Page 1 of 1

typo on todays exam?

Posted: Sat Oct 18, 2008 5:09 pm
by michael879
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...

Re: typo on todays exam?

Posted: Sat Oct 18, 2008 5:14 pm
by Seen
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.

Re: typo on todays exam?

Posted: Sat Oct 18, 2008 5:15 pm
by Imperate
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.

Re: typo on todays exam?

Posted: Sat Oct 18, 2008 5:40 pm
by michael879
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.

Re: typo on todays exam?

Posted: Sat Oct 18, 2008 5:51 pm
by Imperate
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

Re: typo on todays exam?

Posted: Sat Oct 18, 2008 5:58 pm
by Imperate
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?

Re: typo on todays exam?

Posted: Sat Oct 18, 2008 6:51 pm
by michael879
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.

Re: typo on todays exam?

Posted: Sat Oct 18, 2008 6:52 pm
by michael879
yea don't worry we took different tests for sure.

Re: typo on todays exam?

Posted: Sat Oct 18, 2008 9:58 pm
by blueeverest
I wish I had more time today. My timing was terrible.

Re: typo on todays exam?

Posted: Sun Oct 19, 2008 2:06 am
by Helio
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

Re: typo on todays exam?

Posted: Sun Oct 19, 2008 4:51 am
by Imperate
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...

Re: typo on todays exam?

Posted: Sun Oct 19, 2008 12:23 pm
by astrophysicist2b
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).

Re: typo on todays exam?

Posted: Sun Oct 19, 2008 1:31 pm
by cato88
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.

Re: typo on todays exam?

Posted: Sun Oct 19, 2008 1:48 pm
by The_Seer
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. =)

Re: typo on todays exam?

Posted: Sun Oct 19, 2008 1:51 pm
by blueeverest
for the apogee perigee, wasn't there .04 as an option? I think I got .04.

Re: typo on todays exam?

Posted: Sun Oct 19, 2008 2:28 pm
by astrophysicist2b
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.

Re: typo on todays exam?

Posted: Sun Oct 19, 2008 2:38 pm
by YF17A
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.

Re: typo on todays exam?

Posted: Sun Oct 19, 2008 2:56 pm
by YF17A
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?

Re: typo on todays exam?

Posted: Sun Oct 19, 2008 3:02 pm
by michael879
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.

Re: typo on todays exam?

Posted: Sun Oct 19, 2008 5:50 pm
by The_Seer
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.
http://www.physicsforums.com/showthread.php?t=83333

Re: typo on todays exam?

Posted: Sun Oct 19, 2008 5:53 pm
by The_Seer
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/classes/152.mf1i.spring02/DopplerEffect.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.

Re: typo on todays exam?

Posted: Sun Oct 19, 2008 6:33 pm
by G01
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.)

Re: typo on todays exam?

Posted: Mon Oct 20, 2008 12:21 am
by michael879
The_Seer wrote:michael,

Take a look at this site http://galileo.phys.virginia.edu/classes/152.mf1i.spring02/DopplerEffect.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.

Re: typo on todays exam?

Posted: Mon Oct 20, 2008 12:33 am
by casaubon
i'm gonna puke. :shock:

i don't think i can wait a month!

Re: typo on todays exam?

Posted: Mon Oct 20, 2008 6:15 pm
by G01
The answer is f_0/.82 right?

Re: typo on todays exam?

Posted: Mon Oct 20, 2008 9:18 pm
by michael879
G01 wrote:The answer is f_0/.82 right?

yep, we got the right answer for the wrong reason :D.

Re: typo on todays exam?

Posted: Mon Oct 20, 2008 10:31 pm
by G01
michael879 wrote:
G01 wrote:The answer is f_0/.82 right?

yep, we got the right answer for the wrong reason :D.


I stopped reading at "right answer"... :D

Re: typo on todays exam?

Posted: Tue Oct 21, 2008 1:33 am
by Helio
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

Re: typo on todays exam?

Posted: Tue Oct 21, 2008 3:21 am
by Maxwells_Demon
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

Re: typo on todays exam?

Posted: Tue Oct 21, 2008 12:51 pm
by mike164
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.

Re: typo on todays exam?

Posted: Mon Sep 28, 2009 8:33 pm
by noospace
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.
http://www.physicsforums.com/showthread.php?t=83333


That or you could just use conservation of angular momentum (only valid at the apogee/perigee, however).