Physics question thread

[Shrike]

All users may post questions about MCAT and OAT physics here. We will answer the questions as soon as we reasonably can. If you would like to know what physics topics appear on the MCAT, you should check the MCAT Student Manual (http://www.aamc.org/students/mcat/studentmanual/start.htm), though be warned, there are subjects listed there that are rarely tested, or that appear in passages only and need not be learned.

Be sure to check the Physics FAQs and Topic Writeups thread if you have a general question; eventually, many of your answers will be located there. Also, a request: to keep this thread at least somewhat neat, when replying to someone else's post please refrain from quoting anything more than what's necessary for clarity.

Acceptable topics:

• general, MCAT-level physics
• particular MCAT-level physics problems, whether your own or from study material
• what you need to know about physics for the MCAT
• how best to approach to MCAT physics passages
• how best to study MCAT physics
• how best to tackle the MCAT physical sciences section

Unacceptable topics:

• actual MCAT questions or passages, or close paraphrasings thereof
• anything you know to be beyond the scope of the MCAT

Side note: anyone who knows how to post subscripts and superscripts in this system, please PM me the method. I also wouldn't mind knowing how to post some obvious symbols, such as Greek letters and an infinty sign. Should be a matter of changing fonts, but I don't know what's available; again, a PM would be appreciated.

If you really know your physics, I can use your help. If you are willing to help answer questions on this thread, please let me know. Here are the current official contributors to the this thread -- a team to which I hope to add more people:

Thread moderated by: Shrike. Shrike is a full-time instructor for The Princeton Review; he has taken the MCAT twice for no good reason, scoring 14 on the physical sciences section each time. He majored in mathematics, minored in physics, and spent several years accumulating unused school experience (in economics and law).

Also answering questions: Xanthines, a Kaplan instructor. He scored 13 on the PS section of the MCAT and 34 overall.

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[mostwanted]

Hi shrike, can you please go over torque and discuss rotational equilibrium? and how do u approach torque problems? ty

 

[freesolo]

i dread seeing sound passages, i dont get this stuff about open end tube closed end tube string tied on both ends! I dont get any of the sound stuff at all other than v=fwavelength =/. What equations do we use for each situation, and what is n! its not nodes is all i remember

 

[Shrike]

Questions pending about torque and rotational equilibrium, and sound. I'm sorry, I haven't had time to get to these, but I will. Unfortunately, neither is a simple subject; they take some work to write up. May not get to them before the 4th.

 

[blankguy]

I have a pulley question. I understand that pulley is 2 ropes of a pulley supporting the weight mg of an object as opposed to having a single rope with tension T. The pulley is 2 ropes with 1/2 the tension of the single rope supporting weight mg, but I don't understand how to get the force needing to tug the rope in the pulley on the other end of the other end of the pulley.

 

[Shrike]

I have a pulley question...

I reluctantly admit that I have no idea what you're asking. Could you rephrase, please?

 

[blankguy]

I reluctantly admit that I have no idea what you're asking. Could you rephrase, please?

What is the relation between the force applied on the rope to tug it so that a weigh gets lifted on a pulley?

 

[psiyung]

I reluctantly admit that I have no idea what you're asking. Could you rephrase, please?

 

[gujuDoc]

I don't know when you'd need tangent on the MCAT. Sine and cosine, you need only for a few angles:

• sin(30) = 0.5; cos(30) = 0.9
• sin(45) = 0.7; cos(45) = 0.7
• sin(60) = 0.9; cos(60) = 0.5

There is something I wanted to add to this that TPR teaches:

Sin 0 = sq rt.(0)/2 = 0
Sin 30 = sq rt. (1)/2 = .5
Sin 45 = sq rt (2)/2 = .7
Sin 60 = sq rt (3)/2 = .85 or roughly .9 as Shrike pointed out above
Sin 90 = sq rt. (4)/2 = 1

For cosine, it is the exact reverse of the values for Sine. The TPR books listed it this way in the appendix with all the formulas for the Physics section as well as in the math appendix at the end of the Physical sciences book. Thinking of it this way helped me a bit to remember it, so I thought it would be useful to point that out.

The very opposite of that is the trend for Cosine.

 

[Nitya2284]

I was looking at the examkrackers book and it doesn't do go into much detail about the center of mass and center of gravity. what do we have to knwo about the center of mass and the center of gravity?

 

[gujuDoc]

This might be a stupid question, but can you explain the relationship between work and kinetic enery/potential energy?????? A friend of mine studying for the MCAT wanted me to ask you here.

 

[Nitya2284]

This might be a stupid question, but can you explain the relationship between work and kinetic enery/potential energy?????? A friend of mine studying for the MCAT wanted me to ask you here.

Work is defined as force * distance and in other words it is related to potential energy and kinetic energy by a theory called work/energy theory where work = mgh or (1/2 mv^2). Let me know if that helps..

 

[smiley98]

Okay this question deals with circuits... Let E be the strength of the electric field between the plates at the end of the set-up procedure, and let E1 be teh strength of the field after the insertion of the dielectric in Experiment #1. How does E1 compare to E? The set up procedure just charges the capacitor using a battery then exp. 1 just inserts a insulator while the battery is still connected (dielectric of value 4).
THe answer is that they equal each other E=E1 but what I don't understand is when an dielectric is inserted doesn't that decrease the electric field thus decreasing the voltage and increasing the capacitance? How could the electric fields be equal to each other?

 

[dat_student]

Work is defined as force * distance and in other words it is related to potential energy and kinetic energy by a theory called work/energy theory where work = mgh or (1/2 mv^2). Let me know if that helps..

WRONG
work is NOT always equal to the change in potential energy but it is always equal to the change in the kinetic energy

here is the proof for W = delta KE

I assume V1 = 0 to make calculations simpler but the calculations for V1 != 0 is the same:

w = f * d = (m * a) d (eq. #1)
d = [(V1 + V2)/2] * t (eq. #2)
Assuming V1 = 0 & given eq #1 & eq. #2
W = (m * V2 - 0 / t) * (V2 + 0)/2 * t = (m * V2 / t) * (V2) / 2 * t
W = 1/2 m * V2^2

 

[hippocampus]

sound travels faster in what? less dense or denser mediums?

 

[Nutmeg]

Faster in denser media. The more closely packed the atoms, the more quickly the compression will propagate.

 

[Nutmeg]

Okay this question deals with circuits... Let E be the strength of the electric field between the plates at the end of the set-up procedure, and let E1 be teh strength of the field after the insertion of the dielectric in Experiment #1. How does E1 compare to E? The set up procedure just charges the capacitor using a battery then exp. 1 just inserts a insulator while the battery is still connected (dielectric of value 4).
THe answer is that they equal each other E=E1 but what I don't understand is when an dielectric is inserted doesn't that decrease the electric field thus decreasing the voltage and increasing the capacitance? How could the electric fields be equal to each other?

The function of the dielectric is to create an opposing electric field by assuming a polarity in the opposite direction. For a given charge, the electric field would end up lower with the dielectric than without. But in the set up you describe, the voltage drop is controlled by the battery--the voltage drop of the capacitor must remain equal to the voltage drop of the battery (remember Kirchhoff's voltage law: The directed sum of the electrical potential differences around a circuit must sum to zero). Hence the electric field is fixed because the voltage is fixed. The dielectric merely acts to increase the charge on the plates.

 

[gujuDoc]

Work is defined as force * distance and in other words it is related to potential energy and kinetic energy by a theory called work/energy theory where work = mgh or (1/2 mv^2). Let me know if that helps..

No offense, but I was hoping shrike could give a more detailed response which is why I posted it here. I know what the work-energy theorem is but I was asking a theoretical/conceptual question about why it is what it is. Something he or psuying might have better insight on. But thanks.

 

[gujuDoc]

WRONG
work is NOT always equal to the change in potential energy but it is always equal to the change in the kinetic energy

here is the proof for W = delta KE

I assume V1 = 0 to make calculations simpler but the calculations for V1 != 0 is the same:

w = f * d = (m * a) d (eq. #1)
d = [(V1 + V2)/2] * t (eq. #2)
Assuming V1 = 0 & given eq #1 & eq. #2
W = (m * V2 - 0 / t) * (V2 + 0)/2 * t = (m * V2 / t) * (V2) / 2 * t
W = 1/2 m * V2^2

THANKS A BUNCH. That makes more sense.

 

[frany584]

Hi, I've been reading about Standing waves in my Kaplan book and I'm pretty confused! It discusses the equations for finding the wavelength or length of standing waves, which for a wave fixed on both ends the wavelength =2L, 2L/3, 2L/4....2L/n (n=1,2,3....) I know n is the designated harmonic, meaning n=1 is the first harmonic, but if you are given a question with only a diagram of a string, how do u know what n equals? Can the same apply with open/closed pipes?

 

[QofQuimica]

Faster in denser media. The more closely packed the atoms, the more quickly the compression will propagate.

merging this thread with the physics question thread.

 

[Neurolemma]

WRONG
work is NOT always equal to the change in potential energy but it is always equal to the change in the kinetic energy

Actually it depends on what type of work you're talking about. The work done by a force (for MCAT purposes, a linear force) over a given displacement is equal to the change in kinetic energy. But the work done by the gravitational force is indeed equal to the change in potential energy.

 

[dat_student]

Actually it depends on what type of work you're talking about. The work done by a force (for MCAT purposes, a linear force) over a given displacement is equal to the change in kinetic energy. But the work done by the gravitational force is indeed equal to the change in potential energy.

Well, like I said, you can "ALWAYS" say work is equal to change in kinetic energy but you can "SOMETIMES" say work is equal to the change in potential energy. In other words, work is equal to the change in potential energy only when delta KE = delta PE.

 

[Nitya2284]

WRONG
work is NOT always equal to the change in potential energy but it is always equal to the change in the kinetic energy

here is the proof for W = delta KE

I assume V1 = 0 to make calculations simpler but the calculations for V1 != 0 is the same:

w = f * d = (m * a) d (eq. #1)
d = [(V1 + V2)/2] * t (eq. #2)
Assuming V1 = 0 & given eq #1 & eq. #2
W = (m * V2 - 0 / t) * (V2 + 0)/2 * t = (m * V2 / t) * (V2) / 2 * t
W = 1/2 m * V2^2

I'm pretty sure to the work energy theory, its equal to both potential and kinetic energy. I don't quite understand you're so called proof??

 

[Nitya2284]

I'm pretty sure to the work energy theory, its equal to both potential and kinetic energy. I don't quite understand you're so called proof??

k yeah i get it..thanks! that helps..i just didn't look it at right

 

[dat_student]

I'm pretty sure to the work energy theory, its equal to both potential and kinetic energy. I don't quite understand you're so called proof??

tell you what...directly derive "work = f * d = mg* delta h" without setting a to g and I'll agree with you.

 

[hippocampus]

sound travels faster in what? less dense or denser mediums?

so how come exankrackers said that...

when its humid, water vapor goes into the air. air is made up of co2 and n2 that has a greater mass than water. so if water vapor goes into the air, the mass decreases (18g/mol).. which decreases the density. ok, that makes sense. but then they said that sound travels faster in that.. it increases speed in *less* dense material... why?

thats why i was confused, cuz i thought the same as you.

 

[DarkLordofSith]

1 m^3 = 1m x 1m x 1m = 100 cm x 100 cm = 100cm = 1000mm x 1000mm x 1000 mm[/b]

That should be 100 cm x 100 cm x 100cm. Just thought I'd help. I'm pretty stoked, it's 4th of July weekend and Shrike's supposed to be posting some new writeups this weekend

 

[1Path]

Is light a wave, particle or both? When I first took physics 100 years ago, it was a wave only. EK says it's a wave AND particle. Which is right? Thanks!

 

[hippocampus]

Is light a wave, particle or both? When I first took physics 100 years ago, it was a wave only. EK says it's a wave AND particle. Which is right? Thanks!

its neither.

 

[DarkLordofSith]

Actually, EK is right. Light exhibits properties of both waves and particles. For more information, check here:

http://en.wikipedia.org/wiki/Wave-particle_duality

 

[hippocampus]

Actually, EK is right. Light exhibits properties of both waves and particles. For more information, check here:

http://en.wikipedia.org/wiki/Wave-particle_duality

oh i thot he was being sarcastic.

 

[1Path]

oh i thot he was being sarcastic.

I SAID I took physics many,many years ago (14 to be exacrt) so dude, gimmie a break? Now if you ask me about the dysregulation of apoptosis in renal cell carcinoma and my thoughts on whether or not this is a funtion of the extrinsic, intrinsic, or both pathways, I could help you. Unfortunately, this ain't on the MCAT!!

So is this the thread of physics MCAT questions or physics MCAT questions YOU guys think are "good/smart" questions?

And by the way, light being ONLY a wave is from the physics book I used at the time: Physics: Concepts and Connections by Art Hobson.

 

[1Path]

Actually, EK is right. Light exhibits properties of both waves and particles. For more information, check here:

http://en.wikipedia.org/wiki/Wave-particle_duality

Thanks!!

 

[mustangsally65]

Would someone explain what the normal force is supposed to be? Where does it come from, and what does it represent? I never got a clear definition in Physics class, and still don't really understand what it is. I did a search, and didn't find anything defining it. Thanks!

 

[dat_student]

I SAID I took physics many,many years ago (14 to be exacrt) so dude, gimmie a break? Now if you ask me about the dysregulation of apoptosis in renal cell carcinoma and my thoughts on whether or not this is a funtion of the extrinsic, intrinsic, or both pathways, I could help you. Unfortunately, this ain't on the MCAT!!

So is this the thread of physics MCAT questions or physics MCAT questions YOU guys think are "good/smart" questions?

And by the way, light being ONLY a wave is from the physics book I used at the time: Physics: Concepts and Connections by Art Hobson.

Wondering why that physics book said that !!!! It must have been a pretty old book at the time. Einstein proved light has the characteristics of particles and waves before you took physics...

 

[WilliamX2R]

Would someone explain what the normal force is supposed to be? Where does it come from, and what does it represent? I never got a clear definition in Physics class, and still don't really understand what it is. I did a search, and didn't find anything defining it. Thanks!

The normal force is just the reaction force in Newton's Third Law. Remember that if one object exerts a force on another object, the second object will exert an equal and opposite force against the first object. Lets say you placed an object on a table. The object exerts a downward force on the table, which is in this case is the weight of the object. The normal force is the force exerted by the table on the object (the reaction force) and is equal to the weight of the object. Lets say you placed a second object on top of the first. The normal force is then the sum of the weights of the two objects. Weight is also a force, keep that in mind.

 

[WilliamX2R]

Is light a wave, particle or both? When I first took physics 100 years ago, it was a wave only. EK says it's a wave AND particle. Which is right? Thanks!

It can be either one. This phenonemon is called wave-particle duality. In simple terms, light can behave as a stream of particles (photon). But it also possesses properties that are characteristic of a wave - it reflects and refracts like a wave would. Interference and the Doppler Effect are further proof light can exhibit wave-like characteristics.

 

[hippocampus]

I SAID I took physics many,many years ago (14 to be exacrt) so dude, gimmie a break? Now if you ask me about the dysregulation of apoptosis in renal cell carcinoma and my thoughts on whether or not this is a funtion of the extrinsic, intrinsic, or both pathways, I could help you. Unfortunately, this ain't on the MCAT!!

So is this the thread of physics MCAT questions or physics MCAT questions YOU guys think are "good/smart" questions?

And by the way, light being ONLY a wave is from the physics book I used at the time: Physics: Concepts and Connections by Art Hobson.

i thought u were being sarcastic cuz u said u took physics "100 years ago," and they found out the light as wave/particle thing less than 100 years ago. take a chill pill

 

[mustangsally65]

The normal force is just the reaction force in Newton's Third Law. . .

Thanks so much! That makes sense now, and I remember it now.

 

[Neurolemma]

Well, like I said, you can "ALWAYS" say work is equal to change in kinetic energy but you can "SOMETIMES" say work is equal to the change in potential energy. In other words, work is equal to the change in potential energy only when delta KE = delta PE.

I suppose you are right, since a force and displacement are always necessary to perform some work. But I would also think its important to know when to use what equation.

 

[gotgame83]

If you have a converging two lense system and the distance between the two lenses is less then the distance needed to form an image do you still go about the problem the same way. To rephrase, if the second lens intercepts the light from the first lens before it forms an image will this change anything?

 

[Nitya2284]

That should be 100 cm x 100 cm x 100cm. Just thought I'd help. I'm pretty stoked, it's 4th of July weekend and Shrike's supposed to be posting some new writeups this weekend

Ohh cool, that's awesome that he's gonna be posting. I can't wait

 

[smiley98]

The function of the dielectric is to create an opposing electric field by assuming a polarity in the opposite direction. For a given charge, the electric field would end up lower with the dielectric than without. But in the set up you describe, the voltage drop is controlled by the battery--the voltage drop of the capacitor must remain equal to the voltage drop of the battery (remember Kirchhoff's voltage law: The directed sum of the electrical potential differences around a circuit must sum to zero). Hence the electric field is fixed because the voltage is fixed. The dielectric merely acts to increase the charge on the plates.

Okay sorry to stay stuck on this one but how does a dielectric increase the charge without affecting the electric field? I am still not clear on this one... am I missing something?

 

[gujuDoc]

It can be either one. This phenonemon is called wave-particle duality. In simple terms, light can behave as a stream of particles (photon). But it also possesses properties that are characteristic of a wave - it reflects and refracts like a wave would. Interference and the Doppler Effect are further proof light can exhibit wave-like characteristics.

Ugh.....this reminds me of the first damned passage I had on my MCAT this past April. I hated it. It was one of those more verbal type of passages that I kept wanting to change my answers to.

It started with a paragraph about the wave theory, then a paragraph about the particle theory, and finally the dual theory.

 

[Shrike]

Greetings, all. I am on vacation: pretty much away from computers, among other things. I logged on for a quick check this morning, only to find fighting. This is not what I wanted to see before the coffee was done brewing.

Let's be clear about a few ground rules for this:

1. There is absolutely no fighting, no name calling, no unpleasantness permitted. Whether someone is being sarcastic or rude should never be an issue.

2. As one needs permission from the moderators of this thread even to post answers, rule 1 should never be a problem; nor should there be issues with posters correcting each other at all. The reason for rule 2 is that we wanted to have a place for test takers to come for answers they knew they could trust; if we ever have disagreements about either the physics or the way to apply it on the MCAT, that goal will not be achieved.

3. I still need help with the thread; if you want to be approved to answer, please PM me.

4. I say in the thread header that questions will be answered in a reasonable amount of time. I never defined reasonable. I'm not sure what amount of time is reasonable. But I'm trying, as are the other moderators and approved answerers.

Now, have a hot dog, watch a firework, and then gear up for the crucial six weeks.

Thank you,

Shrike.

 

[Shrike]

Okay sorry to stay stuck on this one but how does a dielectric increase the charge without affecting the electric field? I am still not clear on this one... am I missing something?

You're missing a small thing: this relationship holds only if the capacitor with and without the dielectric are being compared while being hooked up to the same-voltage battery (or other source). When there's a fixed voltage source, the voltage across the capacitor is fixed, too, and that means so is electric field. Now, in the equation Q = CV, if voltage is fixed, and capacitace C changes (because that's what a dielectric does -- it raises capacitance) then Q has to go up. Frankly, for the MCAT the reason this happens is irreleveant, but it relates to energy stored in dipoles within the dielectric.

If voltage is not held constant, but charge is (because the dielectric is inserted after the capacitor is charged, then removed from the voltage source), then voltage across the capacitor, and thus field, goes down.

 

[Shrike]

If you have a converging two lense system and the distance between the two lenses is less then the distance needed to form an image do you still go about the problem the same way. To rephrase, if the second lens intercepts the light from the first lens before it forms an image will this change anything?

Basically, you find the lens power of the resulting system by taking the powers of each of the component lenses, and adding them. You then solve normally. Power if defined as 1/f; use the power of the system to get the effective focal length, and pretend the lens has that focal length.

For a more detailed answer about what comes next, see the Physics FAQs thread.

 

[Shrike]

What is the relation between the force applied on the rope to tug it so that a weigh gets lifted on a pulley?

Look at how many sections of rope are pulling up on the weight: how many of them would have to shorten for the weight to move? Divide the weight of the object by that number, and you get force required on the rope (also known as tension).

For more on this topic, see my writeup on simple machines in the Physics FAQs thread.

 

[Shrike]

so how come exankrackers said that...

when its humid, water vapor goes into the air. air is made up of co2 and n2 that has a greater mass than water. so if water vapor goes into the air, the mass decreases (18g/mol).. which decreases the density. ok, that makes sense. but then they said that sound travels faster in that.. it increases speed in *less* dense material... why?

In the two gases being considered, density -- in terms of molecules per cubic whatever -- is the same. I know, you don't usually think of density that way, but sound does. Sound also considers, for gases, how often the molecules are running into each other. In the case described, the addition of water vapor increases the average molecular speed (because it decreases average mass while, presumably, keeping temperature [and therefore average KE] constant), so it increases the number of collisions. Sound will therefore travel faster.

The original question was too vague for the MCAT; don't worry too much about it.

 

[Shrike]

I was looking at the examkrackers book and it doesn't do go into much detail about the center of mass and center of gravity. what do we have to knwo about the center of mass and the center of gravity?

You have to know that, on MCAT problems, they're the same.

You have to know how to find center of mass; the answer is found in the FAQ thread. Basically, you reduce everything to point masses, pick a zero, and use CM = (sum of x * m)/(sum of m). Although any zero works, you should always pput it under the mass on the far left.