Physics question thread

[Shrike]

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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.

 

[liverotcod]

It seems like it sould be zero to me. Don't know what to say about that.

Note that the charges are not balancing. The fields generated by +Q and -Q act in the same direction.

 

[Nutmeg]

Note that the charges are not balancing. The fields generated by +Q and -Q act in the same direction.

You add this field to the field of the same equation with negative Q. Directionality is only applicable to the slope of the field or the voltage. The electric field should be zero, though, like it is at the halfway point in this graph:

Am I missing something?? 😕

 

[liverotcod]

You add this field to the field of the same equation with negative Q. Directionality is only applicable to the slope of the field or the voltage. The electric field should be zero, though, like it is at the halfway point in this graph:

Am I missing something?? 😕

Is that not a graph of potential, rather than e-field? And in your equation, wouldn't r-hat resolve to 1 for one of the charges, and -1 for the other? That seems like common sense to me, but electromagnetism is sometimes pretty counterintuitive to my mind.

Here's my take: imagine that we're calculating electrical force, rather than field. +Q would push on a positive test charge, and -Q would pull on it with the same amplitude as +Q, and the test charge would move toward -Q. The force is just the interaction between the charge and the field, right? So there has to be a net field between the charges.

 

[frankrizzo18]

Questions like that give me gas! I had a question this april on a tarzan passage where the answers were A) Sin(theta) B) Cos(theta) C) Sinsquared(theta) D) Cossquared(theta)

That's just a definition. As stated, An arcsecond is 1/60 of an arcminute, an arcminute is 1/60 of a degree. It's just something you have to know, and if you didn't know, now you do. 🙂

 

[stoleyerscrubz]

I finished studying all of EK and I have the basics down for the most part but there are a number of things mentioned in the AAMC MCAT Topic List that were not covered in EK materials. Could you comment on what we need to know about some of these. Thanks.

WORK-Work and Energy:mechanical advantage
Waves and Periodic motion-sound:attenuation

 

[Caboose]

O.k. so there's this guy - he's just nuts; he wants to get from point A to point B on a river flowing 8m/s east. He's a little off and just assumed he could cross the 1800m wide river directly going at 15m/s. Well, his efforts were in vain as he ended up 2040m downstream, (totally far off from his drinkin' buddies). He forgot his watch and was really into timing things. He turns to you in a panic, his wall-eyed gaze frantically looking to you for help.
Being the brilliant mind that you are, you chuckle and say, "Never you worry. We simply use the equation t=d/v. 1800north / 15m/s yielding the solution 120 seconds, or 2 minutes."

A temporary wave of relieve washes the anxiety from his face, but suddenly is replaced by doubt, "But what if ye jist take 2040east / 8m/s that the river's goin'... 'cause there's only one eastward force, so shouldn't that give ya the same answer... but thien it's like... 255 seconds!"

Suddenly the birds cease to chirp, all is silent and your staggered breath betrays your confident charm... you... you don't know.

Caboose.

 

[Nutmeg]

Is that not a graph of potential, rather than e-field? And in your equation, wouldn't r-hat resolve to 1 for one of the charges, and -1 for the other? That seems like common sense to me, but electromagnetism is sometimes pretty counterintuitive to my mind.

Here's my take: imagine that we're calculating electrical force, rather than field. +Q would push on a positive test charge, and -Q would pull on it with the same amplitude as +Q, and the test charge would move toward -Q. The force is just the interaction between the charge and the field, right? So there has to be a net field between the charges.

Ah ha! Gotcha. The problem with the equation i put up is that I forgot abot the r vector quantity. By superposition, Q is the same for both with the r vector being the same, or Q is opposite with the r vector being the opposite.

Nice work. I was completely wrong on that one, question answered. 🙂

 

[yalla22]

I'm reviewing the periodic motion, waves, and sound portion of my kaplan book and I'm wondering how many of those equations one has to have memorized..There are so many of them and they ALL seem exactly alike!!!

Like for standing waves, wavelength = 2L/n and f = nv/2L
and then for waves we have y=Ysin(kx-wt) where k = 2pie/wavelength, etc etc🙁!!

Any suggestions about how to tease these apart?

 

[PneoDr]

I have a question about physics. It's hard for me to put my finger on what exactly has me upset. But it has to do with the idea of voltage or potential. My first problem: In a simple circuit, say one with just a single resistor and a battery with no internal resistance. If the battery is 6V then there must be a "potential drop" across the single resistor equal to 6V. That seems like such a simple idea, but it is hard for me to see. What is losing potential across the resistor? I have not found a book that makes this idea too clear. I arrived at this conclusion: I just think of it as the charge that is flowing through the resistor collides with the atoms of the resistor they lose some of thier "potential," so at the entry point of the resistor there is a higher potential then the exit point. I usually get circuit problems right, but I just wonder if there is a better way to understand this.

The other idea that has me confused is voltage at a point. The way I am thinking of this is that at a given point there is a given potential and the work required to move a point charge to that point is given by the (voltage times the charge). Is this the correct way to think of this concept?

Thank you for your help.
Bryan

 

[yalla22]

I have a question about physics. It's hard for me to put my finger on what exactly has me upset. But it has to do with the idea of voltage or potential. My first problem: In a simple circuit, say one with just a single resistor and a battery with no internal resistance. If the battery is 6V then there must be a "potential drop" across the single resistor equal to 6V. That seems like such a simple idea, but it is hard for me to see. What is losing potential across the resistor? I have not found a book that makes this idea too clear. I arrived at this conclusion: I just think of it as the charge that is flowing through the resistor collides with the atoms of the resistor they lose some of thier "potential," so at the entry point of the resistor there is a higher potential then the exit point. I usually get circuit problems right, but I just wonder if there is a better way to understand this.

The other idea that has me confused is voltage at a point. The way I am thinking of this is that at a given point there is a given potential and the work required to move a point charge to that point is given by the (voltage times the charge). Is this the correct way to think of this concept?

Thank you for your help.
Bryan

For the first paragraph-I think you are actually thinking too hard about it! As long as you can solve the problems you should be okay...

 

[yalla22]

My question is a bit conceptual but I was thinking about the buoyancy force equation and I dont understand this real life situation in light of this:

If you throw a book into a pool it will sink. BUT if you put the book on top of an inflatable tube it won't? Even though the amount of water displaced, for example, is the same?

I'm sure this is really stupid but i'm just wondering...

 

[Mister Pie]

I hope I don't get in trouble for posting in this thread since I'm not officially a contributor.

To answer your question: think about the density of the inflatable tube. It's rather light despite taking up such a large volume. This means it has a low density.

Now, if you toss a book on top of the tube, it is different from the original situation where you tossed the book in "naked". Now, in this new situation, the book will push down on the inflatable tube (which is light and low in density, yet big in terms of occupying volume). The result? More volume is displaced, for the cost of adding something very light. The equation for buoyant force tells you that the upwards force exerted by the water is proportional only to the volume displaced and the density of the water itself. So since you've managed to displace a far greater amount of liquid with the inflatable tube, you have a much greater buoyant force, and if your book isn't too heavy, it floats.

My question is a bit conceptual but I was thinking about the buoyancy force equation and I dont understand this real life situation in light of this:

If you throw a book into a pool it will sink. BUT if you put the book on top of an inflatable tube it won't? Even though the amount of water displaced, for example, is the same?

I'm sure this is really stupid but i'm just wondering...

 

[Mister Pie]

Remember that all energy (potential, KE, etc.) can, in a sense, be "converted" between different forms. When you shoot a ball up the hill, you are, in a sense, converting the KE to gravitational energy. When the ball rolls down, you are converting potential energy to KE. Etc., etc.

When a charge is flowing through a "perfect" wire, you can liken it to a block sliding across a perfectly smooth, frictionless surface. In the absence of any external forces, that block will slide forever with a constant KE (total energy). But, if you say, toss some glue on the path in front of the block, the total KE of the block will get lower since the block must lose some of it's total energy to get through that puddle of glue you just added. (More technically, the glue exerts a force on the block, and you have to subtract the work term away from the KE term to get the new KE).

That's analagous to what happens on the atomic scale in circuits when charges have to go across a resistor. The resistor is like "glue" and to get across the resistor, energy MUST be lost for the charges to get across, hence you have a drop in potential. I hope that helps a little bit. Now, if you're question was about WHY the drop across a single resistor has to equal the potential of the battery (your 6V example), that's more of a statement of conservation of energy, and another point to consider.

I'm not going to get into the mathematical details regarding voltage, but to quickly address your last question, think about the units of eletric potential. 1 Volt = 1 J/C. So basically, when you have the voltage difference calculated between two points, you have, in a sense, calculated the potential energy difference between the two points. Be careful though. Voltage is not a measure of energy in and of itself, which you can easily see from the units (J/C != J). Howevever, when you multiply the electric potential by a charge, the result is the energy lost (or gained) in moving that amount of charge between those two points.

E&M can be a fairly abstract topic and it's actually pretty hard for me to explain without integrals. Perhaps someone else here can do it more justice.

I have a question about physics. It's hard for me to put my finger on what exactly has me upset. But it has to do with the idea of voltage or potential. My first problem: In a simple circuit, say one with just a single resistor and a battery with no internal resistance. If the battery is 6V then there must be a "potential drop" across the single resistor equal to 6V. That seems like such a simple idea, but it is hard for me to see. What is losing potential across the resistor? I have not found a book that makes this idea too clear. I arrived at this conclusion: I just think of it as the charge that is flowing through the resistor collides with the atoms of the resistor they lose some of thier "potential," so at the entry point of the resistor there is a higher potential then the exit point. I usually get circuit problems right, but I just wonder if there is a better way to understand this.

The other idea that has me confused is voltage at a point. The way I am thinking of this is that at a given point there is a given potential and the work required to move a point charge to that point is given by the (voltage times the charge). Is this the correct way to think of this concept?

Thank you for your help.
Bryan

 

[Mister Pie]

Last question I answer for now. Hope I don't get in trouble for answering stuff, heh heh.

If you look at the standing wave equations you listed: wavelength = 2L/n and f = nv/2L. They look so similar, yet different. That's because they ARE related in another equation you are probably familiar with: speed of wave = wavelength x frequency. So, if you solve the equation: (2L/n) * f = v, you will get f = nv/2L, just like you listed.

If you look closely enough, some of the equations that people tend to memorize are in fact "easily" (a term I use loosely) derived from simpler equations.

My personal opinion (I have not taken the MCAT yet) is that you should memorize equations as little as possible. But, I'm sure there are many who memorized like a bajillion and did super well. Also, deriving formulae on the test (which is probably what I will end up doing) might also push you over the edge if time is a factor on PS, so that's another point to consider.

I'm reviewing the periodic motion, waves, and sound portion of my kaplan book and I'm wondering how many of those equations one has to have memorized..There are so many of them and they ALL seem exactly alike!!!

Like for standing waves, wavelength = 2L/n and f = nv/2L
and then for waves we have y=Ysin(kx-wt) where k = 2pie/wavelength, etc etc🙁!!

Any suggestions about how to tease these apart?

 

[Caboose]

O.k., so there's this OTHER guy who's sittin' pretty on the end of a board on a frictionless surface. The board and the man weigh the same. If the man goes two steps to the left, the board is going to go the same distance to the right, is it not?

Caboose

 

[Nutmeg]

O.k., so there's this OTHER guy who's sittin' pretty on the end of a board on a frictionless surface. The board and the man weigh the same. If the man goes two steps to the left, the board is going to go the same distance to the right, is it not?

Caboose

No. It is going to go half the distance the man goes relative to the board, such that the center of mass of the man and board combined stays stationary relative to the frictionless surface.

 

[Nutmeg]

O.k. so there's this guy - he's just nuts; he wants to get from point A to point B on a river flowing 8m/s east. He's a little off and just assumed he could cross the 1800m wide river directly going at 15m/s. Well, his efforts were in vain as he ended up 2040m downstream, (totally far off from his drinkin' buddies). He forgot his watch and was really into timing things. He turns to you in a panic, his wall-eyed gaze frantically looking to you for help.
Being the brilliant mind that you are, you chuckle and say, "Never you worry. We simply use the equation t=d/v. 1800north / 15m/s yielding the solution 120 seconds, or 2 minutes."

A temporary wave of relieve washes the anxiety from his face, but suddenly is replaced by doubt, "But what if ye jist take 2040east / 8m/s that the river's goin'... 'cause there's only one eastward force, so shouldn't that give ya the same answer... but thien it's like... 255 seconds!"

Suddenly the birds cease to chirp, all is silent and your staggered breath betrays your confident charm... you... you don't know.

Caboose.

The line from point A to point B does not necessarily run North-South.

 

[yalla22]

Thanks for the reply🙂
I think my problem is a little more conceptual. I don't exactly understand the difference between the string and open/closed pipe situations and how that corresponds to differences in the equations. Like I could easily memorize the equations for the respective situations but I just don't "get it".

Last question I answer for now. Hope I don't get in trouble for answering stuff, heh heh.

If you look at the standing wave equations you listed: wavelength = 2L/n and f = nv/2L. They look so similar, yet different. That's because they ARE related in another equation you are probably familiar with: speed of wave = wavelength x frequency. So, if you solve the equation: (2L/n) * f = v, you will get f = nv/2L, just like you listed.

If you look closely enough, some of the equations that people tend to memorize are in fact "easily" (a term I use loosely) derived from simpler equations.

My personal opinion (I have not taken the MCAT yet) is that you should memorize equations as little as possible. But, I'm sure there are many who memorized like a bajillion and did super well. Also, deriving formulae on the test (which is probably what I will end up doing) might also push you over the edge if time is a factor on PS, so that's another point to consider.

 

[Tracy47]

Hi, I am having a difficult time understanding capacitors, I memorized the equations, but I don't see how they work. I know they store energy between the plates, but why do we need capacitors, when we have a battery? Doesn't the battery end up in turn charging the capacitors already? By charging the capacitors, doesn't the battery lose its energy? Sorry, I find electricity so confusing, and I can't seem to apply it to everyday household examples.

 

[Caboose]

The line from point A to point B does not necessarily run North-South.

Hey Nutmeg - thanks for the answers! The nutty fisherman is actually moving from north to south. The basic question is if you have two vectors, distance/direction/velocity should you not be able to calculate the time from both?

north to south: 1800m @ 15m/s

west to east: 2040m @ 8m/s

If there is only one force, (current or motor), in each direction, shouldn't you be able to get the time that it took to travel the given distance from either one?
...maybe it's a typo...

Thanks for taking the time!!

Caboose

 

[medanthgirl]

okey dokey - a kinetmatics problem i just can't solve even though ek gives the answer. it's #s 226 and 227 in the 1001 book and goes like this:

A car moving at 20m/s brakes and slides to a stop. If the coefficient of kinetic friction between the pavement and the tires of the car is 0.1, how far does the car slide?

I've tried to come from this at a few different angles (friction eqns, kinematic eqns) but have not gotten their answer of 200m.

Thanks!

 

[yalla22]

okey dokey - a kinetmatics problem i just can't solve even though ek gives the answer. it's #s 226 and 227 in the 1001 book and goes like this:

A car moving at 20m/s brakes and slides to a stop. If the coefficient of kinetic friction between the pavement and the tires of the car is 0.1, how far does the car slide?

I've tried to come from this at a few different angles (friction eqns, kinematic eqns) but have not gotten their answer of 200m.

Thanks!

don't you need the weight of the car?

 

[Nutmeg]

Hey Nutmeg - thanks for the answers! The nutty fisherman is actually moving from north to south. The basic question is if you have two vectors, distance/direction/velocity should you not be able to calculate the time from both?

north to south: 1800m @ 15m/s

west to east: 2040m @ 8m/s

If there is only one force, (current or motor), in each direction, shouldn't you be able to get the time that it took to travel the given distance from either one?
...maybe it's a typo...

Thanks for taking the time!!

Caboose

Where is the question coming from? I thought it was supposed to be a brain teaser or something. Yes, assuming that you've got speed relative to media in one direction, and media motion relative to external landmarks in a perpendicular direction, you should be able to find the time of the crossing by either direction.

It doesn't make physical sense that you can go faster north than east and make more progress in the easterly direction.

 

[Nutmeg]

okey dokey - a kinetmatics problem i just can't solve even though ek gives the answer. it's #s 226 and 227 in the 1001 book and goes like this:

A car moving at 20m/s brakes and slides to a stop. If the coefficient of kinetic friction between the pavement and the tires of the car is 0.1, how far does the car slide?

I've tried to come from this at a few different angles (friction eqns, kinematic eqns) but have not gotten their answer of 200m.

Thanks!

Mass cancels out.

Take these two equations:

V = Vo + a*t
and
X - Xo = Vo*t + (1/2)a*t^2

and, by solving the first for t and inserting that value in the second, you get

2*a*(X - Xo) + Vo^2 = V^2

where you want to solve for X - Xo (total distance to stop), Vo is known to be 20 m/s, and Vo = 0.

So we need the rate of deceleration "a".

Now, the force of kinetic friction Fk is found by

m*a = -Fk

and we know that

Fk = mu*N = (0.1)*m*g

so combining these, and approximating g as 10 m/s gives

-m*a = (0.1)*m*g ==> -a = (0.1)*g

Hence,

(X - Xo) = (V^2 -Vo^2)/(2*a)
(X - Xo) = (0 - 400 m^2/s^2)/(2*(-0.1*10 m/s^2))
(X - Xo) = 400/2 = 200m


Note: I'd never be able to do that on the MCAT. The way to think about this is that your rate of deceleration from friction is contingent upon you normal force, so that deceleration = mu*g when you only have normal force causing friction, etc. Hence deceleration is 0.1*10 m/s^2, or one meter per second per second. That can be done pretty easily. Then, you find your time of deceleration by looking at (20 m/s)/(1m/s^2) = 20s

Then, you mutliply the time by the average speed. If deceleration is linear, the average speed is simply (20 m/s - 0 m/s)/2 = 10 m/s

Hence, X = 10 m/s * 20 s = 200 m.

 

[Caboose]

Where is the question coming from? I thought it was supposed to be a brain teaser or something. Yes, assuming that you've got speed relative to media in one direction, and media motion relative to external landmarks in a perpendicular direction, you should be able to find the time of the crossing by either direction.

It doesn't make physical sense that you can go faster north than east and make more progress in the easterly direction.

By God, you're right! Thank the Christ I'm not taking the August Beast. I still have time to make fundamental connections don't I? I mean, one could make a human child in this much time. Surely I can connect a few neurons...
Thanks Nutmeg! The question came from the first mini mcat in the EK book, (question #12, in case you were just looking through it).
Sigh. Back to the books. Thanks again - I'm sure I'll be lurking about posting random questions. l8er g8er

Caboose

 

[kimijohni]

Im really having dificulty understanding the explanations given about mirrors and how the light rays has three or so possible directions. Within focal length and such. When they say that I have no idea where the focal length of a mirror is or should be. I understand the way the eye works but have no clur on mirrors can anyone help?
Thank you
Robyn

 

[freesolo]

i dont understand the whole diffraction concept at all. Ive read through my kaplan book, and i just dont get it. What formula must i know? there was a whole passage on young's slit on aamc 8 that just killed me.

 

[Mister Pie]

i dont understand the whole diffraction concept at all. Ive read through my kaplan book, and i just dont get it. What formula must i know? there was a whole passage on young's slit on aamc 8 that just killed me.

To understand diffraction you should try to understand Huygen's Principle. The phenomena of diffraction arises from the fact that no slit in real life is infinitely thin. What happens then is that when waves hit a screen with a slit (that is finite in thickness), multiple wavelets can form, and interfere with each other. In a sense, interference and diffraction are really closely related and both result from wave interference. In interference, it is the waves from separate slits interfering with each other and canceling (or reinforcing). In diffraction, it is waves originating from the same slit interfering in some form. I'm not sure which equations you need to know, but if you MUST memorize one, memorize the one that gives the diffraction minima.

 

[Will Ferrell]

I was wondering if an Alpha particle has charge. I always thought it was no more than a neutral helium atom, but i recall from a practice question that it experienced more force in an electric/magnetic field than a proton or electron because it had more charge. Doesn't a neutral atom not go through a field (like how the mass spec. works)?

Thanks!

 

[Nooro]

An alpha particle is 2 protons and 2 neutrons, thus having a charge of plus 2.

 

[perfectmoment]

question about capacitance...

according to EK, capacitance is determined by seperation of the plates, and height and width of the plates, but is NOT determined by thickness, due to the charge being held outside of the plates anyway. however, in one of the kaplan tests, one of the free standing questions had thickness as the answer, and their reasoning was that because the plates were thicker, they would have had less plate seperation, which sounds like a really stupid thing to assume, given that one would expect that everything else would have been held constant anyway. which of these companies is correct?

 

[hippocampus]

what is specific gravity? why do we need it? how do you calculate it?

on test 4R number 27, there is a specific gravity question... can you please explain it to me?

#27: on object with 15g mass is immersed in benzene and suffers an apparent loss of mass of 5 g. what is the approximate specific gravity of the object? (sp gravity of benzene = 0.7)

 

[Mister Pie]

question about capacitance...

according to EK, capacitance is determined by seperation of the plates, and height and width of the plates, but is NOT determined by thickness, due to the charge being held outside of the plates anyway. however, in one of the kaplan tests, one of the free standing questions had thickness as the answer, and their reasoning was that because the plates were thicker, they would have had less plate seperation, which sounds like a really stupid thing to assume, given that one would expect that everything else would have been held constant anyway. which of these companies is correct?

EK is correct. If you look at the equation for the capacitance for a parallell plate capacitor, there is no dependence on thickness, only surface area and distance. It sounds like Kaplan made a really lame assumption that by thickening the plates, you made the plates closer as well.

 

[Shrike]

#27: on object with 15g mass is immersed in benzene and suffers an apparent loss of mass of 5 g. what is the approximate specific gravity of the object? (sp gravity of benzene = 0.7)

The buoyant force is 5g, so that's the weight of the fluid (benzene) displaced. Benzene is 0.7 times as dense as water, so if it were in water it would displace 5g/0.7 or about 7.5g of water. Because you have memorized the density of water, you know instantly that 7.5 grams of it occupies 7.5 cm^3. The object has a mass of 15g, so a density of 15/7.5 = 2g/cm^3. This is twice as much as the density of water, so the specific gravity is (by definition) 2.

Specific gravity is the density of the object divided by that of water. It is necessary when the problem talks about it; it is highly useful for many other buoyancy problems. See the FAQ.

 

[Futuredoctr]

Okay, I searched, but I didn't find it, it could be there. However, if I could get some help with the eye and optics I would greatly appreciate it! I always miss questions reguarding farsightedness or nearsightedness and what kinds of lens are needed to correct the problem and why(how they work, and in relation to where the image is formed before and after on the retina ect...)? Shrike, could you help me out and explain the eye and optics concepts here? Thanks!!!

 

[Righty123]

Hey I was wondering if anyone could just summarize when frequency stays constant and when is does change? I think freq of sound stays constant regardless of medium and so does freq of a wave, and that freq of a light changes as it changes medium. Any clarification is much appreciated. Thanks!

 

[Andrew99]

I keep getting confused when it comes to those problems that deal with a fluid's velocity. When there's less area, (A1v1 = A2V2) it flows faster? When there's less pressure it flows faster?

 

[Futuredoctr]

I keep getting confused when it comes to those problems that deal with a fluid's velocity. When there's less area, (A1v1 = A2V2) it flows faster? When there's less pressure it flows faster?

You have it right. Think about about your garden hose( and assume that the amount of water coming out in a given time is a constant), if you turn it on and let it run, a certain amount of water will flow out each second. Now if you place your tumb over the hole, it doesn't change the amount of water flowing out in that amount of time so the velocity of the water must increase to keep the total amount flowing a constant.

The less pressure question is releated to Bernoulli's equation. Since one side is equaled to another if you have less pressure on one side then velocity has to increase to maintain the equation relationship. I didn't post the equation here because I don't know how to in typing without being confusing, but it should be in any of your physics review notes. I hope that helps, I couldn't help but try to answer your question as I once struggled with this concept myself.

 

[mostwanted]

Shrike can you please go over single slit diffraction, thin films, and x-ray diffraction? i understand double slit, but not quite sure about single slit.

 

[curlykid]

Hi all, I have a few questions. As I was reading over the Examkrackers book, I came across a question that I'm not sure that I fully understand after having looked at the answer. So I made up a similar question of my own (with more parts) and tried to answer it. I was wondering if anyone would mind trying to answer my question or check to see if my response to the question makes sense. Thank you.

Questions: Ann decides to practice dancing around circular track of 1/2 km circumferance. She runs into her friend Ellie, who bought just bought a shiny Swiss stop watch and wants to test it out, so she times Ann as she dances and finds that Ann danced 1.25 km in 10 minutes along the track. (a) How fast, on average, is Ann’s velocity? She decides to continue, but after 20 minutes decides to rest. (b) If she traveled 2km in 20 minutes, what is her average velocity now? average speed?

I understand part (b) a lot better than part (a). In part (b) she travels 2km in 20 minutes. The track has a circumferance of .5 km, so if she travels 2km around the track she has gone 4 times around the track and she has stopped exactly where she has started, so her net displacement is zero. If her net displacement is zero, then her average velocity is zero (because(from my understanding) velocity = net displacement / time). Speed = net distance / time. Her net distance is 4 km so her average speed = .1 km / min.

For part (a) she dances 1.25 km in 10 minutes. If the track has a circumference of .5 km, she travels around the track exactly two and a half times, however every time she goes around the track, she is back to where she started, so her net displacement is half of the circumference or .25 km (because that is her net displacement from the origin. Her velocity then is net displacement / time = .25km / 10 min = .025 km / min. This is my logic, I’m not exactly sure if this is how you are supposed to solve it, but this was my thought process in trying to figure it out.Does it make sense?

 

[hippocampus]

T or F: sound travels faster in denser things (for example, solids).

so what's up with the equation v = (1/density)^(1/2)... that says the opposite?

when they say sound travels faster in solids, they meant air molecules right?

it is a hot day, does sound travel slower? cold day, faster?

thanks

 

[Mutt]

This is from an old TRP (Flowers and Silver) practice test; I'll summarize for the pertinent information.

A hollow tube with one closed end can be used to set up a standing sound wave. When a standing wave forms, the column of air inside the tube is said to resonate.

A standing wave will form if the length of the tube is an odd-integer multiple of one-quarter the wavelength of the sound entering the tube.


Pictured is a tube with fluid in the bottom and a valve that allows the fluid level to be decreased. They show a tuning fork struck and held near the open end of the tube. The tuning fork emits sound waves at 400Hz.
As the water level is lowered, the air in the tube resonates at multiples of a fixed length.


The question:

As the water level in the tube is lowered, resonance occurs at intervals of 0.4m. This implies that the speed of sound in air is:
a) 160 m/s
b) 320 m/s < CORRECT ANSWER
c) 390 m/s
d) 440 m/s

Now, I knew that the speed of sound is roughly 340m/s, so I got it right only because of that. I did not understand how to find the answer; they give the following as the solutiong, which I can't follow:

lambda/2 = 0.4m

Can someone explain why this equation is used, where it came from and whether is somehow follows from the only closed-end tube equation that I know of:

lambda=4L/n


Much appreciated!

 

[hippocampus]

1) if you're in a roller coaster, and there is a loop... if you're at the highest point of the loop, why is the cart not touching the loop? can you please explain me the normal force in this?

2) i know how to use to the right hand rule, but i'm not sure when they want to make it the opposite. for example, i know for the electron, you switch the force when using the right hand rule.
... but you also have to switch it when you want to resist the change in magnetic field, so you make the current go the opposite way (switching something) etc. can you give me an example of how they would ask for these two... the regular one (no switching needed), and the opposite one (switching needed)?

3) if there is a pendulum swinging, and a mass attached to the pendulum, which is it the lightest? at equilibrium, or where?

thanks

 

[05med05]

Hi,

Can someone explain to me the difference between the following linear motion / constant accelleration equations:

X = Xi + Vot + (1/2)at(squared)

and

Vf = Vi + at


This is from a question in the 1001 physics book (I think the q # is 293?). Anyway, the explanation in the back instructed me to use the first equation, since x,a and t were known (or could be figured out). The question asked for the initial velocity of a ball being thrown upwards by a person riding up 6 floors in an elevator.

The upward distance travelled by the ball and person was 25m and the answer was Vi = 52.5m/s (the answer choice listed 55m/s as the correct option).

Using the equations, if t = 10 seconds and a=gravitational force, then:

If Vf and Xi are zero, then why is there a difference between the first and second equation and when should one be used and not the other and vice versa?

Vf=Vi + at
0=vi + (-10) (10)
Vi = 100m/s

X=Xi + vit + 1/2 at squared
25=0 + Vi(10) + 1/2 (-10) (10) squared
Vi = (25 - (1/2) (-10) (100)) / 10
Vi = 52.5

 

[Mister Pie]

Something that's been bothering me for a while:

if light slows down when entering a new medium (originating from a vaccuum), shouldn't the light "build up" somewhere? I'm assuming that since light is an electromagnetic wave, this "build up" basically just translates into heat from exciting the material that the medium is made up of. What's really going on?

 

[vince99]

Just wondering exackly what we need to know about these topics.
What formulas?

Asintheta = nlamba ???

I checked the Physics FAQ, but nothing was mentioned about this topic

 

[deuist]

Hi,

Can someone explain to me the difference between the following linear motion / constant accelleration equations:

Your final velocity is not zero. Recall that you are travelling in an elevator that is moving at a constant speed. There's your disparity.

 

[deuist]

Something that's been bothering me for a while:

if light slows down when entering a new medium (originating from a vaccuum), shouldn't the light "build up" somewhere? I'm assuming that since light is an electromagnetic wave, this "build up" basically just translates into heat from exciting the material that the medium is made up of. What's really going on?

Now you're thinking like a physicist. Here's the deal with light: it has energy and momentum, but no mass. The term "build up" puts us in a confusing position. Instead, think of the light way as dissipating energy---which it does. As light passes through a material, it can excite the electrons in the medium. Sometimes, so many electrons get excited that we can witness a physical event. The motion detectors that stores use to automatically open doors whenever someone approaches are based upon the photoelectric effect.

 

[hippocampus]

T or F... if you throw a ball (projectile) on earth and on the moon... it will go higher in the moon cuz there is lower gravity there... equation: h = v^2/2g

if that is true.. then wouldn't it contradict equation h = 1/2at^t...?? confused...

 

[Mister Pie]

T or F... if you throw a ball (projectile) on earth and on the moon... it will go higher in the moon cuz there is lower gravity there... equation: h = v^2/2g

if that is true.. then wouldn't it contradict equation h = 1/2at^t...?? confused...

It will go higher, yes. There is no contradiction because the full equation is:

d = v(initial)t - 1/2at^2. (There's also d(initial) if you are superhardcore).

There's two unknowns: time and distance traveled. Fortunately, we know that at the maximum height, final velocity is zero. We can use this to solve for time:

0 = V(initial) - at ==solve==> t = V(initial)/a

Plugging our expression for time back into the distance equation:
d = V(initial) * V(initial)/a - 1/2 * a * [V(initial)/a]^2
= V^2(initial)/a - V^2(initial)/(2a)
= V^2(initial)/2a

So in fact, going the hardcore derive-from-scratch way gives you the same result as the equation you started out with. Pretty cool, huh?

(I hope I didn't make any algebraic mistakes- that would be embarassing)