copy physics posts

[Shrike]

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

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

Acceptable topics:

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

Unacceptable topics:

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

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

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

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

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

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[Futuredoctr]

So I'm reading my kaplan books (and others) and for example snell's law ( or any thing with a trig function) will come up and Ill derive it down (because after all I do have all equations memorized ) and I'll get something like Sin@=.3/1.8 (@=theta) and then the next thing I know the answer reads @=9.5 degrees. How do I do that? I believe its inverse sin, but in my head, come on?? Any help on how to figure problems like this out in a hurry would be great! Thanks!

 

[Futuredoctr]

Thanks Shrike!!! I appreciate you setting this up!

 

[Futuredoctr]

Next question if you have time. What equations are a must for the MCAT, as my review material contains seriously A LOT of equations, and if I need to commit them to the small memory I have left I should start now 😉 .

 

[Shrike]

What equations are a must for the MCAT, as my review material contains seriously A LOT of equations, and if I need to commit them to the small memory I have left I should start now 😉 .

That one's going to take some time to put together; give it a while, please, but eventually I'll have it in the FAQs.

Re my editing of your second post: I'm just trying to keep the thread fairly uncluttered, so I took out the big quotation of my answer.

Thanks for moving your question in here, and congratulations on being the first to take advantage of the new forum. I appreciate the opportunity to help.

To others: I'll be looking for help. If you know your physics and want to lend a hand, please PM me.

 

[psiyung]

Hey Shrike, I can put together some equations if you want. I scored a 13 on the PS section a few years back. Just tell me if you are busy

 

[DarkLordofSith]

Hey Shrike,
I guess my question to you would be about how best to tackle Physics.....it's been a loooong time since I took physics, and I'm having a hard time trying to learn all this stuff again. I guess the stuff that's giving me the most trouble is Newtonian Mechanics, but I really do need to go over everything. So what would you suggest is the best way to tackle it? Thanks.

 

[Shrike]

I guess my question to you would be about how best to tackle Physics.....it's been a loooong time since I took physics, and I'm having a hard time trying to learn all this stuff again. I guess the stuff that's giving me the most trouble is Newtonian Mechanics, but I really do need to go over everything. So what would you suggest is the best way to tackle it? Thanks.

You will need to two things by the time you're done: (1) the basic equations, a list of which I'll be compiling (there are somwhere between thirty and fifty that I believe you absolutely must know) and some related definitions; and (2) an understanding of the major concepts. For example, with Newtonian mechanics, you need F = ma, F(G) = GMm/R^2, the equations for uniformly accelerated motion (e.g., d = v(0)t + 0.5at^2), and an understanding of three concepts: velocity and acceleration can be in the same or different directions; all forces and motion can be broken into x- and y-components, which you then deal with separately; and with projectiles, once you've broken the motion down into components the x-component is trivial, because v(x) remains constant always.

That's the general idea, though of course it doesn't really answer your question. Give me a while; I'll be writing it up. Essentially, I need to put nine two-and-a-half-hour lectures into print, and I don't type fast. Meanwhile, the more specific questions are easier to address, but keep asking whatever you want answers to.

 

[DarkLordofSith]

Thanks for the help......Just a suggestion, but could you do a thread that's "General Physics Concepts" and put it in there......this thread could be more for specific questions and the other one could be a thread exclusively for mods where you add general info that you feel would be beneficial for the average test taker to know.

 

[freesolo]

alright torque problem:
a massless plank with a length of 4 m has two weights placed on it. One with a mass of 10 kg on the left edge of the plank, while the other mass of 20 kg is placed 1.5 m inward from the opposite right edge. Where should the fulcrum be located if the plank with the weights on it were to remain horizontal?
Also do i have to use the CCW and CW to determine whats negative and positive torque?

 

[Shrike]

a massless plank with a length of 4 m has two weights placed on it. One with a mass of 10 kg on the left edge of the plank, while the other mass of 20 kg is placed 1.5 m inward from the opposite right edge. Where should the fulcrum be located if the plank with the weights on it were to remain horizontal?

The plank is massless, so only the two weights matter. Never mind the equation for this problem; one's twice as heavy, so the fulcrum needs to be only half as far from it. With the numbers given, one third of the distance between the two weights is 0.83m from the larger mass, 1.67 from the smaller mass (and therefore the left edge).

When using the center of mass equation/center of gravity equation, I recommend always putting the zero at the left-most mass or force (because you're used to zero being on the left). If you do that, you don't have to mess with the signs so much when handling torques: up is one sign, down is the other; no clockwise or counterclockwise.

The center of mass (which is where you want the fulcrum) is at

CoM = (m1X1 + m2X2)/(m1 + m2) = (10kg x 0m + 20kg x 2.5m)/(20kg + 10kg) = (5/3)m = 1.67m​

 

[Shrike]

Also do i have to use the CCW and CW to determine whats negative and positive torque?

Don't worry about using a different sign for different directions. In most problems, you care only about equilibrium, so everything zeros out anyway. If you have to calculate a torque, figure out its magnitude and then ask yourself what direction it's going.

 

[Shrike]

Just a suggestion, but could you do a thread that's "General Physics Concepts" and put it in there......this thread could be more for specific questions and the other one could be a thread exclusively for mods where you add general info that you feel would be beneficial for the average test taker to know.

We may eventually arrange it that way; we'll see what we come up with by way of general concepts (and how quickly).

Thanks for the suggestion.

 

[blankguy]

I could really use some help in understanding the buoyancy force in relation to figuring out the density of an submerged object. The archimedes principle.

 

[Shrike]

Why is it that no current runs through a capacitor in a circuit if the circuit has been on for a while?

Because that's the nature of a capacitor. The way a capacitor stores charge, and therefore energy, is by keeping charges crossing from one part (one plate, in the usual parallel plate capacitor) to the other, even though they want to. In the parallel plate capacitor, the plates are separated by a non-conductor, or by vacuum (which doesn't conduct current either). As long as the capacitor doesn't fail, charges can't cross through it.

The reason there is current in the short run is that charges are filling up the capacitor -- none of them is going through it.

When a capacitor fails -- it's often called dielectric breakdown -- the voltage has become high enough that those charges can get across the non-conductor (or vacuum) between the plates. Then, current flows, and all bets are off. This doesn't happen on the MCAT, as far as I know.

 

[Shrike]

The easiest way to solve MCAT optics problems -- all of them -- is to know three equations (which must be memorized):

• 1/o + 1/i = 1/f
• m = -i/o
• p = 1/f

... nine easy definitions (try them; you already know most of them):

• o = distance (from the lens/mirror) to the object
• i = distance to the image
• f = focal length (sometimes hiding in the problem, as Radius of Curvature/2)
• m = magnification (which, oddly, includes being right side up and upside down)
• p = power
• converging = bringing together
• diverging = spreading apart
• real = light goes there
• virtual = light does not go there

... three things you already know about the world, for translating MCAT problems and figures into the proper form:

• magnifying glasses, which are convex, make light come together
• mirrors are different from lenses (they have the opposite effect on light)
• concave is different from convex (it has the opposite effect on light)

... four easy, but perhaps novel, principles for setting up and interpreting the equations, based on the idea that positive is good:

• object o --> always + (because it's always real, and real is good)
• image i --> real is + (because real is good)
• focal length f --> converging is + (because it's good to come together)
• magnification m --> upright is + (because it's good to be upright)

... and one sensible method:

• to find the effect of mutiple lenses or mirrors, add their powers to get the power of the combination.

 

[Shrike]

In the subject of machines(ramp, pulley, lever). Isn't the ramp inefficient in the way that the work(which leads to force) is being wasted on the horizontal component as opposed to lifting an object straight up in which everything goes to the vertical component?

Not really. Pushing and pulling is conservative on the MCAT: no energy is lost. On the other hand, if energy is used to accelerate the block (up the ramp, or otherwise), then that energy isn't available to lift the block while the speed is increased (relative to its starting speed). On a complex problem this could matter, but in a simple problem we would assume either constant or negligible speed. In either of these simpler cases, all of the energy that goes into pushing or pulling is translated into potential energy by lifting the block, rather than kinetic energy by speeding it up.

 

[freesolo]

im confused with the topic if of weightlessness
a 5kg object is considered weightless when:
I chose when its accelarting up at 9.8 m/s^2
the answer is down, im just thinking weightless means mg is 0 so Forces acting on it should cancel out right? I dont know what im trying to say the answer and explanation they gave is confusing.

 

[freesolo]

thats what im saying if mg is pointing down and the answer i chose is the opposite of g than why isnt it weightless. Instead the answer is if it accelerates downward with g???

 

[Shrike]

Thanks for the help......Just a suggestion, but could you do a thread that's "General Physics Concepts" and put it in there......this thread could be more for specific questions and the other one could be a thread exclusively for mods where you add general info that you feel would be beneficial for the average test taker to know.

Check out the new Physics FAQs and Topic Writeups thread; I'm pretty much following your suggestion to the letter. Thanks for the help.

 

[Shrike]

I could really use some help in understanding the buoyancy force in relation to figuring out the density of an submerged object. The archimedes principle.

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.

 

[mrhealth]

for some reason, i cant figure out why a car is pushed away from the center of a circle when it goes around in a circle. the centripital force and i want to say that the static friction force are both facing into the middle (preventing the car from skidding away from the center)... so, i guess this should be some type of equilibrium situation. what force am i missing that pushes away from the center?

 

[mrhealth]

thanks for the quick reply. so when what force is greater than the inward static frictional force does the car start skidding away?

 

[mrhealth]

thats why this is confusing to me. i dont know if this is beyond the scope of the MCAT or what. so if you draw a free body diagram (ignore the centrifugal force of the car on the ground because the car is the object, not the ground) with a car moving in a circle at constant velocity, then the only one force vector would be inward (=friction=centripital force), right? im sure this has to do with the velocity because intuitively i know that at a higher velocity, it would be harder for a car to stay in its circle. this is more out of curiousity now more than worrying that something like this will be on the mcat. thanks again for all your help.

 

[xanthines]

Pretty much!

The freebody diagram should show mg pointing downwards, the Normal Force (Fn) going up, and the Friction force (Ff) going towards the center of the circle. The centripetal force (Fc) is the force required to keep the car in a circular path at that velocity. Your intuition is correct. The fast you go, the higher the Fc. When the required Fc surpasses the Frictional force, the car begins to skid and circular motion is no longer maintained. When this happens, you'd better be wearing your seatbelt!

-X

 

[DarkLordofSith]

Check out the new Physics FAQs and Topic Writeups thread; I'm pretty much following your suggestion to the letter. Thanks for the help.

Thanks Shrike....I've been looking over it and it's really good stuff. Keep up the good work.

 

[Shrike]

... with a car moving in a circle at constant velocity...

A minor point, but just in case: the car may be moving at constant speed, but its velocity is changing because the direction is changing.

 

[riceman04]

All these threads will be up at least until August, right?
I know it is a dumb question, but I am debating about whether or not I should retake the exam.
This study area is great!

 

[blankguy]

A minor point, but just in case: the car may be moving at constant speed, but its velocity is changing because the direction is changing.

Hard to picture that because in real life we'd have to slow down to change directions(ie turn) while driving the car. An object that is being swung in a ciscular motion at constant speed would be easier to picture. Speed constand but the direction is changing.

 

[Shrike]

Hard to picture that because in real life we'd have to slow down to change directions(ie turn) while driving the car.

You apparently haven't driven with me.

 

[Shrike]

I could really use some help in understanding the buoyancy force in relation to figuring out the density of an submerged object. The archimedes principle.

Look here, and tell me whether it answers your question.

 

[QofQuimica]

All these threads will be up at least until August, right?
I know it is a dumb question, but I am debating about whether or not I should retake the exam.
This study area is great!

They will be up permanently, as far as we know. I'm glad that you are finding them helpful. Best of luck for those taking the test in Aug.

 

[Kussemek]

TPR asks in one of its hw problems the following:

In a series of experimental trials, a projectile is launched with a fixed speed, but with various angles of elevation. As the angle is increased from 0 to 90, the vertical component of the velocity:

Now, I picked the option that said: increases, while the horizontal component remains constant.

The answer, accodring to TPR is: increases, while the horizontal component decreases.

I thought horizontal velocity was constant during the duration of the flight.

 

[speranza]

For some reason, I have a lot of trouble with some of the more basic physics subjects i.e. kinematics, Newtonian mechanics, conservation of energy, etc. I don't have an intuitive understaning of these topics and on MCAT problems that are more conceptual, I often waste a lot of time trying to overanalyze them and often end up picking the wrong answer. I'm not sure what do do about it, because I understand the basic concepts and know the formulas but I just can't seem to do well on the more conceptual problems -- an example would be how changing the angle of inclination would affect velocity, acceleration, etc. Basically problems that are really easy if you have an intuitive understanding of the concepts and how they are interrelated but extremely difficult if you do not.

I'd appreciate any advice on how I can improve on this. Thanks.

 

[xanthines]

Horizontal velocity IS contant during the duration of a flight (assuming no air resistance), but it is only constant with regard to your starting angle of elevation. Put another way, going from 0 to 90 degrees in one degree increments, will give you 90 different horizontal velocities, but they will be constant, ie they won't slow down or speed up.

As for your specific problem. Descriptively, at zero degrees all of the energy from the cannon will be directed in the horizontal component, making the projectile travel in a straight line parallel to the ground. Whereas at 90 degrees all of the energy will go into the vertical component, making the projectile travel straight up. The horizontal velocity would HAVE to decrease since you know that firing anything straight up into the air will come straight back down again. It won't move left or right. At 45 degrees both components will have equal values. This is why 45 degrees give you the best distance in the presence of gravity and the absence of aire resistance.

Remember that sin and cos of theta (ie angle of elevation) will tell you how much of the total starting velocity is composed of its horizontal and verical components. Just give an arbitrary starting velocity of 10 m/s and compare the sin and cos values as go from 0 to 90 degrees in 15 degree increments.

Example:
Vertical Component --> 10 m/s * sin(0) = 0 m/s
Horizontal Component --> 10 m/s * cos(0) = 10 m/s

Shrike is a TPR teacher. Perhaps he can comment better since he will probably know the HW Question you are talking about.

-X

TPR asks in one of its hw problems the following:

In a series of experimental trials, a projectile is launched with a fixed speed, but with various angles of elevation. As the angle is increased from 0 to 90, the vertical component of the velocity:

Now, I picked the option that said: increases, while the horizontal component remains constant.

The answer, accodring to TPR is: increases, while the horizontal component decreases.

I thought horizontal velocity was constant during the duration of the flight.

 

[TheGuy2000]

Which of the follwing gives the percent change to the Young's Modulus for a substance when its cross sectional are is increased by a factor of 3?

0
33%
300%
900%

I put 33% since the equation is F/A/ (Delta L/L)

But of course it was wrong and the answer was 0.

Also for another stress q:
The sole of a certain tennis shoe has a shear modulus of 4x 10 to the 7th, I f the height of the sole is double the strain will.

Decrease by factor of 2
Same
Increase by f of 2
Increase by f of 4

I put increase by f of 2 since, which is wrong, but is it 0 because young's modulus is height, and shear is length? Thanks
__________________

 

[Shrike]

Shrike is a TPR teacher. Perhaps he can comment better since he will probably know the HW Question you are talking about.

Naah. We don't look at homework problems until someone asks about them, and then we usually promptly forget the problem -- it's just not a big part of our class.

Xanthine's explanation is fine. For a given set of launch conditions, vx remains constant, but you're changing launch conditions.

 

[Shrike]

Which of the follwing gives the percent change to the Young's Modulus for a substance when its cross sectional are is increased by a factor of 3?

Young's modulus is a property of the material, like coefficient of friction or index of refraction. It doesn't change just because there's more stuff.

The equation is then used, with this fixed value, to figure the strain for a given stress.

 

[Shrike]

...I don't have an intuitive understaning of these topics and on MCAT problems that are more conceptual, I often waste a lot of time trying to overanalyze them and often end up picking the wrong answer. I'm not sure what do do about it.

Tough problem. Some solutions ideas:

• Move to Dallas, and take my class.
  
• Arrange to get better instruction at your school.
  
• Keep an eye on the FAQs and Topic Writeups thread, where I'll be trying to give ways to deal with conceptual problems.
  
• Find a self-study source that promotes conceptual understanding. I'm told Examkracker's physics text is good for this, but I can't tell you from firsthand knowledge. As an extreme solution, you could get a book of the Feinman lectures.

 

[xanthines]

Well, a less ideal way would be to just memorize what happens if you do X. Since you already know the euquations and concepts, you could memorize a few key "happenings" despite the lack of intuition. Again, it would be better if you could "just get it", but in the absence of that (or Shrike's suggestions) this may help you through the physics Q's.

-X

For some reason, I have a lot of trouble with some of the more basic physics subjects i.e. kinematics, Newtonian mechanics, conservation of energy, etc. I don't have an intuitive understaning of these topics and on MCAT problems that are more conceptual, I often waste a lot of time trying to overanalyze them and often end up picking the wrong answer. I'm not sure what do do about it, because I understand the basic concepts and know the formulas but I just can't seem to do well on the more conceptual problems -- an example would be how changing the angle of inclination would affect velocity, acceleration, etc. Basically problems that are really easy if you have an intuitive understanding of the concepts and how they are interrelated but extremely difficult if you do not.

I'd appreciate any advice on how I can improve on this. Thanks.

 

[mostwanted]

is torque covered on the mcat? it's not listed in the aamc list, but it's still discussed in most mcat review books.

 

[Shrike]

Yes, torque is covered. And it is listed on the MCAT topic list, as is rotational equilibrium, which requires an understanding of torque.

 

[Jezzielin]

Does anyone one know tricks to figuring out tangent, sine, and cosines in your head?

 

[mostwanted]

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

 

[freesolo]

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

 

[Shrike]

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

 

[blankguy]

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

 

[Shrike]

I have a pulley question... 😕

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

 

[blankguy]

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

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

 

[psiyung]

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