it is connecting the two or three objects.

Is that so hard?

#### stoleyerscrubz

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I remember how amazing it felt to see how effortless it was using a cheater pipe to take off the nut(frozen by time) holding my brake drum. I use to waste a lot of time fighting to take off nuts & bolts on my VW beetle.

cfdavid said:
If you try changing a tire with a short shafted wrench, and then switch to a longer one, you'll no longer have a problem intuitively understanding torque!

#### NRAI2001

##### 3K Member
15+ Year Member
Shrike said:
The trouble is probably that the way you were taught center of mass has something to do with torque.

For c.o.m.:

1. if it's a uniform object, the c.o.m. is at the center.

2. If there's more than one point mass, you need the one formula (sorry): sum of (distance times mass) divided by (sum of mass). Distance from where? You get to pick. Pick one you like.

3. If there's more than one mass and they're not point masses, use rule 1 first, and then 2; on the MCAT this is enough for the hardest problems.

4. In the very rare case where you need c.o.m. in two dimensions, do one at a time, just as described above.

5. On the MCAT the center of gravity and the center of mass are identical, so always do center of mass, because the equation is less scary. Yes, I hereby give you permission to cross out the c.o.g. equation in your book, and never to look at it again.​

The tricks, to the extent there are any, are, first, to pick a nice happy origin, ideally under one of the masses so one of the (distance times mass) terms goes away, and even better would be to have it under the mass on one end or the other, so you don't screw with negative signs (even better than that would be always to put it under the left-hand mass, so the problem always looks the same to you); second, to be sure to remember to add all the masses in the bottom of the equation, including the one at the origin that you picked; and third, to be sure you use all the masses -- it's easy to forget the mass of the meter stick or board or whatever it is connecting the two or three objects.

Is that so hard?
Wow, thanks a lot!

#### futuredoc10

##### Senior Member
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Shrike said:
The trouble is probably that the way you were taught center of mass has something to do with torque.

For c.o.m.:

1. if it's a uniform object, the c.o.m. is at the center.

2. If there's more than one point mass, you need the one formula (sorry): sum of (distance times mass) divided by (sum of mass). Distance from where? You get to pick. Pick one you like.

3. If there's more than one mass and they're not point masses, use rule 1 first, and then 2; on the MCAT this is enough for the hardest problems.

4. In the very rare case where you need c.o.m. in two dimensions, do one at a time, just as described above.

5. On the MCAT the center of gravity and the center of mass are identical, so always do center of mass, because the equation is less scary. Yes, I hereby give you permission to cross out the c.o.g. equation in your book, and never to look at it again.​

The tricks, to the extent there are any, are, first, to pick a nice happy origin, ideally under one of the masses so one of the (distance times mass) terms goes away, and even better would be to have it under the mass on one end or the other, so you don't screw with negative signs (even better than that would be always to put it under the left-hand mass, so the problem always looks the same to you); second, to be sure to remember to add all the masses in the bottom of the equation, including the one at the origin that you picked; and third, to be sure you use all the masses -- it's easy to forget the mass of the meter stick or board or whatever it is connecting the two or three objects.

Is that so hard?
Well that about sums it up.

#### gujuDoc

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jtank said:
include torque and electromagnetism.
does anyone else find these 2 topic difficult?

I'm just posting to say that I love your signature about mkittens vs. mcat. That's cute.

#### Daichi Katase

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i have nothing insightful to add.

i think the topic i hate the most is optics.

#### gujuDoc

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DetectiveChubby said:
i have nothing insightful to add.

i think the topic i hate the most is optics.

Aw, optics isn't that bad. Actually i prefer it to dynamics and other such physics one topics.

#### cfdavid

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stoleyerscrubz said:
I remember how amazing it felt to see how effortless it was using a cheater pipe to take off the nut(frozen by time) holding my brake drum. I use to waste a lot of time fighting to take off nuts & bolts on my VW beetle.

Good old cheater pipes.. In high school, I worked for a landscaping company. We'd have to latch down the loaders to the trailer with these chain latches, and you could only do it with a cheater bar. I too, remember struggling with the measley 14" lever. Then, the driver would get this big a.s cheater bar from behind the cab, and problem solved.

#### Shrike

##### Lanius examinatianus
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Now I'm thinking of actually taking my torque wrench to class. It's a cheapo, so we could show the stress/strain relationship (it's a bar gauge), and the effect of the length of the handle (it's really long, about half a meter -- I use it as a breaker bar) and the importance of that length (the handgrip pivots, so the force is always applied the same distance from the business end).

Seriously, I think nearly everyone understands torque, they just get freaked out by the frightening-looking equations, with Greek letters even. So in my classes we just talk about it for a while, and get the equation later. And the lever-arm method -- it doesn't really matter, as it's just a convenient way to do the math, so I make sure people understand that they can feel free to ignore it. Now there's just one equation which, if you think about it for a while, happens to make pefect sense.

If only I could do that for magnetism. The teacher who can make cross products intuitive is truly a genius.

Shrike
TPR physics et al; always learning how best to teach

#### Shrike

##### Lanius examinatianus
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DetectiveChubby said:
i think the topic i hate the most is optics.
I haven't prepped my optics lecture since last summer. Once I have, I will post the quick version of how to do MCAT optics problems, and all will become clear. Clear, and with a fairly low index of refraction.

#### jigglyboo

##### Member
10+ Year Member
EK's 1001 physics ?'s has some great inertia and torque questions. I'm learning a whole lot more from it than from my university or kaplan. here's a good one for you to get your hands on that i had a hard time with:

A one meter board with uniform density, hangs in static eqbm from a rope with tension T. A weight hangs from the left end of the board as shown. What is the mass of the board?

------ ]T
-.2m- ]
===================
]
[3kg]

PM if you really want the answer

#### Topper Harley

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I hate general chemistry questions...

alot!

#### Topper Harley

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Shrike said:
I haven't prepped my optics lecture since last summer. Once I have, I will post the quick version of how to do MCAT optics problems, and all will become clear. Clear, and with a fairly low index of refraction.
ROTFLMAO

#### drguy22

##### 1K Member
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jtank said:
include torque and electromagnetism.
does anyone else find these 2 topic difficult?

i know its difficult to understand..i hate physics with a passion...however, trust me on this when i say F*** the understanding just memorize memorize memorize....i tried to understand physics the first time i took the MCAT and its didnt work out soo well (7) the second time however, i just MEMORIZED the stuff, and i got a 10....Good luck!

#### Shrike

##### Lanius examinatianus
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drguy22 said:
. . . trust me on this when i say F*** the understanding just memorize memorize memorize....i tried to understand physics the first time i took the MCAT and its didnt work out soo well (7) the second time however, i just MEMORIZED the stuff, and i got a 10....Good luck!
Congratulations, but. . .

Your single experience says that rote memorization has worked; on that basis you advise others to follow your example. My experience, working with hundreds of MCAT takers, says it's not so simple. Some memorize and do better than they had been doing. However, my impression is that the memorizers don't do as well as those who learn (1) the concepts, in areas susceptible to conceptual analysis, and (2) the proper approaches to MCAT-style questions, in all areas. Also, the memorizers seem to plateau at around 10; to do better, conceptual knowledge seems to be necessary.

Of course, most readers of this forum would be pleased to score 10 in physical sciences; the bigger problem is that most memorizers never get even that good. Consider: if I offered you a 100% chance at an 8, or a 45% chance at a 12, which would you take? Most applicants would take the gamble, because the difference between almost getting accepted and not even coming close, is nil. Even if the choice were between a guaranteed 9 and a 45% shot at a 12, a lot would still take the gamble.

I believe, based on having worked with hundreds of students in past couple years, that a majority will maximize their expectations of scoring high enough to get interviewed by focusing on conceptual knowledge and MCAT problem structure, with a safety net of 20-30 equations (this being most important in the non-conceptual areas, particularly E&M).

Look at the salt with which people should consider our respective positions. Your experience is good, solid data -- the same person tried two different approaches, each approach being available to other MCAT takers. The relationship may well be causal, and the data are precise. But it's a sample size of one. I can't impute causation as easily because I am not comparing the two approaches for individual students, and the data are also not terribly precise. Also, there may be a big difference between taking a conceptual approach on one's own and doing so in my class*; a forum lurker considering what strategy to adopt is unlikey to have the latter option available. On the other hand, I've seen a whole lot of students, probably too many to be completely off base.

The question is what is likely to be best for someone else. We have different views. I can't prove it, you can't prove it. Readers should consider carefully before taking anyone's advice, my own included: what does the poster's experience say about what is likely to work for me?

Shrike
TPR physics, verbal, bio

--------------
* Why should this make such a difference? My class is 22.5 hours of physics lessons and passages, plus 4-6 hours of review sessions, during all of which I focus exclusively on how to solve MCAT problems. I have great flexibility in what I teach and how I teach it, partly because TPR allows some of this and partly because I do what I think is right, almost regardless what's officially sanctioned. And I've done this before, many times, each time refining the method based on how I see students responding to what I do.

MCATers who don't have such a class available to them are making a decision that is at least somewhat different. I don't know how different.

OP
T

#### tank you

##### 2K Member
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Shrike said:
Congratulations, but. . .

Your single experience says that rote memorization has worked; on that basis you advise others to follow your example. My experience, working with hundreds of MCAT takers, says it's not so simple. Some memorize and do better than they had been doing. However, my impression is that the memorizers don't do as well as those who learn (1) the concepts, in areas susceptible to conceptual analysis, and (2) the proper approaches to MCAT-style questions, in all areas. Also, the memorizers seem to plateau at around 10; to do better, conceptual knowledge seems to be necessary.

Of course, most readers of this forum would be pleased to score 10 in physical sciences; the bigger problem is that most memorizers never get even that good. Consider: if I offered you a 100% chance at an 8, or a 45% chance at a 12, which would you take? Most applicants would take the gamble, because the difference between almost getting accepted and not even coming close, is nil. Even if the choice were between a guaranteed 9 and a 45% shot at a 12, a lot would still take the gamble.

I believe, based on having worked with hundreds of students in past couple years, that a majority will maximize their expectations of scoring high enough to get interviewed by focusing on conceptual knowledge and MCAT problem structure, with a safety net of 20-30 equations (this being most important in the non-conceptual areas, particularly E&M).

Look at the salt with which people should consider our respective positions. Your experience is good, solid data -- the same person tried two different approaches, each approach being available to other MCAT takers. The relationship may well be causal, and the data are precise. But it's a sample size of one. I can't impute causation as easily because I am not comparing the two approaches for individual students, and the data are also not terribly precise. Also, there may be a big difference between taking a conceptual approach on one's own and doing so in my class*; a forum lurker considering what strategy to adopt is unlikey to have the latter option available. On the other hand, I've seen a whole lot of students, probably too many to be completely off base.

The question is what is likely to be best for someone else. We have different views. I can't prove it, you can't prove it. Readers should consider carefully before taking anyone's advice, my own included: what does the poster's experience say about what is likely to work for me?

Shrike
TPR physics, verbal, bio

--------------
* Why should this make such a difference? My class is 22.5 hours of physics lessons and passages, plus 4-6 hours of review sessions, during all of which I focus exclusively on how to solve MCAT problems. I have great flexibility in what I teach and how I teach it, partly because TPR allows some of this and partly because I do what I think is right, almost regardless what's officially sanctioned. And I've done this before, many times, each time refining the method based on how I see students responding to what I do.

MCATers who don't have such a class available to them are making a decision that is at least somewhat different. I don't know how different.
thanks for the clarification.
do you have any advice on torque/EM problems?