Physics extra 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.

---

Members don't see this ad.

 

[QofQuimica]

question about the torque equations for both center of mass problems and physical pendulum. So the F*R, what is the R. For a center of mass problem, it's the pivot point to the center of that mass, but what about physical pedulum, is it still the same? If the pendulum was a uniform stick with one end tied to a beam, then R would be half the stick length right? Is the true for all the torque eqations?

If you mean the T = F x R x sin theta equation, then R is the distance from the fulcrum to the mass. Assuming that the pendulum was uniform, then its center of mass should be in the center, and the fulcrum is on one end, so I think you're right. DrChandy or Nutmeg, if you're reading this, will you confirm?

 

[runnora]

Sorry; I've posted a fairly comprehensive answer to this somewhere, but now I can't find it. Be patient and I will get to it.

shrike you are awsome!! wow, i am amazed at all your posts here! thanks for all your help!

 

[Madcoomofo]

Circuits and light bulbs....

Circuit A: Let's say you have a 12 V battery, and a resistor (light bulb) with 2 Ohms resistance.

Circuit B: 12V battery with two 2 Ohm light bulbs connected in series.

Circuit C: 12V battery with two 2 Ohm light bulbs connected in parallel.


Could you please discuss the overall brightness of each circuit, and also the brightness of each light bulb?

Thanks!

 

[Dave_D]

Circuits and light bulbs....

Circuit A: Let's say you have a 12 V battery, and a resistor (light bulb) with 2 Ohms resistance.

Circuit B: 12V battery with two 2 Ohm light bulbs connected in series.

Circuit C: 12V battery with two 2 Ohm light bulbs connected in parallel.


Could you please discuss the overall brightness of each circuit, and also the brightness of each light bulb?

Thanks!

Well lets look at the equations you'd need to solve this. They are

P=IV
V=IR

solving for I in the first equation we get P=V^2/R.

Ok, so what's the total resistance? In the first one we're given it, it's 2 Ohms. Plug the values in and we get

P=12^2/2 = 72Watts

What about the second? The rule for resisters in series is just add them up and that's the total resistance. So

Rtotal= R1 + R2 = 2 Ohms + 2 Ohms = 4 Ohms

So just plug that in we get

P=12^2/4 = 36 Watts

So together the bulbs put out a total of 36 watts, or 18 watts each

The last one remember the rule for getting the total resistance with parallel circuits. It's

1/(Rtotal) = 1/R1 + 1/R2 = 1/2 + 1/2 = 1


So 1/Rtotal = 1. So we take the inverse to get Rtotal = 1 Ohms. BTW, be very careful when doing parallel circuits that you remember to take the inverse, otherwise you'll get the wrong total resistance. So plug in the values.

P=12^2/1=144Watts.

So the bulbs put out a total of 144 Watts, or each one puts out 72 Watts.

So lets sum up. In A the bulb puts out 72 Watts, in B each puts out 18 Watts, in C each bulb each puts out 72 watts. So in A and C the bulbs are the same brightness.(Which is to be expected, they're in parallel and can draw current separately. So the bulbs should affect each others brightness.) However in B since we link them in series we affect the bulbs brightness and find the bulb now puts out 1/4 the energy as before. As for total brightness B is the least bright and is half as bright as A. C is twice as bright as A.

Anyway hope I got that right.(Haven't done circuits in a few months so I'm doing this off the top of my head. Take what I say with a grain of salt.)

 

[kevin86]

so brightness is just another way to say power, would intensity be power as well
? my book only mentions intensity, never brightness.

 

[Dave_D]

so brightness is just another way to say power, would intensity be power as well
? my book only mentions intensity, never brightness.

Well in my physics course brightness was synonymous with intensity which was just power.(Watch me be wrong though.) I think they might mention "intensity" because people might think brightness = how bright it looks to me.(Which brings up the issue in what spectrum is the light being emitted in. I'll clarify, suppose some of these bulbs emitted mostly in infrared and others mostly in visible. So to the naked eye we might see the infrared bulb dimmer than expected even though it's emitting as much energy as the equations suggest it should. These questions never really care about that, they just care about how much energy total is coming out, regardless of frequency or anything. Anyway more than you need to know.)

 

[poppytart]

Just wondering if anyone knew when momentum is conserved and when total energy is conserved. Also, when the momentum is conserved that means that m1v1=m2v2 and that v1 doesn't necessarily have to equal v2 right? They just need to balance out in the end?

 

[QofQuimica]

Just wondering if anyone knew when momentum is conserved and when total energy is conserved. Also, when the momentum is conserved that means that m1v1=m2v2 and that v1 doesn't necessarily have to equal v2 right? They just need to balance out in the end?

Momentum will always be conserved on the MCAT. KE is only conserved for elastic collisions, not for inelastic. You are right that the final momentum has to equal the initial momentum in order for momentum to be conserved.

 

[Dave_D]

Just wondering if anyone knew when momentum is conserved and when total energy is conserved. Also, when the momentum is conserved that means that m1v1=m2v2 and that v1 doesn't necessarily have to equal v2 right? They just need to balance out in the end?

Wait, are you asking is total energy in a system conserved or kinetic? Q has it right, KE is conserved in totally elastic collision but not in inelastic. However in general the total energy (and I mean EVERYTHING) is always conserved as far as the MCAT is concerned.(It's just transformed. So for example in an inelastic collision that lost KE gets turned into thermal energy if I remember correctly.) Of course this is probably more than you need to know.

 

[poppytart]

wow you are guys are fast...thanks! that really helped! i also had another question (sorry I just realized that there was this discussion board and i am super stoked!) i am confused when to answer if a particle is accelerating or not. for instance, two charges with opposite signs would accelerate towards each other because they have a force acting on them, right (kqq/r2)? But, how about like an object rolling down an incline plane...

For example, after a block begins to slide, how does it speed vary with time? (assuming that tension and kinetic frictional forces are constant in magnitude)

 

[dbhvt]

Momentum will always be conserved on the MCAT.

Sorry Q, beg to differ. Momentum is conserved when there are no external forces. If you have external forces, you're not going to be able to conserve momentum. I seem to remember this being an issue in at least one MCAT question.

Remember, the second law says the net force on a particle is equal to the product of it's mass and acceleration (F=ma). Another way Newton's 2nd is presented, and another way you can think of it, is that the net force on a particle is equal to the rate of change in that particles momentum (F=[p2-p1]/[t2-t1])

For the curious:
If you don't know calculus, just change the little d's to delta signs. I don't know how to make delta signs here.

Remember, F=ma
since a=dv/dt, we can sub in the def of acceleration.
F=m(dv/dt)
recalling p=mv, we see now how net force = rate of change in p.
F=dp/dt

 

[dbhvt]

wow you are guys are fast...thanks! that really helped! i also had another question (sorry I just realized that there was this discussion board and i am super stoked!) i am confused when to answer if a particle is accelerating or not. for instance, two charges with opposite signs would accelerate towards each other because they have a force acting on them, right (kqq/r2)? But, how about like an object rolling down an incline plane...

For example, after a block begins to slide, how does it speed vary with time? (assuming that tension and kinetic frictional forces are constant in magnitude)

Newton again. Sum the force vectors. That will be equal to the magnitude of the mass times the acceleration and point in the direction of the accel.

F=ma Live it, breathe it, be it.

Count the forces. Things you're touching, gravity if you need it, friction if you need it, electric or mag if you need it.

So with the block
Things you're touching: the plane (normal force)
Gravity: check
Friction: check (kinetic).
There's no tension unless it's touching something like a string.

If those forces cancel out, no accel. If they don't cancel out, you're accelerating.

Edit: You said "after" the block starts to slide. Realize the "just beginning to slide" is code for static friction at the breaking piont (set F=ma to 0, because you haven't started moving yet).

Q, is this allowed? Me answering these questions? You mentioned something, so I just kind of dove in...

 

[QofQuimica]

I seem to remember this being an issue in at least one MCAT question.

Really? Was it a test prep company question, or an AAMC one? Every MCAT-like momentum question I've ever seen has been a scenario where there are no external forces acting on the system, like two hockey pucks colliding or something like that. Anyway, it's a good point that what I said in my last post assumes that there are NOT any external forces acting on the system. Thanks for clarifying.

 

[QofQuimica]

Q, is this allowed? Me answering these questions? You mentioned something, so I just kind of dove in...

Definitely. We're always looking for good people to help answer questions. That's why I invited you. If you feel that physics is your strong point, I will more or less leave this thread to you, and I'll take the chem ones.

 

[Dave_D]

Edit: You said "after" the block starts to slide. Realize the "just beginning to slide" is code for static friction at the breaking piont (set F=ma to 0, because you haven't started moving yet).

Actually I think you should clarify this a little. Once it "just begins to slide" we stop worrying about static friction and start looking at dynamic friction.(Static friction> dynamic) So we have to do all the work, calculating the normal force, force down the plane, etc to see what the net force is. (Just an aside to the original asker, do you know how to calculate a normal with trig? It's not hard, I can run through the thought process if you want but then again loads of people here can as well.)

Oh BTW (again to the original asker.) you need to be careful to remember that acceleration and velocity are vectors.(dbhvt is being careful to point this out but it's really important to remember since it can get students far too often.) I'm just bringing it up because dbhvt points out that the acceleration in directed in some direction but he is not saying the velocity is in any particular direction.(Because he doesn't have enough info to say what direction the velocity is in, it could be in a completely different direction than acceleration.)

 

[QofQuimica]

(Just an aside to the original asker, do you know how to calculate a normal with trig? It's not hard, I can run through the thought process if you want but then again loads of people here can as well.)

Dave, if you want to write up a general explanation of how to determine the forces for blocks sliding down inclines, that would be great. We'll add it to the explanations thread.

 

[poppytart]

Well, the answer to that block question is that its speed increased linearly with time....I thought that since there was a consant force that acceleration would be constant leading to no change in speed?

 

[st.exupery]

I'm getting confused about waves. Is this correct?

Sound waves: needs a medium to travel. Velocity increases as it travels through a medium, while f stays the same, thus wavelength increases? But the Doppler effect can change just the observed frequency?

EM waves: does not need a medium to travel. Velocity decreases depending on the medium, frequency stays the same, so wavelength changes?

Light (EM): Higher frequency waves take longer to travel through a medium and are thus dispersed more than lower frequency wavelengths. So does this mean that velocity and wavelength are the only things that change also?

Lastly, in reference to the galvanic/electrolytic cell, electrons always go to the anode. This might be the wrong thread, but in o-chem, anions always go to the anode, correct? (regarding lab techniques)

Thanks all!

 

[dbhvt]

Actually I think you should clarify this a little. Once it "just begins to slide" we stop worrying about static friction and start looking at dynamic friction.(Static friction> dynamic)

FINE POINTS:

This is what I meant by 'code for...'

If it's actually moving, and you're worried about what is happening while it's moving, then yes you do stop worrying about static friction and start looking at dynamic friction (or kinetic friction, whichever term you want to use).

If the problem says "JUST BEGINS to slides", almost always, the moment of interest is the change from sitting there and not moving to moving. This moment is best described in equations as static friction maxed out.

(I wish I knew how to do greek letters, subtext and supertext)

Recall that F(static friction) is < or = (the coefficient of static friction)*(the normal force).

Because it's an inequality, you generally can't solve the equation definitively if you are told the block is just sitting there and not moving.

But if they tell you it is at the point where it JUST STARTS moving, they're interested in the moment when the net force vector overcomes the force of static friction. Set F=ma to zero, and F(static friction) to it's maximum value in the inequality.

There are a couple of buzz words in physics problems that are like this, where we go from one state to another state, each with different force vectors. They can be a little tricky and counter intuitive, but generally want to look at the state BEFORE the change.

 

[dbhvt]

Definitely. We're always looking for good people to help answer questions. That's why I invited you. If you feel that physics is your strong point, I will more or less leave this thread to you, and I'll take the chem ones.

Q-

How about if I just answer things here when I'm bored?

Now that I know where I'm going, I'm in shedding responsibility mode.

 

[PBandJ]

Lastly, in reference to the galvanic/electrolytic cell, electrons always go to the anode. This might be the wrong thread, but in o-chem, anions always go to the anode, correct? (regarding lab techniques)

Thanks all!

regarding your electrochemistry question (which probably belongs in the general chemistry thread):

electrons always flow from anode to cathode. the cathode is where reduction occurs (gaining electrons); the anode is where oxidation occurs (losing electrons). in a galvanic cell (spontaneous), the anode is negative while the cathode is positive. in an electrolytic cell (non-spontaneous), the anode is positive while the cathode is negative (hence why an applied voltage is needed to move the electrons TOWARD the negative charge).

 

[st.exupery]

hi scentimint, thanks, I was afraid I wrote that. I meant, electrons go to the cathode. The thing I wanted to clarify was that in regards to o-chem lab techniques (i.e. electrophoresis), the anions migrate to the anode, right? I've been keeping that in my head, and then I read the posts about electrolytic/galvanic cells and realized that I should separate these thoughts in my head. Hope that made sense.

 

[poppytart]

ACCELERATION AND VELOCITY
(this is a key point, so let me know if you got it)

Acceleration is defined as a change in velocity over some time.

So if the net force is not zero, then every single time without fail no matter what, the acceleration is not zero and the velocity is changing.

If the net force is constant, the acceleartion is constant and the velocity is changing at a constant rate.

Say you have a constant acceleration of 10 m/s^2 down.

Every second, you add 10 m/s down to your previous velocity vector. If you start out going 5 m/s down, in 1 second you'll be going 15 m/s down, in two you'll be going 25 m/s down, etc....

If you have non-zero accelaration, but it is NOT constant, your velocity vector will change, but it will change different amounts every second. Eg, start out at 5 m/s down, next second your at 40 m/s down, the next second you'r at 2 m/s to the left.


SPEED vs VELOCITY

As far as speed is concerned, remember that speed is a scalar (just a number) and velocity is a vector (a number and a direction). You could come up with a situation where there is no change in speed but a change in velocity. That situation would be a particle moving in a circle without speeding up or slowing down (ie uniform circular motion). The velocity will be changing, because the particle is not moving in a straight line. Even though the number part of the velocity isn't changing, the direction part is changing.

Oh okay, I think I got it...does this make sense? Since the force is constant...the acceleration is constant, meaning no change in velocity. But, velocity is a change in either speed or direction and only one of them have to remain constant. So, since the acceleration is constant, a=F/m...its speed will be equal to the equation v=at. While t increases v (its speed) must also increase with time since acceleration remains constant. It kinda reminds me of those V=IR questions because we know R remains constant (depends on pL/A)...so if V increases so does I because R is constant.

Thanks so much for your help! One more quick question...If you throw an object up in the air the time it takes for it to reach the top is longer than the time it takes during its descent right? Is it because on the way down it has the force of gravity pulling it down so its velocity is greater than the way going up? If not, can you tell me a scenario when this would be true?

 

[poppytart]

Thanks so much for your help dbhvt ...that really clears things up! I really appreciate you taking the time to help me with these questions!

 

[dbhvt]

Thanks so much for your help dbhvt ...that really clears things up! I really appreciate you taking the time to help me with these questions!

No problem.

 

[Dave_D]

Dave, if you want to write up a general explanation of how to determine the forces for blocks sliding down inclines, that would be great. We'll add it to the explanations thread.

You mean just how to calculate a normal or a force down a ramp? It's pretty straight forward but the normal is F=mg*cos(x) and the force down the ramp is F=mg*sin(x) where x is the incline of the ramp. I can write something up later today about what all that means.(Or do you mean how do you go about attacking the problem in the first place?)

 

[Dave_D]

FINE POINTS:

This is what I meant by 'code for...'

If it's actually moving, and you're worried about what is happening while it's moving, then yes you do stop worrying about static friction and start looking at dynamic friction (or kinetic friction, whichever term you want to use).

If the problem says "JUST BEGINS to slides", almost always, the moment of interest is the change from sitting there and not moving to moving. This moment is best described in equations as static friction maxed out.

(I wish I knew how to do greek letters, subtext and supertext)

Recall that F(static friction) is < or = (the coefficient of static friction)*(the normal force).

Because it's an inequality, you generally can't solve the equation definitively if you are told the block is just sitting there and not moving.

But if they tell you it is at the point where it JUST STARTS moving, they're interested in the moment when the net force vector overcomes the force of static friction. Set F=ma to zero, and F(static friction) to it's maximum value in the inequality.

There are a couple of buzz words in physics problems that are like this, where we go from one state to another state, each with different force vectors. They can be a little tricky and counter intuitive, but generally want to look at the state BEFORE the change.

Oh point taken. I agree, it can be a bit tricky since wording can make a world of difference.(Yes, been burnt by that before.)

 

[Dave_D]

I'm getting confused about waves. Is this correct?

Sound waves: needs a medium to travel. Velocity increases as it travels through a medium, while f stays the same, thus wavelength increases? But the Doppler effect can change just the observed frequency?

I'm not sure what you're asking here. Yes sound requires a medium and it travels at a constant speed through a given medium. It doesn't speed up or slow down traveling through a given medium. It will change speed going from one medium to another but frequency stays the same. Also the doppler effect can change the frequency.(Relative motion between observer and emitter will do that.)

EM waves: does not need a medium to travel. Velocity decreases depending on the medium, frequency stays the same, so wavelength changes?

Light (EM): Higher frequency waves take longer to travel through a medium and are thus dispersed more than lower frequency wavelengths. So does this mean that velocity and wavelength are the only things that change also?

Lastly, in reference to the galvanic/electrolytic cell, electrons always go to the anode. This might be the wrong thread, but in o-chem, anions always go to the anode, correct? (regarding lab techniques)

Thanks all!

Yes, light does not need a medium to travel. Velocity does depend on the medium although frequency stays the same. I hate to admit but I'm a little iffy on how refraction relates to frequency.

Actually here's a rule of thumb, waves going from one medium to another don't change frequency. Speed and wavelength change.(There's an example of a wave going down a string crossing from a light to heavy string that really illustrates this. I think I can find a picture of the set up and I can give the explaination of why it's true. It's apparently is true of any wave.) Do you want me to go through why frequency doesn't change between mediums?

 

[st.exupery]

DaveD,
Thank you for taking the time to answer my questions. You don't have to go into the explanation of why freq. doesn't change. I was just having a hard time keeping the relationships straight. I was looking through EK physics and it seems that they state some steadfast rules and then add to it saying something along the lines of 'except for light,' or 'except for sound'.
So i will just keep it at: frequency does not change depending on the medium, only velocity and wavelength, for all types of waves. (unless there is a doppler shift).

 

[93106]

Stupid question:

If the SI unit for elec pot is volts, how is it equal to an amount of Work? Work has units of Joules, right?


Ridiculous question:

Will it help me for the mcat to be very familiar with the SI units and their derivatives for potential electricity and magnetism questions?


Last question:

What would you say is the most often misunderstood sort of question about elec and magn on the mcat?

 

[BoyGenius]

Let's say someone is standing in a frictionless ice rink and someone tosses them a ball. Who would experience a greater impulse, someone that catches it and holds on or someone that catches it starts to move and then drops it? Please explain in detail using F, dt, and dp. Thanks! Assume that both times the balls were thrown with the same constant speed.

 

[QofQuimica]

Q-

How about if I just answer things here when I'm bored?

Now that I know where I'm going, I'm in shedding responsibility mode.

Well, I'll take whatever I can get; this forum is a lot of work, as I'm sure you can imagine. I don't know what it is with you physics people, but I sure go through a lot of you.

BTW, I'm going to use your a vs. v post for the explanations thread. Thanks for writing that.

 

[Madcoomofo]

Bernoulli's equation:

Okay, so it says that where when velocity of the fluid increases, the pressure decreases.

Would it also be correct to say that pressure is directly proportional to area?

EK says that pressure is inversely proportional to area, and this just doesn't make sense to me...

Thanks!

 

[Mr. Three-Wiggle]

Pressure = Force/Area. An increase in area results in a decrease in pressure; a decrease in area results in an increase in pressure.

 

[Madcoomofo]

Pressure = Force/Area. An increase in area results in a decrease in pressure; a decrease in area results in an increase in pressure.

But what about fluid flow going from a fat pipe into a thin pipe. The velocity increases when fluid enters the thin pipe, and the pressure decreases. Hasn't the cross sectional area also decreased?

You see, the reason for my confusion is that in the EK bio review book (edition 5, page 142), they try to make the point that blood flow does NOT follow Bernoulli's equation. In trying to make this point, they say, "Bernoulli's equation tells us that pressure is inversely proportional to cross-sectional area, but in blood vessels this is not the case..."
But it IS, like that in blood, right? Capillaries have big overall cross-sectional area, low pressure.
I can't quite make the connection though because Pressure is low in capillaries, which have the greatest area...


EDIT: ohhh but the capillaries do have more pressure than venules.. I get it.. So it kinda follows Bernoulli's equation, but not completely...

 

[DesiMcatAcer]

But what about fluid flow going from a fat pipe into a thin pipe. The velocity increases when fluid enters the thin pipe, and the pressure decreases. Hasn't the cross sectional area also decreased?

You see, the reason for my confusion is that in the EK bio review book (edition 5, page 142), they try to make the point that blood flow does NOT follow Bernoulli's equation. In trying to make this point, they say, "Bernoulli's equation tells us that pressure is inversely proportional to cross-sectional area, but in blood vessels this is not the case..."
But it IS, like that in blood, right? Capillaries have big overall cross-sectional area, low pressure.
I can't quite make the connection though because Pressure is low in capillaries, which have the greatest area...


EDIT: ohhh but the capillaries do have more pressure than venules.. I get it.. So it kinda follows Bernoulli's equation, but not completely...

I am sorry if I am not supposed to answer this, but from what I understand Bernoulli's equation applies to ideal fluid flow(there is a variation to apply to non-ideal system, but that is beyond scope of MCAT). Blood is a non-ideal fluid, it is considered Non-newtonian fluid and hence, that bernoulli's equation for ideal flow does not apply to blood. If anyone wants to add or correct me feel free to do so. I hope that clarifies a little bit

 

[putamen]

m2sin(theta2) = m1sin(theta1)

Which equation is NOT used in the derivation?


A. F = mg
B. F = Fsin(theta)
C. F = ma
D. F = Fcos(theta)