## more probs from GR8677

Nova
Posts: 7
Joined: Thu Sep 29, 2005 2:00 am

### more probs from GR8677

#12 A source of 1-kHz sound is moving straight toward you at speed 0.9 times the speed of sound. The freq you recieve -
(a) 0.1 kHz
(b) 0.5 kHz
(c) 1.1 kHz
(d) 1.9 kHz
(e) 10 kHz

I used the Doppler relation -
f_L = [(v + v_L)/(v + v_S)] * f_S

where f_L = freq of listener, v_L = speed of listener
f_S = " " source, v_S = " " source
v = seepd of sound in air

The answer I get is (b), but obviously incorrect. The source is approaching, so f_L > f_S. Is v negative in this equation? Why?

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#15, #39, #40, #99, #100 - Can anybody help me with these please?

danty
Posts: 19
Joined: Fri Sep 30, 2005 6:40 pm
Both V_L and V_s are considered positive if they have a direction from L to S, and negative in the opposite case. So in this problem you should take V_s negative.

danty
Posts: 19
Joined: Fri Sep 30, 2005 6:40 pm
Question 15:
The Probability is: 1- Prob(find only 1) -Prob(find 2) -.....-Prob(find N)

Im not quite sure about how to calculate the above quantity but it has the form P=1-.... , so when i came up to this problem i chose D , which is correct.

Question 39:

Since log(Gain) is linearly dependent on log(w) , G must be exponentialy dependent on w, and due to the decline of G with w, their relation has the form G=Kexp(-aw), a>0. You are not over! You must calculate a.

G=Kexp(-aw) => log(G)=log(K)-alog(w)=>dlog(G)/dlog(w)=-a

By taking a space of dlog(w) and its corespondive dlog(G) in the linear area, you find that a=2. So the correct answer is E.

Question 40:

In decay experiments the standard deviation is defined as the square root
of counts(or rate of decay). So the correct answer is sqrt(9934)~100 , A.

Question 99:

The energy equation in hydrogenic atoms has the form:
E=-13,6*m'*Z^2/n^2

where m'=m1*m2/(m1+m2) is the "reduced mass"of the system (i am not sure if the term "reduced" is correct) and Z the atomic number.
For the electron - positron pair m'=(1/2)mh , where mh is the reduced mass of electron - proton system in the hydrogen atom,and Z=1. So for the ground state (n=1) the correct answer is B :(1/2)*13,6 eV.

Question 100:

The correct answer is B: sqrt(l*D). However you will probably won't have the slightest idea about it , just as i did I when i took this test.

However, i thought that the diameter d should not be either to large or too small. l^2/D , l, l/10 should make a very small diameter for the pinhole, and D^2/l an extremely large one. The only possible answer giving a rational diameter is sqrt(l*D).

kolndom
Posts: 19
Joined: Tue Oct 25, 2005 11:29 am

If we assume that v, v_L, and v_s are measure in the same direction, the Doppler relation would be:
f_L=[(v-v_L)/(v-v_s)]*f_S
Then the condition v_L=0; v_s=0.9v gives f_L=10*f_S, so the answer is (e).

danty
Posts: 19
Joined: Fri Sep 30, 2005 6:40 pm
That's correct. Just remember that the speed of sound v, is always considered positive. You only need to determine the sign of VL and VS.

yosofun
Posts: 87
Joined: Mon Oct 31, 2005 5:50 am