TBR test 2 physical sciences 11 questions

[2010premed]

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4. All of the following would result from using a longer effusion tube EXCEPT:
A.. .an increase in the relative abundance of H2 (g) in the first gas sample from the tube
B.. .an increase in the relative velocities of D2(g) and H2 (g)

Answer: B
I get how B is the answer, but I don’t get how a longer tube would result in A? The explanation says, As a result of this better separation, the first sample of gas from the tube would be more enriched in H2 (g).
8. Given M2 > M1, which of the following pendulum systems has the GREATEST change in potential energy from a state of rest?

Answer: D
In the passage it states that “it is found that the greatest PE is always associated with the system whose center of mass is displaced away from its resting state by the greatest change in height.” So this makes it easy to eliminate A and C, but how can you be sure that the answer is D? I guessed D b/c it’s bottom heavy… but not sure if this is the right explanation. The explanation they give seems to be backwards.

13. The perception of sound waves is affected by the relative motions of the source and the receiver toward or away from each other. This phenomenon (also observed in light waves) is known as the Doppler effect. The relationship between the speed of a sound wave and its frequency for a moving source detecting a reflected wave off of a stationary object is expressed as:

​

• incident frequency of the sound wave increases
• incident frequency of the sound wave decreases
• observed frequency of the sound wave increases
• observed frequency of the sound wave decreases

I used the Doppler effect equation of fL=[ (v+vL) / (v-VS)] Fs. I see now that as vS decreases, fL increases. But why does it say in the explanation that incident frequency cannot change? Is it b/c they’re talking about the speed of the wave after it leaves the source?

14. This is just a question about something they said in the explanation:

​

​

How does this graph show that the boiling point is greater than the melting point at all pressured above the triple-point pressure? I mean I know that theoretically it is, but was just wondering if someone could explain how I could see it from the graph.

17. Just have a math question. How do you know that 1.4^4 is about the same as 2^2?

19. The greatest flow rate is exhibited by a solution with:
A. η = 1.2 flowing through a 10.0-cm long tube with a diameter of 2.0 cm and a ∆P of 1000 mmHg
B. η = 2.0 flowing through a 15.0-cm long tube with a diameter of 1.5 cm and a ∆P of 500 mmHg
C. η = 1.5 flowing through a 5.0-cm long tube with a diameter of 2.0 cm and a ∆P of 800 mmHg
D. η = 2.0 flowing through a 10.0-cm long tube with a diameter of 1.0 cm and a ∆P of 2000 mmHg

The equation

is given in the passage

I was able to eliminate B and D b/c of the smaller radius, which plays the most significant role. But is there a fast way to narrow down between A and C? I felt like it would take too much time, and just took a guess…

21. How can one explain a different flow rate for blood through veins in the neck and veins in the legs of a human body?
Answer: The combined effects of gravity and potential energy produce different flow rates

I got this answer by process of elimination, but could someone please elaborate on why this is correct?

23. What is the ratio of the speed of a beta particle to the speed of an alpha particle if they both have the same kinetic energy?
A. 21 : 1
B. 84 : 1
C. 1800 : 1
D. 7200 : 1

We’re given that m alpha = 6.6x 10^-27kg and m beta = 9.1 x 10 ^-31kg
Is this the right way to go about the math?

Malpha=10,000 mbeta

MBVB^2= 10,000MBVA^2
MB’s cancel out
VB^2/VA^2 = 10,000
Take the square root of everything,
VB / VA = 100
?
In the solution they know that the mass ratio is 7200… do you think that this is necessary?

25. An impurity in an organic sample that would cause a scintillation counter to estimate the age of the sample as younger than it really is could include foreign material that can undergo:
A. beta-decay
B. Beta- Capture

Answer: A

I’m having a hard time seeing this. In the explanation, it says that as the beta-particle decay rate per unit mass deceases, the age is assume to be greater (because there is less carbon-14 remaining.)
In the passage it says that Carbon-14 is unstable and undergoes spontaneous, first-order beta-decay to form nitrogen-14. The half-life of 14C is approximately 5,700 years. The age of an artifact can be estimated from the decay rate of beta particles.

So, if the beta decay rate decreases, doesn’t that mean that MORE C14 remains (b/c it’s not being converted to N14 as fast? Also, how exactly do they measure the age? Do they look at the amound of C14 remaining? Or the decay rate of beta particles?

31. According to Figure 1, an increase of 15 mph in speed would result in a threefold increase in the wind resistance against a car whose initial speed was:
A. 20mph
B. 35mpg
C. 50 mph

D. 65 mph​

​

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Figure 1. Friction and wind resistance as functions of car speed.​

In the explanation, they just eyeball that there was a threefold increase in resistive force only for D… is there an easy way of seeing this? Or a different way of looking at it?

37. Why does it take longer to cook noodles in boiling water at higher elevations than at sea level?
A. Water's boiling point is lower at higher elevations, so its boiling temperature is higher than at sea level
B. Water's boiling point is lower at higher elevations, so its boiling temperature is not as high as at sea level
C. Water's boiling point is higher at higher elevations, so its temperature is higher than at sea level
D. Water's boiling point is higher at higher elevations, so its temperature is not high as at sea level

Answer: B

I know that at higher elevation, Patm is less, so the BP (when Patm = P vapor) is less. So why would it take longer to cook noodles at higher elevations? I don’t know how to differentiate between A and B, except maybe that since Pressure is less, then temperature is less…

40. What ideal properties should a tissue struck by an x-ray have in order to produce a good image?

• High thermal conductivity, a high melting point
• Low thermal conductivity, high melting point

Answer:A

Why should it be able to conduct heat well? In the passage it states that most of the energy from the accelerated electron produced by an x-ray machine is given up as heat in the target organ, but I don’t see why that means it should have high thermal conductivity… maybe someone can better define thermal conductivity for me?

 

[fizzgig]

40. if you dump a bunch of heat into an object and you don't want it to heat up and damage the material, it needs to 1) not change phase, because that is bad for my organs, obviously, and 2) be very willing to dump that heat to surroundings so it doesn't change temp due to the heat. if you dump lots of heat to an organ and it can't conduct that heat to its surroundings very easily, the heat stays in the organ. if it has a high heat capacity so it takes lots of energy to change temp by a degree, that's good (water has a high heat capacity, so that's good), but even with that, the heat has to GO somewhere. so you want high conductivity.

also energy in the target organ is only indirectly from an accelerated electron, wtf. xray machines don't send accelerated electrons into your body. they send xrays. there is some wrongness in their wording right there.

 

[2010premed]

thank you =)

 

[shiite7]

Hey, I'm reviewing this exam now, so can answer some of your questions.

4) D2 travels slower because of its bigger mass. Because H2 travels faster, more particles will reach the tube.
8) I chose B on this question, and spent some time thinking about it. D is correct because greatest change in PE will be greatest change in mgh, which will then just be greatest change in h, for a given sweep. For example, for a "sweep of 90 degrees" (45 degress to the left to 45 degress to right of vertical), then the lower the center of mass, the higher the change in vertical height. I thought of this as the "height" of the arcs made by the center of mass as it "sweeps" through in its motion. It's like having concentric circles but one has a greater radius. Try drawing a quarter of a circle. Then draw another "concentric" quarter-circle. Look at the height difference between the two concentric arcs. The one with a bigger radius (corresponding to the pendulum with the lower center of mass) has a bigger change in vertica height...I know, this is probably more confusing than helpful, lol...
17) 1.4 is about square root of 2, which can be written as 2^1/2. So 1.4^4 = (2^1/2)^4 = 2^(1/2 * 4) = 2^2 (you multiply the exponents, because (x^y)^z = x^(yz)

...I'll see if I can answer some of your other questions later.

 

[shiite7]

19. The greatest flow rate is exhibited by a solution with:
A. η = 1.2 flowing through a 10.0-cm long tube with a diameter of 2.0 cm and a ∆P of 1000 mmHg
B. η = 2.0 flowing through a 15.0-cm long tube with a diameter of 1.5 cm and a ∆P of 500 mmHg
C. η = 1.5 flowing through a 5.0-cm long tube with a diameter of 2.0 cm and a ∆P of 800 mmHg
D. η = 2.0 flowing through a 10.0-cm long tube with a diameter of 1.0 cm and a ∆P of 2000 mmHg

Choosing between A and C:

This is how I was able to pick C:
A is like 1000/1.2 but since the tube is twice as long as C it's like 1000/2.4
C is like 800/1.5
We are now comparing 1000/2.4 and 800/1.5
1000/2.4 is a little above 400, since 1000/2.5= 400, but we are dividing by a slightly smaller number, so number must be slightly bigger
800/1.5 is above 500, since 500 * 1.5 = 500 + 250 = 750

 

[shiite7]

25. An impurity in an organic sample that would cause a scintillation counter to estimate the age of the sample as younger than it really is could include foreign material that can undergo:
A. beta-decay
B. Beta- Capture

Answer: A

I'm having a hard time seeing this. In the explanation, it says that as the beta-particle decay rate per unit mass deceases, the age is assume to be greater (because there is less carbon-14 remaining.)
In the passage it says that Carbon-14 is unstable and undergoes spontaneous, first-order beta-decay to form nitrogen-14. The half-life of 14C is approximately 5,700 years. The age of an artifact can be estimated from the decay rate of beta particles.

So, if the beta decay rate decreases, doesn't that mean that MORE C14 remains (b/c it's not being converted to N14 as fast? Also, how exactly do they measure the age? Do they look at the amound of C14 remaining? Or the decay rate of beta particles?

YES, if beta decay rate decreases, there is MORE C-14. But the way age is determined is by how much C-14 is remaining. The less C-14, the older the sample, since more half-lifes have gone by. Hence, a beta-decay will yield an OLDER AGE estimate because then there is LESS C-14 remaining, meaning the analysis indicates that the sample is OLDER, since there has been more decays/half-lives.

31) I just eye-balled it as well. You can immediately eliminate A and B. If you look at the y-intercept for x=50mph and x=50+15, the y-intercepts don't seem to be triple each other. But for x=65 and x=65+15, there is clearly a greater differential.

 

[shiite7]

I picked A on Q37 as well. At first I was really confused by their explanation, but now I think I understand why B is correct. First off, they are considering water that IS boiling. It is correct that water at higher elevations boils SOONER because the atmospheric pressure is lower and hence it is "easier" for the vapor pressure of water to equal (and exceed) Patm. However, this also means that this temperature at which Patm = Pvapor is LOWER. Hence, the boiling temperature is LOWER. At a lower temperature, there is not as much heat being transferred to the noodles, since there is less kinetic energy. Because of this (and only this), the cooking time is longer. It would be like, hypothetically, cooking noodles in water in 30 degree C water (that is boiling) and cooking noodles in 50 degree C water (that is boiling). The water that is boiling at 30 degree C will take longer to cook the noodles.

 

[icewave667]

anyone know the answer to #13? that question is bothering me :/

Thanks

 

[sps27]

anyone know the answer to #13? that question is bothering me :/

Thanks

The question # 13 is talking about Echo phenomenon. So we stand infront of a mountain, say Hello aloud, that gets reflected off the mountain, meanwhile we have started walking away from the mountain. So what will be our observed frequency of the wave that gets reflected off a stationary mountain. So we can imagine the mountain to be the source of reflected wave, the detector to be us, and they are moving apart, hence the observed frequency will be less. If in fact we did not move and stayed there, the observed frequency and incident frequency (meaning the frequency of wave hitting the mountain) would in fact be the same (assuming no damping), because both the source and detector are stationary.

f'(p) = f(o) [ v - v(d) ] / [ v + v(s) ] <<<<< source and detector are moving away >>>>>>
where
f(p) = observed frequency
f(o) = original frequency
v(d) = velocity of detector
v(s) = velocity of source. v(s) = 0 in this case as the mountain is stationary.

So if we rearrange the terms f(p) / f(0) = v - v(d) / v which will be less than 1. Which means, f(p) is less than f(0) i.e.., perceived frequency will always be less than original frequency if the object and source are moving away from one another.

The eqn they have given us
f(0) = original frequency which they call as incident frequency,
f = observed frequency
2v is the velocity of the detector
The numerator is more than denominator and so always > 1. Now if the flip all of that, i.e., make f as numerator and f(0) as denominator the value will be < 1 i.e,, observed frequency is less than incident frequency.

So I don't know exactly what the question is asking ha ha ha.......but choice (d) seemed good to me.