## typo on todays exam?

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michael879
Posts: 13
Joined: Thu Sep 18, 2008 4:37 pm

### typo on todays exam?

Im not going to repeat the question but it involved the classical doppler shift of a sound wave. Im 99% sure they made a typo on the answer choices so that the correct answer was not given as a choice. Did anyone else notice this? If this was a typo, what will happen? The answer was off by a factor of .82/.81...

Seen
Posts: 13
Joined: Mon Sep 15, 2008 1:07 pm

### Re: typo on todays exam?

Indeed. I thought I might I have done that one wrong, but even if that wasn't the correct choice, it still looked like a typo.

Imperate
Posts: 105
Joined: Thu Jul 03, 2008 7:01 am

### Re: typo on todays exam?

I got an answer that matched one of the choices using f=[(v_s+v_L)/v_s] f_0 (listener moving toward stationary source) where v_s is speed of sound, v_L speed of listener......If I remember rightly.

michael879
Posts: 13
Joined: Thu Sep 18, 2008 4:37 pm

### Re: typo on todays exam?

v_L is negative if the source is moving towards the listener, and Im pretty sure its f = v_s/(v_s+v_L)f_0. If you accidently inverted the equation, you would have gotten one of the choices listed. Otherwise, like I said it was very close to the right answer and you may have not noticed (I was going through it so fast I barely noticed). Also you had to apply the equation twice because of reflection. If you didn't do that then the answer you got would have been listed also. Then again I could just be wrong about the right answer and like Seen said, the typo was just on an incorrect answer.

Imperate
Posts: 105
Joined: Thu Jul 03, 2008 7:01 am

### Re: typo on todays exam?

I thought it was stationary source moving listener, and the listener was moving towards the source? I don't remember this reflection bit! although memory is very hazy now as I've been awake for about 15hrs.

If the source is moving listener stationary, the formula changes altogether to f=v_s/(v_s+v)f_0. (v is speed of source, v_s speed of sound again)

The gen form is f=(v_s+v_l)/(v+v_s) f_0 w/ v_l:listener, v_s speed of sound, v speed of source.

these formula correspond to v_l +ve when listener is moving toward source, v +ve when source moving away from listener, i.e.

L-------> S--------> would both be positive

Imperate
Posts: 105
Joined: Thu Jul 03, 2008 7:01 am

### Re: typo on todays exam?

Seriously do not remember anything whatsoever about reflection not even the mention of it, you have me worried now.....unless my test was different and we're talking about different Doppler questions. All I remember was they say listen has velocity blah toward source, source has freq blah, speed of sound is \$\$\$ what is the freq heard....was yours different?

michael879
Posts: 13
Joined: Thu Sep 18, 2008 4:37 pm

### Re: typo on todays exam?

it doesnt really matter which is moving (classical relativity). All that matters is the relation between the vector between the source and the listener, and the velocity vector. I may have been mixing up v_L and v_s though, sorry about that. I can see how that would be confusing.

You may be talking about a different question. There were at least 2 different tests given, so although that was the only doppler shift on mine and Seen's test, there may have been a different one on urs. In fact thats probly the case. In the question Im talking about a source is moving towards a wall and they ask about the frequency heard back at the source.
Last edited by michael879 on Sat Oct 18, 2008 6:53 pm, edited 1 time in total.

michael879
Posts: 13
Joined: Thu Sep 18, 2008 4:37 pm

### Re: typo on todays exam?

yea don't worry we took different tests for sure.

blueeverest
Posts: 38
Joined: Fri Mar 28, 2008 3:09 pm

### Re: typo on todays exam?

I wish I had more time today. My timing was terrible.

Helio
Posts: 809
Joined: Thu Mar 15, 2007 7:11 pm

### Re: typo on todays exam?

the beats questions seemed weird too or did i just misread that....

I also think the answer to that doppler was really weird because i used two formulae and got different answers. i also could not get numbers on some of them i dunno why... ohh well i answered most of them and skipped the ones i couldn't get the numbers too answered somewhere around 85 i think

Imperate
Posts: 105
Joined: Thu Jul 03, 2008 7:01 am

### Re: typo on todays exam?

You may be talking about a different question. There were at least 2 different tests given, so although that was the only doppler shift on mine and Seen's test, there may have been a different one on urs. In fact thats probly the case. In the question Im talking about a source is moving towards a wall and they ask about the frequency heard back at the source.
yeah def a different question, phew...

astrophysicist2b
Posts: 44
Joined: Sun Nov 18, 2007 4:14 pm

### Re: typo on todays exam?

Anyone think that the perigee/apogee velocities question didn't have any correct answer choices? sqrt(8000/40000)=0.45 which was not even close to an answer choice (0.2 and 1 were the closest).

cato88
Posts: 420
Joined: Wed Sep 03, 2008 12:46 am

### Re: typo on todays exam?

you choose .2 because its closest by a factor of 2 and you would probably get a number around .2 I figure by estimating but I do think thats a bit underhanded if your right.

The_Seer
Posts: 3
Joined: Sun Oct 19, 2008 1:42 pm

### Re: typo on todays exam?

For the doppler effect problem,

The problem gives a car moving at 1/10 the speed of sound emitting frequency f_0 to a stationary wall, and asks what the car observes when it listens to the reflection of the emitted frequency. In this case, the frequency is equal to f_0 (1+v)/(1-v), which is f_0 / 0.82. It wasn't a typo.

For the perigee - apogee problem,

Recall Kepler's law that states at any given moment of an orbit, the line connecting the orbiting object to the center of the orbit sweeps an equal amount of area. The perigee and apogee distances had a factor of 5 between them, so the velocities should also differ by a factor of 5.

This was how I answered those questions, don't know if they're right tho. =)

blueeverest
Posts: 38
Joined: Fri Mar 28, 2008 3:09 pm

### Re: typo on todays exam?

for the apogee perigee, wasn't there .04 as an option? I think I got .04.

astrophysicist2b
Posts: 44
Joined: Sun Nov 18, 2007 4:14 pm

### Re: typo on todays exam?

nevermind, if you use vis-viva you get 0.2... I just wasn't thinking when I answered that question. still, expecting us to do those fractions by hand isn't cool.
Last edited by astrophysicist2b on Sun Oct 19, 2008 9:28 pm, edited 1 time in total.

YF17A
Posts: 94
Joined: Sat Aug 30, 2008 4:42 am

### Re: typo on todays exam?

I think it's actually (8000/40000)^2 for the apogee-perigee question...since area is 1/2 R^2 theta, and theta is approximately v delta t, equal areas in equal times means (v_2/v_1) = (R_1/R_2)^2.

YF17A
Posts: 94
Joined: Sat Aug 30, 2008 4:42 am

### Re: typo on todays exam?

come to think of it, actually, i had a similar issue about the C_P vs. C_V question...i don't remember the exact numbers, but i ended up with 2.9.../2.1.... = 1 + R/C_v, and the closest was C_v = 1.5R, which didn't quite match. anyone else remember this?

michael879
Posts: 13
Joined: Thu Sep 18, 2008 4:37 pm

### Re: typo on todays exam?

The_Seer wrote:For the doppler effect problem,

The problem gives a car moving at 1/10 the speed of sound emitting frequency f_0 to a stationary wall, and asks what the car observes when it listens to the reflection of the emitted frequency. In this case, the frequency is equal to f_0 (1+v)/(1-v), which is f_0 / 0.82. It wasn't a typo.
ok first, I was trying not to repeat the exact question since we DID all sign a nondisclosure agreement. Second, where did you get that formula? I applied the doppler shift equation twice and got f = f_0/.81. The other options were f_0/.9, f_0 * .9 and f_0 * .81 (forget the 5th). .81,.9,1/.9,1/.82? Seems like a typo to me.. If not on the right answer than on one of the wrong answers.

The_Seer
Posts: 3
Joined: Sun Oct 19, 2008 1:42 pm

### Re: typo on todays exam?

YF17A wrote:I think it's actually (8000/40000)^2 for the apogee-perigee question...since area is 1/2 R^2 theta, and theta is approximately v delta t, equal areas in equal times means (v_2/v_1) = (R_1/R_2)^2.
YF17A, theta would be equal to (v delta t) / R radians, not just v delta t, hence making the ratio (8000/40000), not (8000/40000)^2.

Take a look at this site if you need detailed explanation.

The_Seer
Posts: 3
Joined: Sun Oct 19, 2008 1:42 pm

### Re: typo on todays exam?

michael879 wrote:
The_Seer wrote:For the doppler effect problem,

The problem gives a car moving at 1/10 the speed of sound emitting frequency f_0 to a stationary wall, and asks what the car observes when it listens to the reflection of the emitted frequency. In this case, the frequency is equal to f_0 (1+v)/(1-v), which is f_0 / 0.82. It wasn't a typo.
ok first, I was trying not to repeat the exact question since we DID all sign a nondisclosure agreement. Second, where did you get that formula? I applied the doppler shift equation twice and got f = f_0/.81. The other options were f_0/.9, f_0 * .9 and f_0 * .81 (forget the 5th). .81,.9,1/.9,1/.82? Seems like a typo to me.. If not on the right answer than on one of the wrong answers.
michael,

Take a look at this site http://galileo.phys.virginia.edu/classe ... Effect.htm to see why the above formula holds. Basically, for the source moving its f(1+v), and for the observer moving it's f/(1-v), so in this case, where the source and the observer is moving, we get the combined result.

G01
Posts: 75
Joined: Thu Aug 21, 2008 7:54 pm

### Re: typo on todays exam?

michael879 wrote:
The_Seer wrote:For the doppler effect problem,

The problem gives a car moving at 1/10 the speed of sound emitting frequency f_0 to a stationary wall, and asks what the car observes when it listens to the reflection of the emitted frequency. In this case, the frequency is equal to f_0 (1+v)/(1-v), which is f_0 / 0.82. It wasn't a typo.
ok first, I was trying not to repeat the exact question since we DID all sign a nondisclosure agreement. Second, where did you get that formula? I applied the doppler shift equation twice and got f = f_0/.81. The other options were f_0/.9, f_0 * .9 and f_0 * .81 (forget the 5th). .81,.9,1/.9,1/.82? Seems like a typo to me.. If not on the right answer than on one of the wrong answers.
I got the same answer as you Michael. I think it was a typo. I picked f_0/.82. (It was the only thing close.)

michael879
Posts: 13
Joined: Thu Sep 18, 2008 4:37 pm

### Re: typo on todays exam?

The_Seer wrote: michael,

Take a look at this site http://galileo.phys.virginia.edu/classe ... Effect.htm to see why the above formula holds. Basically, for the source moving its f(1+v), and for the observer moving it's f/(1-v), so in this case, where the source and the observer is moving, we get the combined result.
o ok I guess your right, its not a typo.. It seems the "typo" was on one of the wrong answers. Im lucky 1/(1-v) ~ (1+v) lol. I was thinking theres no difference between the source moving and the observer moving, but I see thats wrong now. If the source is moving at the speed of sound, the frequency goes to infinity. If the observer is moving at the speed of sound the frequency just doubles. thanks for the clarification.

casaubon
Posts: 22
Joined: Sun Sep 07, 2008 6:42 pm

### Re: typo on todays exam?

i'm gonna puke.

i don't think i can wait a month!

G01
Posts: 75
Joined: Thu Aug 21, 2008 7:54 pm

### Re: typo on todays exam?

michael879
Posts: 13
Joined: Thu Sep 18, 2008 4:37 pm

### Re: typo on todays exam?

G01 wrote:The answer is f_0/.82 right?
yep, we got the right answer for the wrong reason .

G01
Posts: 75
Joined: Thu Aug 21, 2008 7:54 pm

### Re: typo on todays exam?

michael879 wrote:
G01 wrote:The answer is f_0/.82 right?
yep, we got the right answer for the wrong reason .

Helio
Posts: 809
Joined: Thu Mar 15, 2007 7:11 pm

### Re: typo on todays exam?

well i got the wrong answer, ohh well another one of the list.

Suppsedly this exam was already given before from what i hear from the voices out of the question committee

Maxwells_Demon
Posts: 101
Joined: Fri Dec 21, 2007 6:09 am

### Re: typo on todays exam?

Yeah, I went over that equation the night before, so I got it.

Thinking about the test in my dreams, etc., I figured I got 2 wrong so far. ugh... But it's hard for me to recall any questions to think about them in particular because I flew so fast taking the test....

Maxwell's Demon

mike164
Posts: 23
Joined: Sun Apr 13, 2008 6:45 pm

### Re: typo on todays exam?

Maxwells_Demon wrote:Yeah, I went over that equation the night before, so I got it.

Thinking about the test in my dreams, etc., I figured I got 2 wrong so far. ugh... But it's hard for me to recall any questions to think about them in particular because I flew so fast taking the test....

Maxwell's Demon

If you take your 0177 booklet and start writing down all the questions that came in the test which was similar to the one that is in 0177 you should be able to recall most of it. You might find this hard to believe, but, I almost wrote down 81 of them. Now, It might be too late. I did this as soon as I came back home after the exam. I also took the april exam and I found almost 6-7 questions reappeared in the Oct test.

noospace
Posts: 46
Joined: Fri Feb 22, 2008 9:14 pm

### Re: typo on todays exam?

The_Seer wrote:
YF17A wrote:I think it's actually (8000/40000)^2 for the apogee-perigee question...since area is 1/2 R^2 theta, and theta is approximately v delta t, equal areas in equal times means (v_2/v_1) = (R_1/R_2)^2.
YF17A, theta would be equal to (v delta t) / R radians, not just v delta t, hence making the ratio (8000/40000), not (8000/40000)^2.

Take a look at this site if you need detailed explanation.