Physics question thread

[Shrike]

---

Members don't see this ad.

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.

 

[Creightonite]

Hi, I got a question. I was reading about speed of waves when moving from media to media. I understand that frequency does not change but wavelength is the one that changes affecting the speed of light in the media. Saw I was wondering if you were to arrahcnge this compund in terms of the speed of the sound wave (or any other type of wave) from having the lowest speed to the highest: water, metal, air, glycerol (a viscous compound (less dense than water) and kerosine (less dense than water)

Here is how I would arrange them (in the order of increasing speed of sound):

air -> kerosine --> glycerol --> water --> metal

Am I right?

A little follow up to this. How about the speed of light in this case? I was reading that the speed of light goes down as it goes into a media with a greater refraction coeficient (like from air to water for example) so it would be opposite to what is described above. The only explanation I could think of is that sound is pure wave phenomenon but light is a particle+wave phenomenon. Any input on this?

 

[gridiron]

A little follow up to this. How about the speed of light in this case? I was reading that the speed of light goes down as it goes into a media with a greater refraction coeficient (like from air to water for example) so it would be opposite to what is described above. The only explanation I could think of is that sound is pure wave phenomenon but light is a particle+wave phenomenon. Any input on this?

Usually, the trend for the speed of sound is the opposite for light (there are exceptions of course) For example, the speed of sound increases in glass whereas the speed of light decreases. Light travels fastest in a vacuum---at 3 *10^8 m/s and slows down in mediums with a greater bulk modulus. Again, to answer your question about the trend, you need to know the density of kerosine vs. that of glycerol. Usually, but not always, materials that are more dense have a greater bulk modulus--are more incompressible (there are exceptions of course!!). I hope this helps!!

 

[reds_maniac]

So...why is the pressure of the water coming our of your shower the same as atmospheric pressure? Does this pressure change if the water pressure in your water pipe changes?
Sigh.

 

[DesiMcatAcer]

So...why is the pressure of the water coming our of your shower the same as atmospheric pressure? Does this pressure change if the water pressure in your water pipe changes?
Sigh.

When you are saying that it means as the water comes out of the tap (just think of it as the moment it comes out of the tap), so it 'adapts' itself to the atmospheric pressure. Even if the pressure inside the pipe were to change to anything higher or lower, still as soon as the water comes out of the tap, it will still be atmospheric pressure. Hope it helps!

 

[gridiron]

When you are saying that it means as the water comes out of the tap (just think of it as the moment it comes out of the tap), so it 'adapts' itself to the atmospheric pressure. Even if the pressure inside the pipe were to change to anything higher or lower, still as soon as the water comes out of the tap, it will still be atmospheric pressure. Hope it helps!

I would be careful to assert that the pressure of the water exiting the tap is atmospheric pressure. If you were to use a high resolution digital manometer, you would find the pressure to be less than atmospheric pressure. The reason for this is because the pressure is governed by bernoulli's equation and factors such as cross sectional area, height differences and resistance must be taken into consideration. If for example, the water was in a tank with its surface exposed to the atmosphere, then the pressure of the water, at the surface, would be atmospheric pressure (with slight deviations).

 

[DesiMcatAcer]

I would be careful to assert that the pressure of the water exiting the tap is atmospheric pressure. If you were to use a high resolution digital manometer, you would find the pressure to be less than atmospheric pressure. The reason for this is because the pressure is governed by bernoulli's equation and factors such as cross sectional area, height differences and resistance must be taken into consideration. If for example, the water was in a tank with its surface exposed to the atmosphere, then the pressure of the water, at the surface, would be atmospheric pressure (with slight deviations).

I agree on that, however I was answering to his/her situation..obviously, if you put monometer, you have changed the situation now...

 

[neurodoc]

If the "atmospheric" pressure were greater than the pressure of the water in the pipe, it wouldn't come out of the pipe, right?

 

[DesiMcatAcer]

If the "atmospheric" pressure were greater than the pressure of the water in the pipe, it wouldn't come out of the pipe, right?

Pressure goes from higher to lower so you are correct it would not come out of pipe if atm pressure were greater than pressure of water in the pipe, unless some other pressure or force is acting on it

 

[87138]

Ok, so here's one that confused me. I happened to be looking at Kaplan's MCAT 45 book and came across a physics question. I apologize for not having the actual question in front of me, but I'll do my best to paraphrase.

A crane lifts a 1000kg beam 10m vertically and sets it on top of a building. Which one of the following statements is false?

A. The net work done on the beam is 0J
B. The net work done on the beam is 98100J
C. The work done on the beam by the crane has a magnitude of 98100J
D. The work done on the beam by gravity has a magnitude of 98100J



Now their explanation of the correct answer is A, because Work = change in KINETIC ENERGY and the beam's kinetic energy is the same at the beginning as at the end!

Am I stupid, or is that a horrible explanation? Work also takes into account potential energy, yes? Its potential energy obviously increased.

 

[123dan321]

Hi everyone -

Bernoulli's equation K = P + (1/2)(rho)v^2 + (rho)gh can be applied to the classic "spigot at the bottom of a water tower" problem, and used to find the velocity of water out of the spigot, reducing to v = sqrt(mgh) because we say that the height at the level of the spigot is 0, and the velocity at the surface of the water in the tower is 0. My question is, why do we say that velocity is equal to 0, if the water level is dropping?

I know I'm not thinking of something correctly... does it have to do with the net movement of the water molecules equalling 0 in the water tower, but not in the spigot (since water flows out)? (Even in this case, my first instinct is to say that there's net movement downwards...) - thanks in advance.

 

[gridiron]

Ok, so here's one that confused me. I happened to be looking at Kaplan's MCAT 45 book and came across a physics question. I apologize for not having the actual question in front of me, but I'll do my best to paraphrase.

A crane lifts a 1000kg beam 10m vertically and sets it on top of a building. Which one of the following statements is false?

A. The net work done on the beam is 0J
B. The net work done on the beam is 98100J
C. The work done on the beam by the crane has a magnitude of 98100J
D. The work done on the beam by gravity has a magnitude of 98100J



Now their explanation of the correct answer is A, because Work = change in KINETIC ENERGY and the beam's kinetic energy is the same at the beginning as at the end!

Am I stupid, or is that a horrible explanation? Work also takes into account potential energy, yes? Its potential energy obviously increased.

The explanation isn't entirely wrong, but it isn't appropriate (strictly my opinion, I would use a different explanation). Actually, A is the correct answer because the net work isn't zero. The beam is being lifted against the earth's gravitational field. When you use the equation for work:

work = fdcostheta​

The force is the weight of the book, d is the displacement the beam undergoes and theta will be 180 degrees making the work done negative. Potential energy will be the opposite of work. This makes sense because the beam isn't doing what it normally would do and the energy of the book increases.

 

[gridiron]

Hi everyone -

Bernoulli's equation K = P + (1/2)(rho)v^2 + (rho)gh can be applied to the classic "spigot at the bottom of a water tower" problem, and used to find the velocity of water out of the spigot, reducing to v = sqrt(mgh) because we say that the height at the level of the spigot is 0, and the velocity at the surface of the water in the tower is 0. My question is, why do we say that velocity is equal to 0, if the water level is dropping?

I know I'm not thinking of something correctly... does it have to do with the net movement of the water molecules equalling 0 in the water tower, but not in the spigot (since water flows out)? (Even in this case, my first instinct is to say that there's net movement downwards...) - thanks in advance.

The reason the velocity is assumed to be zero is two fold. First, I believe the equation is derived from the lagrangian point of view--this is a method often used in fluid dynamics that derives flow and flow rates from the point of view of the observer. Second, consider a differential area near the top of the surface. As water flows out of the spigot, the height level of the water drops. Now, try to view the differential area as the water drops----you will see that the height of the fluid drops leading to a pressure drop. The velocity is the same because over time, the differential area is only losing height. This is only an assumption however because the water moleculed do have some velocity but it is so small that can be assumed to be zero---differential area. The reason the velocity changes from the tank exiting the spigot is because it encounters a pressure gradient. Another way I would recommend to go about this is to map the differential area over time. You will see that if you take the average velocity at two different heights it will come out to be zero--thus indicating that as the height of the fluid drops from the top, its pressure changes (the velocity doesn't change in the y direction, the pressure does). I hope I have helped and haven't confused you even more!!!!

 

[legobikes]

I'm more than a little confused about electric fields. For an electric field, is the movement simply dictated by the direction of the E field and the charge of the particle?


If this were an E field with 'e' being an electron, would the direction of the travel of the electron simply be the vector sum of its original velocity and the force of the electric field?

|--------------------------->
|--------------------------->
|-----------e--------------->
|--------------------------->
|--------------------------->

I mean, regardless of whether the electron were coming out of the screen, going into the screen, moving left, right, up or down, would the electric field's net force simply point to the LEFT?

I know in magnetic fields you use the right hand rule, or if a charged particle is moving perpendicular to the field, then it will experience no force. Is the same true for electric fields?

Thanks in advanced, I hope my post isn't too convoluted.

 

[sotired]

is freezing a spontaneous process?

what about boiling? i know boiling is endothermic. and freezing exo...

 

[osjx-82]

I have a question about the photoelectric effect. Let me babble on about what I THINK is correct, and please correct me if I'm not.

The equation is E = hf, where E is the energy of the SINGLE photon and f is the frequency of the incident LIGHT that contains many of these photons. I hope this is correct.

If you increase "f" (that is to say, if you increase the frequency of the incident LIGHT) then you also increase the energy of the SINGLE photons porportionally.

Is it also true that increasing the frequency of the light will increase the number of photons as well?

 

[gridiron]

I have a question about the photoelectric effect. Let me babble on about what I THINK is correct, and please correct me if I'm not.

The equation is E = hf, where E is the energy of the SINGLE photon and f is the frequency of the incident LIGHT that contains many of these photons. I hope this is correct.

If you increase "f" (that is to say, if you increase the frequency of the incident LIGHT) then you also increase the energy of the SINGLE photons porportionally.

Is it also true that increasing the frequency of the light will increase the number of photons as well?

Yes, a quantum of a light wave of frequency f has energy of the form: E=hf. The least energy a light wave of frequency f can have is hf--which is the energy of a single photon. If you increase the frequency, by an integer multiple, then yes the energy of a single photon is increased. However, increasing the frequency of light does ot increase the number of photons. This is because the frequency is characteristic of the velocity and wavelength of the wave and gives the energy of the wave. Intuitively, you would think that if you view light as an electromagnetic wave, you must expect that no matter what the frequency is that electrons can be ejected. This is not what happens. The frequency of light must be above the materials cut-off potential in order to liberate electrons. The number of photons can be increased by increasing the intensity of light, but the photon energy is governed by E=hf.

 

[gridiron]

I'm more than a little confused about electric fields. For an electric field, is the movement simply dictated by the direction of the E field and the charge of the particle?


If this were an E field with 'e' being an electron, would the direction of the travel of the electron simply be the vector sum of its original velocity and the force of the electric field?

|--------------------------->
|--------------------------->
|-----------e--------------->
|--------------------------->
|--------------------------->

I mean, regardless of whether the electron were coming out of the screen, going into the screen, moving left, right, up or down, would the electric field's net force simply point to the LEFT?

I know in magnetic fields you use the right hand rule, or if a charged particle is moving perpendicular to the field, then it will experience no force. Is the same true for electric fields?

Thanks in advanced, I hope my post isn't too convoluted.

The electric field above is one for a positive charge---the arrows point out. Any charge put into such an electric field will feel an electrostatic force: F=qE where E is the electric field at the point and q is the charge of the particle. When the electrostatic force is negative, you have attraction and when it is positive you have repulsion. To relate this with your question above: since the E field is produced by a positive charge, than E will be positive. But q is an electron, so the electrostatic force is negative. That means the electron will oppose the E field due to attraction to the proton and will accelerate toward the proton. Will it accelerate with increasing acceleration? Yes, because as it nears the source, it feels a greater force. Since F, the force, is increasing, than according to F=ma, the acceleration is as well. To relate this concept to electric potential: Since the electron is doing what it wants to do, move toward the positive charge, the work will be positive and the potential energy will decrease---in fact it is converted to kinetic energy. What would happen if the electron was pulled away? Obviously, the electron feels a electrostatic force toward the source, so it would be doing something it normally wouldn't do. In this case, and external force is required to pull the electron away to do work against the electric field. This makes the work negative and means the potential energy increases! I hope this helps!

 

[gridiron]

is freezing a spontaneous process?

what about boiling? i know boiling is endothermic. and freezing exo...

I think this question is better off in the chemistry thread where Q can help and advise you better. All I can say is that freezing is an exothermic process because heat is expelled. However, spontaneity is dependent on the value of G--the gibbs free energy. The entropy of the system is decreasing, so S would be negative. G= change in H - Tchange in S. So, I think this is dependent on the temperature of the medium. Since different materials freeze at different temperatures, than it would depend on the material. I always think that a process is not favorable where its entropy decreases---I COULD be wrong. The best person to ask is QofQuimica!!

 

[okcomputer]

Hi, I was hoping someone could explain the difference between diffraction and interference in terms of the patterns they create (bright and dark)

All I know (which could easily be wrong) is that the equation that refers to the bright spots in an interference graph seems to refer to the dark spots in diffraction. Is this correct?

Thanks for any help

 

[metastasis]

Two cartes are moving along a frictionless horizontal surface. The first crate, of mass M=100 kg, is being pushed by a force of 300N. The first crate is in contact with a second crate, of masss m=50kg.

Diagram

b)What's the force exerted by the larger crate on the smaller one?
c) What's the force exerted by the smaller crate on the larger one?

Question:
I don't understand yfor b they used ma and not Ma, and for c how they did it.

Thanks.

 

[Cloudcube]

Hi there. Quick question. I'm a little confused with parallel resistors. I know that a voltage drop on one equals a total voltage drop, but what does that mean, exactly, in practice? For example, if I had a certain voltage would that mean that the voltage of one of the resistors would be the same as if I connected two resistors in parallel. So, regardless of whether I have one or two resistors I'd still have a voltage of 120 volts running through?

Thanks!

 

[legobikes]

Thanks BioMedEngineer!

Hi there. Quick question. I'm a little confused with parallel resistors. I know that a voltage drop on one equals a total voltage drop, but what does that mean, exactly, in practice? For example, if I had a certain voltage would that mean that the voltage of one of the resistors would be the same as if I connected two resistors in parallel. So, regardless of whether I have one or two resistors I'd still have a voltage of 120 volts running through?

Thanks!

I think so, if they're in parallel then regardless of their resistance they have the same voltage drop across. So whether there were a 5 ohm and 10 ohm resistor, two 10 ohm resistors, or one 2 ohm resistor, they'd each have 120V across.

 

[QofQuimica]

I think this question is better off in the chemistry thread where Q can help and advise you better. All I can say is that freezing is an exothermic process because heat is expelled. However, spontaneity is dependent on the value of G--the gibbs free energy. The entropy of the system is decreasing, so S would be negative. G= change in H - Tchange in S. So, I think this is dependent on the temperature of the medium. Since different materials freeze at different temperatures, than it would depend on the material. I always think that a process is not favorable where its entropy decreases---I COULD be wrong. The best person to ask is QofQuimica!!

This is basically right. The only thing I'd add is that you have to also consider the effects of pressure. Think of a phase diagram; those show you what phases will spontaneously exist for a given temperature and pressure. So in answer to the original question, yes, freezing or boiling will be spontaneous at certain temperatures and pressures for any substance, and you can actually manipulate temperature and pressure to get the phase that you want. For example, I can make water boil at room temperature by putting it under a vacuum. We use decreased pressure all the time in the lab to remove our solvents so that we don't have to heat the compounds and risk decomposing them. 🙂

BME, will you go to the gen chem thread and see if you can help this student? I'm not sure whether I totally understand the question.

http://forums.studentdoctor.net/showpost.php?p=4013867&postcount=589

 

[5moreminutes]

Is there always an accelration on a particle moving with a net force?no matter what the system? This is because of newtons second law right?

 

[5moreminutes]

In a capacitor, what is it that tells us whether the particle is moving at a constant velocity or not?

thanks ..

 

[grapeflavorsoda]

Is there always an accelration on a particle moving with a net force?no matter what the system? This is because of newtons second law right?

yes.

yes.

sort of yes.

 

[grapeflavorsoda]

In a capacitor, what is it that tells us whether the particle is moving at a constant velocity or not?

thanks ..

net force = 0.

 

[googlinggoogler]

I'm sorta confused..for a floating object, the weight of the object is equal to the buoyant force. So, is an object that hovers underneath the fluid considered to be floating? And if it is not, then its wieght is the apparent weight, right? Thanks!

 

[grapeflavorsoda]

I'm sorta confused..for a floating object, the weight of the object is equal to the buoyant force. So, is an object that hovers underneath the fluid considered to be floating? And if it is not, then its wieght is the apparent weight, right? Thanks!

yes. the object has same density as the fluid.

nope. apparent weight would be actual weight minus the bouyant force.

so

if we let o = density of object, v=volume of the object that is submerged,
f = density of the fluid, and g = gravitational acceleration constant, then

apparent weight = ovg - fvg = (o-f)vg.

 

[Cloudcube]

There's a person on a scale in an elevator. Scale reads 60 kg (or some other random value). Say this person accelerates at... 3 meters per second squared. What is this person's scale weight when accelerating. I don't think I'm solving this problem right.

Fn=ma + mg
Fn=ma + 60
Fn= (60/gravity)(3)+60
Fn=78kg

What am I doing wrong?

Thanks!

 

[Cloudcube]

Does work depend on the path taken to get there or no? If work is dependent on path then why is it that two blocks that are lifted up against gravity (one with a straight-line path, the other with a zig-zag path) will do the same amount of work?

Thanks!

 

[grapeflavorsoda]

There's a person on a scale in an elevator. Scale reads 60 kg (or some other random value). Say this person accelerates at... 3 meters per second squared. What is this person's scale weight when accelerating. I don't think I'm solving this problem right.

Fn=ma + mg
Fn=ma + 60
Fn= (60/gravity)(3)+60
Fn=78kg

What am I doing wrong?

Thanks!

is it accelerating upward or downward?

anyway, it doesnt matter.

let's just define the usuals.

let N = force applied by spring.
a = acceleration of the elevator.
m = mass of the object of the interest.
g = magnitude of the gravitational constant.

define upward as positive and downward as negative.

notice that net force = ma; here a is the acceleration of the object which is equal to acceleration of the elevator.

so,

N + (m)(-g) = ma

scale weight is equal to N. therefore,

N = scale weight = m(g+a)

 

[grapeflavorsoda]

Does work depend on the path taken to get there or no? If work is dependent on path then why is it that two blocks that are lifted up against gravity (one with a straight-line path, the other with a zig-zag path) will do the same amount of work?

Thanks!

work is numeric value you obtain by performing line integral with force vector and the path(which is also a vector).

if the force vector is a derivative of a function, then when you perform the line integral, it will just be the antiderivative of the force vector function in terms of the path function which is in turm could be expressed in parametrization.

so if the force vector in question is just a derivative of a function, then work will not depend on the path. such a vector function is called the conservative function.

when the force vector is not a conservative function then you have to perform the line integral.

static electric force and gravitational force vector are conservative functions, so when you perform work against these forces it will be path independent.

but other forces in nature are not conservative functions so work performed against these forces are path dependent.

 

[Cloudcube]

Thanks for both of those! That helps a lot.

 

[Cloudcube]

I know you guys have addressed this before, but I'm still confused with electric fields. How do I determine where the electric field is pointing?

 

[harrypotter]

Hi, will someone please tell me the difference between flow speed and flow volume?

If you have a pipe where one end is wide and the other end is more narrow, where is flow speed the greatest and flow volume the greatest?

Thank you!

 

[grapeflavorsoda]

I know you guys have addressed this before, but I'm still confused with electric fields. How do I determine where the electric field is pointing?

for static electrical charges, away for positive and toward for negative.

 

[grapeflavorsoda]

Hi, will someone please tell me the difference between flow speed and flow volume?

If you have a pipe where one end is wide and the other end is more narrow, where is flow speed the greatest and flow volume the greatest?

Thank you!

flow speed at a point is a numeric value obtained by taking norm of the flow vector at that point; i.e. it just assigns a speed value of fluid at a particular point.

flow volume is rate of volume passing through an area at a given period time.
so flow volume is related to flux of fluid at an arbitrary surface.

for mcat, it is assumed that fluid movement is in steady states.

this implies all the functions that describes the fluid is time independent. therefore fluid speed at a given point will obviously remain constant under steady state.

for the condition you have described, the flow speed will be larger at the surface having smaller area, but flow volume(rate) will remain same for both sides.

 

[harrypotter]

flow speed at a point is a numeric value obtained by taking norm of the flow vector at that point; i.e. it just assigns a speed value of fluid at a particular point.

flow volume is rate of volume passing through an area at a given period time.
so flow volume is related to flux of fluid at an arbitrary surface.

for mcat, it is assumed that fluid movement is in steady states.

this implies all the functions that describes the fluid is time independent. therefore fluid speed at a given point will obviously remain constant under steady state.

for the condition you have described, the flow speed will be larger at the surface having smaller area, but flow volume(rate) will remain same for both sides.

Thank you for the explanation. Just to check, so flow speed is the actual speed that the liquid is coming out of the pipe and flow volume is how much liquid is coming out. And in regards to the funky pipe, the small end is like holding your thumb over the hose and having the water come out faster. Flow volume has to be the same because what goes in must come out?

 

[grapeflavorsoda]

Thank you for the explanation. Just to check, so flow speed is the actual speed that the liquid is coming out of the pipe and flow volume is how much liquid is coming out. And in regards to the funky pipe, the small end is like holding your thumb over the hose and having the water come out faster. Flow volume has to be the same because what goes in must come out?

sort of yes.

by the way, constant flow volume can hold in steady state only if the fluid is assumed to be incompressible; it is a consequence of constant flow of mass in steady state when fluid is not compressible.

 

[Cloudcube]

So, just to make sure, if I have a POSITIVE particle moving this way:

-------------------------------> then the electric field is in the same direction.

If the particle is going in the same direction as the positive particle than the e-field would be going...

<---------------

?

 

[tik-tik-clock]

So, just to make sure, if I have a POSITIVE particle moving this way:

-------------------------------> then the electric field is in the same direction.

If the particle is going in the same direction as the positive particle than the e-field would be going...

<---------------

?

Depends on what direction the electron is in. The positive charge will always move in the direction of the electron. ... I m not sure I understood your question..

 

[87138]

Speaking of electrons and fields . . .

Ok this question is linked to a passage (not AAMC) about charged oil droplets in an electric field. The background really isn't necessary I don't think.

A drop of oil of mass 5x10^-16 kg is at rest on the bottom of a plate of a parallel plate combination when the electric field is zero. An electric field of 4x10^3 V/m is then applied between the plates, accelerating the drop towards the top plate. What will be the resultant acceleration of the drop if it carries a negative charge of 3x10^-18 C?

A) 9.8 m/s^2
B) 14.2 m/s^2
C) 24.0 m/s^2
D) 28.4 m/s^2


I answered C, but the "correct" answer is B. Here was my logic.

F = Eq, and F = ma, so Eq = ma.

(4x10^3)(3x10^-18) = (5x10^-16)(a)


Solve for a, and you get 24 on the dot.

Where am I going wrong here?

 

[Cloudcube]

Depends on what direction the electron is in. The positive charge will always move in the direction of the electron. ... I m not sure I understood your question..

If the electron is going right, --->, then the electric field would be pointing the opposite way? I'm just having trouble determining where the electric field points and my books say to use electric field lines but I don't know how.

 

[w1ll]

I have a more general question. How exactly has been your successful strategies in tackling the Physical section? (ie. read passage CAREFULLY, or with speed, -> then go back 2x or w/e -> Read each question & try to think of answer before looking at choice? -> spend 1 minute max, or guess -> or what???)

How much info are you using from passage vs knowledge usually?

I don't know why but I've gotten the same physics score on every single AAMC practice test (This is even AFTER I went back and actually remembered and reviewed the material well). I just can't seem to bring up my Physical score at all!! Although my foundation of the information could always use some work, I feel like I pretty much know everything for the MCAT's. Yet whenever I go over my answers, it seems to be either 1) I didnt realize something I should've known from the passage info only 2) I made a dumb mistake. 3) tricky question, guessed the wrong one out of 2 choices. And 1&2 happen alot!

I even have problems finishing physics on time! I always leave atleast one passage all C's.... Yet I can finish verbal with 10 minutes to spare and Bio on time.

And while I'm taking the practice MCAT's I have 1-2 passages I have trouble figuring out what they're talking about in the passage! In those cases I just try to use my background info to solve the problems

Sorry if this is a rant, just frustrated with MCAT's being so close and finishing all the AAMC's with little success.

 

[wannaberockstar]

Is the temperature of light related to its intensity?

 

[Dave_D]

Is the temperature of light related to its intensity?

Umm, light doesn't have a temperature. Are you asking how the temperature of an object relates to the radiation emits?(If so you really want to read up on black body radiation unless someone else here wants to talk about that. I'm a little weak on that.)

 

[tracy34]

Hi,
I have a lot of trouble with the concept of tension in physics questions. In the exam crackers text it shows a mass hanging on a string and asks what the tension is...that's pretty straight forward, T=mg. It then asks, albeit rhetorically, when T would equal 2mg or when it would equal 0. I have no idea how to figure that one out.

 

[peterpansy]

Hi,
I have a lot of trouble with the concept of tension in physics questions. In the exam crackers text it shows a mass hanging on a string and asks what the tension is...that's pretty straight forward, T=mg. It then asks, albeit rhetorically, when T would equal 2mg or when it would equal 0. I have no idea how to figure that one out.

I would say that if the mass was accelerating upwards with a=g, then T would equal 2mg.

 

[Cloudcube]

IYet whenever I go over my answers, it seems to be either 1) I didnt realize something I should've known from the passage info only 2) I made a dumb mistake. 3) tricky question, guessed the wrong one out of 2 choices. And 1&2 happen alot!

I have the exact same problem. And tips would certainly be appreciated.