AAMC PS Self-Assessment Official Q&A

MD
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This thread shall serve as the official site for discussion of the AAMC Self-Assessment in MCAT Physical Sciences.

danny89

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I'll start:
For #80, anybody have a quicker strategy to get to the answer?
Since level of intensity for sound B is 20 db greater than sound A, I just did the following:
20=10*log(Intensity of B/Intensity of A)
2=log(Intensity of B/Intensity of A)
10^2=Intensity of B/Intensity of A
100(Intensity of A)=Intensity of B

Not sure if this is the proper way to doing this type of question...

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Jepstein30

Full Member
I'll start:
For #80, anybody have a quicker strategy to get to the answer?
Since level of intensity for sound B is 20 db greater than sound A, I just did the following:
20=10*log(Intensity of B/Intensity of A)
2=log(Intensity of B/Intensity of A)
10^2=Intensity of B/Intensity of A
100(Intensity of A)=Intensity of B

Not sure if this is the proper way to doing this type of question...

I mean.. just know natural relationships.

10x intensity = 10^1 = 10 dB
100x intensity = 10^2 = 20 dB
1000x intensity = 10^3 = 30 dB

just memorize the first two and you can pretty much do any dB question.

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Did anyone do the Physics Self-Assessment yet? And how about Gen Chem Self-Assessment?

I did both of them (and Ochem) and I'm kinda pissed about the score

Lets discuss the scores so I get some peace of mind that I'm not the only one doing bad.

Vurday

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Did anyone do the Physics Self-Assessment yet? And how about Gen Chem Self-Assessment?

I did both of them (and Ochem) and I'm kinda pissed about the score

Lets discuss the scores so I get some peace of mind that I'm not the only one doing bad.

Edit: didn't realize you were the one that posted the other thread also!

Here's my scores so far:
Physics: 87%
Gen Chem: 89%
OChem: 90%
Biology: (Will do tomorrow)
Verbal: 81%

I'm not really sure where these scores fall compared to others though.

danny89

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Can someone explain #62 in the general chemistry SA?

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FeinMS

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#107
Electric power for transmission over long distances is "stepped up" to a very high voltage in order:
A) to produce currents of higher density
B) to produce higher currents in the transmission wires
C) to make less insulation necessary.
D) to cut down the heat loss in the transmission wires.

The answer is D and the explanation given is a bit confusing.

Is this basically saying since we can maximize the power at user's end by maximizing either V or I, and Power lost is also V^2/R=I^2*R, it's better to maximize V than I to minimize heat loss? Since R is constant and P_lost is minimized by having R in the denominator?

tennisalex

Full Member
#107
Electric power for transmission over long distances is "stepped up" to a very high voltage in order:
A) to produce currents of higher density
B) to produce higher currents in the transmission wires
C) to make less insulation necessary.
D) to cut down the heat loss in the transmission wires.

The answer is D and the explanation given is a bit confusing.

Is this basically saying since we can maximize the power at user's end by maximizing either V or I, and Power lost is also V^2/R=I^2*R, it's better to maximize V than I to minimize heat loss? Since R is constant and P_lost is minimized by having R in the denominator?

I'm not great at explaining things, but I'll try my best to get the point across.

So the formula for Power you know is: P=(I)(V) a.k.a. Ohm's Law.

If you think of Power as a constant, you can manipulate it by either one of two ways: (1) Increasing current and therefore decreasing voltage, or (2) Decrease current and thereby increasing voltage.

Ok. So now that we know we have two options, we look at another equation for power.

Equation 2: P=(I^2)(R): Equation for power lost in a transmission line.
What the explanation is basically stating is that resistance is fixed and based off of the material used as well as it's dimensions; the transmission wire can only provide so much resistance. Glancing at equation 2, you'll notice that if you decide to increase the current in a wire, you'll be increasing the amount of power lost, hence "power lost in a transmission line" formula.

Equation 3: P=(V^2)/(R)
Because we just saw how increasing current will also cause an exponential lost in power sent through the transmission line, the only other option we are left with is to increase voltage instead (thereby decreasing current which results in minimal power loss). Stated again, by increasing voltage (as opposed to increasing current), we end up decreasing current. In this way, we minimize the amount of power lost through transmission (as heat) and maximize the amount of power able to be provided.

Hope this helps!

D

deleted380953

What is the significance of the van t hoft factor in question 95? Why do you not take it into account here?

Gonna try and revive this question, I'm also confused why the solution didn't account for the "i" factor

Based only on the information in the passage, which of the following expressions should be used to best estimate the temperature at which ice will form

A) 0 - 1.86(6.020)
B) 0 - 1.86(1.025)
C) 0 - 1.86(4.184)
D) 0 - 1.86(1.142)

The answer is D, but I chose A since it's the closest to 5*1.142.

i = 5, since there's 5 ions in solution of seawater

Anyone care to explain this?

Blueprint MCAT Tutor

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7+ Year Member
Gonna try and revive this question, I'm also confused why the solution didn't account for the "i" factor

Based only on the information in the passage, which of the following expressions should be used to best estimate the temperature at which ice will form

A) 0 - 1.86(6.020)
B) 0 - 1.86(1.025)
C) 0 - 1.86(4.184)
D) 0 - 1.86(1.142)

The answer is D, but I chose A since it's the closest to 5*1.142.

i = 5, since there's 5 ions in solution of seawater

Anyone care to explain this?

Heya, my response here:

Just scroll to the bottom

inasensegone

Full Member

So essentially, because we were given the molality of each ion species, we can omit the van't hoff factor simply because its already contained in the molalities of each ion?

For reference, here is the data table provided:

The van't hoff factor applies when something is dissociating into solution. For example, let's say you have 1 mole NaCl dissociating into water. In this case, the van't hoff factor is needed because for every single NaCl you get 1 Na+ and 1 Cl-. Now you have 1 mole Na+ ions and 1 mole Cl- ions.

In the AAMC question however, you don't have a bunch of stuff dissolving, you have stuff that is already dissolved. They give you the pertinent molalities of each ion. Add them up and you get 1.142 (total molal) as is reflected in the correct answer.

dudewheresmymd

Slowly Drifting...

Couple of qs:

1) For RC circuits, does the resistor always go before the capacitor when current flows from the (+) end of the battery?
2) if the resistor is placed after the capacitor (going from + to - end fashion), is it still an RC circuit? In this example, it looks like the resistor is placed closer to the (-) end of the battery?
3) if the resistor is placed after a capacitor going + to -, is the work done by the battery equal to the potential energy a capacitor gains?

Is there an error in this question? p *v*vx is not the same as pvx^2 ?

Thanks!

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milski

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1) No, the resistor can be "before" or "after" the capacitor, there is no real ordering in this case.
2) Due to 1) - yes, it does not change anything (as long as it does not change its place with relation to other connections in the circuit).
3) No, the loss of energy has nothing to do with the placement of the resistor. The loss is due to current flowing through the resistor. Regardless of where the resistor is, the current will flow through it and there will be loss of energy.

106) That does not make much sense. What's in the passage? Do they give you a specific angle or do they talk in general? If they are not talking about an angle, the answer is probably the most reasonable estimate. It is not exactly correct but it is within an order of magnitude (v is substituted by vx) within the precise amount. All other three answers are close to correct only for given angles.

justalittlestar

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• Did anyone find their scores increasing after brushing up on weak areas using the PS self-assessments?

My test is July 24th. I just finished up General Chem and got %58 - 8% below avg. I broke the chem test into parts over the span of a week doing them untimed. I'm only asking because I took a few practice tests a couple months ago and scored 6 at the highest in PS with AAMC tests. I haven't taken another AAMC practice test since but I've been done content review for a pretty long time and now I'm still brushing up on areas that I keep forgetting like periodic table trends and thermodynamics.

mtravis2190

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"The solubility of calcium sulfate would be expected to be low in water because both ions are highly charged." Can someone please elaborate on this? I don't understand why calcium sulfate is poorly soluble in water even though it is an ionic compound.

Thank you everyone!

patrickmbyrne

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Force holding them together is K(Q1)(Q2)/(r^2) so you can see Ca+2 and sulfate -2 = 4. This is a larger electrostatic force.

Teleologist

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"The solubility of calcium sulfate would be expected to be low in water because both ions are highly charged." Can someone please elaborate on this? I don't understand why calcium sulfate is poorly soluble in water even though it is an ionic compound.

Thank you everyone!

Brah, this is a poorly worded question. NaF is ionic; its ions are highly charged (Na+ is rather small as far as metals goes and so it has a high positive charge density; same with fluorine - it's the most charge dense element!). Yet it dissolves in water because upon dissolution, the polar solvent can readily stabilize the ionic particles. Also, dissolution confers entropic advantages.

What the question is trying to convey is that the lattice energy of calcium sulfate is large, so it isn't likely to dissolve in the first place.

MD
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"The solubility of calcium sulfate would be expected to be low in water because both ions are highly charged." Can someone please elaborate on this? I don't understand why calcium sulfate is poorly soluble in water even though it is an ionic compound.

Thank you everyone!
Merged with Official PS Self-Assessment Thread.

shoopshoop

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this question is driving me insane. how is p.v.vx and p.vx^2 the same? please let me know!

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nudelove

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I'm having a tough time with question #26 on this self-assessment. Basically the question is:

Which of the following compounds can be used as a base catalyst to cleave triglyceride into methyl ester and glycerine?
A) HCl
B) NaCl
C) NaOH
D) Na2SO4

I chose Na2SO4 and got it wrong, but the reasoning for the correct answer (NaOH) only says that "NaCl and Na2SO4 are salts. Only NaOH is a base and can catalyze the saponification of an ester."
I don't understand how Na2SO4 and NaOH are both sodium salts and therefore need to be dissociated before the base can accept hydrogens (as SO4^2- and OH^1-) but for some reason, in this case, "only NaOH is a base" and Na2SO4 is only a salt. Can anyone explain this?

Abbas012

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I was surprised by the difficulty of #102 in the Physics Self Assessment, although I knew the target formula I would have never discerned what they actually wanted in this question, and would have likely wasted a great deal of time on the test looking for the resistivity variable in the passage, this has me worried. Is my surprise reasonable or should I actually expect questions in the PS (physics, particularly) portion to be this difficult?

Direction of Time

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There is always one passage in the PS section of the real MCAT that has exceptionally tricky questions (about 5 of them). Getting these right requires you to be familiar with the topic and read the passage very carefully.

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golfman7

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On my MCAT ps was pretty straightforward and probably easier than the SA questions. With that said, everyone's mcat experience is different and instead of a rough ps, the bs section was much more challenging than normal. You will be fine for ps with plenty of practice problems, especially from Berkley review.

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jared_the_great

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I didn't remember the exact formula, but still found the problem straightforward. You just have to rely on intuition about the system and what's being asked.

All you had to do is remember the relationship between resistance and cross-sectional area. It's much like a water pipe. The wider the pipe, the less resistance to flow there is. As area increases, resistance decreases by the same factor. If the area is 4x as big, the resistance will be 1/4.

There were other problems that really stressed me out and had me crunching numbers for way too long, but after looking at the answers I always realized it was not because the question was extraordinarily difficult, but because I just needed to review that topic and understand it better.

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MD
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Merging with Official PS Self-Assessment thread in Q&A.

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Vino24

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A nucleus splits into two fragments that have equal charge but unequal mass. Which of the following is equal for the two fragments as they move apart?

A) Magnitude of the force of one fragment on the other
B) Magnitude of acceleration
C) Speed
D) Kinetic energy

The answer was A. I was able to cross off B and C pretty quickly, however I do not understand why D is incorrect. If the two fragments have different masses, they can still have the same kinetic energy (as long as they have different speeds). Am I wrong in assuming this? Any input would be great.

IslandStyle808

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A nucleus splits into two fragments that have equal charge but unequal mass. Which of the following is equal for the two fragments as they move apart?

A) Magnitude of the force of one fragment on the other
B) Magnitude of acceleration
C) Speed
D) Kinetic energy

The answer was A. I was able to cross off B and C pretty quickly, however I do not understand why D is incorrect. If the two fragments have different masses, they can still have the same kinetic energy (as long as they have different speeds). Am I wrong in assuming this? Any input would be great.

To answer the question their accelerations are different, even if their forces are the same. This means the difference in their velocities will be greater as time progresses. Hence the reason their kinetic energies are also different.

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Vino24

New Member
To answer the question their accelerations are different, even if their forces are the same. This means their velocities will be different as well as time progresses. Thus the reason their kinetic energies are also different.

Oh, that's right. I think I was confusing momentum with kinetic energy. Two objects with equal momentums can still have different kinetic energies. Thanks for the reply!

IslandStyle808

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Oh, that's right. I think I was confusing momentum with kinetic energy. Two objects with equal momentums can still have different kinetic energies. Thanks for the reply!

Yep. Also remember that even if the momentums are the same, the difference in their respective velocities will be magnified when calculating the KE due to the v is being squared (v^2).

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Winterbourne

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Need hep with Passage VII. I am totally lost on this one. Especially questions 32 and 33.

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Gauss44

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Physics Self Assessment
#39

Modern nuclear reactors typically have several boron rods that can be inserted varying degrees into the reactor. These rods control the rate of the reaction by:

A. producing neutrons
B. speeding up neutrons
C. reflecting neutrons
D. absorbing neutrons

How were we suppose to know the answer to this? (I know what the answer is. My question is how were we suppose to figure it out?)

Gauss44

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Physics Self Assessment #70

When a nucleus emits a 2.5 MeV gamma ray, by how much does the nuclear mass decrease?

Why can't you go from MeV to Joules to mass using E=mc^2 and 1 amu=931 Mev? I tried solving it that way and the numbers didn't work out. I know AAMC has an alternative way of solving which is fine. My question is why didn't MY METHOD work?

Gauss44

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Physics Self Assessment
#91

Because comets shine predominately by reflected sunlight, what one sees when viewing a comet is:

How did you resist NOT picking A or C after reading the last paragraph of the passage?

(I would have got it right if it were NOT for that last paragraph.)

Gauss44

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Physics Self Assessment
#93

What mechanism supplies the tension in the string at the molecular level?

Why isn't the answer magnetic forces?

Stretching bond lengths seems to be the RESULT of the "mechanism" that supplies tension in the string, NOT the cause. Please explain if you can.

Gauss44

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General Chemistry Self Assessment
#82

Limestone, marble, caulk, Iceland spar are all composed of CaCO3 crystals. Which of the following could cause differences in the properties of the material:

I. Crystal Grain Size
II. Conditions of Formation
III. Molecular Formula
IV. Impurities

Doesn't it stand to reason that if "impurities" are present, the molecular formula may vary?

Gauss44

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General Chemistry Self Assessment
#91

The addition of Ca2+ to a solution containing Ca2+ and (CaO3)2- ions causes CaCO3 to precipitate because:

C. The reaction quotient Q would be lower than the Ksp
D. The reaction quotient Q would be greater than the Ksp

Doesn't the answer to this depend on which direction you write the reaction? If so, how do you determine whether the test maker wants you to write CaO3 ---> Ca2+ + (CaO3)2- OR Ca2+ + (CaO3)2- ---> CaO3 ????

MD
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Physics Self Assessment
#39

Modern nuclear reactors typically have several boron rods that can be inserted varying degrees into the reactor. These rods control the rate of the reaction by:

A. producing neutrons
B. speeding up neutrons
C. reflecting neutrons
D. absorbing neutrons

How were we suppose to know the answer to this? (I know what the answer is. My question is how were we suppose to figure it out?)
No idea lol, I remember I missed this one too.
Physics Self Assessment
#93

What mechanism supplies the tension in the string at the molecular level?

Why isn't the answer magnetic forces?

Stretching bond lengths seems to be the RESULT of the "mechanism" that supplies tension in the string, NOT the cause. Please explain if you can.
The chemical bonds that actually hold the atoms together are electrostatic interactions; they have nothing to do with magnetism.
General Chemistry Self Assessment
#82

Limestone, marble, caulk, Iceland spar are all composed of CaCO3 crystals. Which of the following could cause differences in the properties of the material:

I. Crystal Grain Size
II. Conditions of Formation
III. Molecular Formula
IV. Impurities
Doesn't it stand to reason that if "impurities" are present, the molecular formula may vary?
Impurities aren't considered part of the substance itself; that is, they won't be represented in the molecular formulae. If they were, the precise amount of impurity would determine the formula since the empirical formula would depend directly on the ratio of mineral : impurity, and you'd have infinite molecular formulae for varying degrees of impurity.
General Chemistry Self Assessment
#91

The addition of Ca2+ to a solution containing Ca2+ and (CaO3)2- ions causes CaCO3 to precipitate because:

C. The reaction quotient Q would be lower than the Ksp
D. The reaction quotient Q would be greater than the Ksp

Doesn't the answer to this depend on which direction you write the reaction? If so, how do you determine whether the test maker wants you to write CaO3 ---> Ca2+ + (CaO3)2- OR Ca2+ + (CaO3)2- ---> CaO3 ????
The question tells you you're adding Ca2+ to a solution of Ca2+ and (CaO3)2–, so those are the reactants and the product being formed in CaCO3. Also, this is the standard format for a solubility equilibrium from which the Ksp expression is written. Edit: I misspoke here. Solubility equilibria represent the dissolution, going left to right, of solid to ions. See next post for for info.

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Gauss44

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No idea lol, I remember I missed this one too.

The chemical bonds that actually hold the atoms together are electrostatic interactions; they have nothing to do with magnetism.

Impurities aren't considered part of the substance itself; that is, they won't be represented in the molecular formulae. If they were, the precise amount of impurity would determine the formula since the empirical formula would depend directly on the ratio of mineral : impurity, and you'd have infinite molecular formulae for varying degrees of impurity.

The question tells you you're adding Ca2+ to a solution of Ca2+ and (CaO3)2–, so those are the reactants and the product being formed in CaCO3. Also, this is the standard format for a solubility equilibrium from which the Ksp expression is written.

Thank you very much. These are very difficult questions and I think you did an excellent job of explaining all but the first one and maybe except the last one* I listed as well. I figured that there was a chance that no one knew the answer, so I feel relieved.

*I think #91's answer key said, "CaCO3 ---> Ca + (CaO3)2"

MD
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Thank you very much. These are very difficult questions and I think you did an excellent job of explaining all but the first one and maybe except the last one* I listed as well. I figured that there was a chance that no one knew the answer, so I feel relieved.

*I think #91's answer key said, "CaCO3 ---> Ca + (CaO3)2"
I misspoke on the last question, sorry. The Ksp expression is always just a product of ion concentrations, so the equilibrium is of course written from solid to aqueous phases going left to right. Because of this convention the Q exceeds Ksp leading to precipitation of the solid.

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Gauss44

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No idea lol, I remember I missed this one too.

The chemical bonds that actually hold the atoms together are electrostatic interactions; they have nothing to do with magnetism.

Impurities aren't considered part of the substance itself; that is, they won't be represented in the molecular formulae. If they were, the precise amount of impurity would determine the formula since the empirical formula would depend directly on the ratio of mineral : impurity, and you'd have infinite molecular formulae for varying degrees of impurity.

The question tells you you're adding Ca2+ to a solution of Ca2+ and (CaO3)2–, so those are the reactants and the product being formed in CaCO3. Also, this is the standard format for a solubility equilibrium from which the Ksp expression is written. Edit: I misspoke here. Solubility equilibria represent the dissolution, going left to right, of solid to ions. See next post for for info.

Eureka! I think I might have figured out #39.

The wording of the question stem says, ¨control,¨ the reaction. I think that might imply slowing down this sort of fast reaction or some chain reaction. And maybe, the wrong answer choices would either result in speeding up the reaction or make no difference. I'm not sure this is right. Still working on it...

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Gen Chem #102

Just for curiosity sake, does increasing the size of an anode increase current at all (since I thought it might increase surface area for the reaction)?

I assume the size of the cathode would not matter?

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To answer the question their accelerations are different, even if their forces are the same. This means the difference in their velocities will be greater as time progresses. Hence the reason their kinetic energies are also different.

Are you able to make any conclusions about which object (higher mass object or lower mass object) would have greater KE given the same initial force?

Or does it depend on the numbers given?

IslandStyle808

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7+ Year Member
Are you able to make any conclusions about which object (higher mass object or lower mass object) would have greater KE given the same initial force?

Or does it depend on the numbers given?

Sorry I am just trying to think back to this question. I believe you can conceptually figure out which one has the higher kinetic energy. By newton's third law we know the forces will be equal. However, the one with the lighter mass will have the greater acceleration. Now apply this to speed, the small fragment has the greater velocity and will have the greater kinetic energy since the velocity is squared. Remember, that the smaller fragment is accelerating and thus will have a velocity that greatly increases over time. So in the end the kinetic energy will be greater for the smaller fragment, in all standard fission scenarios.

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Physics #13

The question asks about a way to "lower the tone" of a violin. According to the answer explanation, you want to lower frequency.

Is frequency really the same as "tone"? I've looked online and I seems that the two are distinct things.

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Physics #23

I understand that rotational equilibrium is when the external torques = 0. But does having zero angular momentum mean that something is on rotational equilibrium too?

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Physics #19

If you use the equation given for black body radiation, you'd get that the wavelength of light is 483.

I'm just curious why it doesn't emit red/orange light?

Oh_Gee

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could someone clarify the answer for me?

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pr2med4lf

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Hi everyone, I am having trouble with Physics #27 and #28 and unfortunately do not have access to the explanations any longer. I would really be grateful for any insight!

#28 (ANSWER: A) : The glass that is used as a beam splitter is replaced with glass that is identical except it has a 10% higher index of refraction. Which of the following changes will occur to the pinhole image?

A. It will move
B. It will become larger
C. It will become smaller
D. It will become more clear

Also, if anyone could upload the answer explanations to the Physics Self-Assessments, it would be much appreciated!

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