Practice Exam Difficulty

[cornsnicker3]

One thing that helped me recently with my effort on the NCEES practice exam was to ignore the data and focus on what was asked. I was always took a physics approach of start with what I needed to solve for and then fanning outward to find the missing constituents. One thing that really bugs me about Lindeburg's solutions for his MERM practice problem is that he works backwards. His methods are like throwing up all over the place and then coming together at the end. Not my favorite style.

 

[sambisu]

One thing that helped me recently with my effort on the NCEES practice exam was to ignore the data and focus on what was asked. I was always took a physics approach of start with what I needed to solve for and then fanning outward to find the missing constituents. One thing that really bugs me about Lindeburg's solutions for his MERM practice problem is that he works backwards. His methods are like throwing up all over the place and then coming together at the end. Not my favorite style.

I agree 100%. My method is always to first try to identify the equation that involves the parameter I am looking for. Then I identify what I do and don't know in that equation and work backwards from there.

 

[Vel2018]

Anyone else saw that the key answer on problem 10 SlayPE is a typo? It is also like one of the merm companion problem but show different solution, specially the last step made in the slayPE solution. So which one is which to follow? Anybody have an idea? 

 

[MikeGlass1969]

Anyone else saw that the key answer on problem 10 SlayPE is a typo? It is also like one of the merm companion problem but show different solution, specially the last step made in the slayPE solution. So which one is which to follow? Anybody have an idea? 

Which Companion problem?

MERM Practice Problem 25-6?

 

[Vel2018]

Which Companion problem?

Chapter 25 #6, this also velocity behind shockwave. Where if following SlayPE solution I can get the velocity as well after the shockwave. But Lindeburg did not do that to get the velocity behind the shockwave. 

 

[cfm03]

Imo, both problems are correct. I think the text of problem 10 might need to be revised. Velocity (V1-V2) is actually induced velocity behind shock wave and V2 is the velocity behind shock wave. There are two different velocities. The problem 10 text needs to make the clear distinction between the two velocities...Let's wait for Slay's input to shed more light.

 

[Slay the P.E.]

Imo, both problems are correct. I think the text of problem 10 might need to be revised. Velocity (V1-V2) is actually induced velocity behind shock wave and V2 is the velocity behind shock wave. There are two different velocities. The problem 10 text needs to make the clear distinction between the two velocities...Let's wait for Slay's input to shed more light.

When our problem states that "A normal shock wave travels ... through stagnant ...air." and then we say "the velocity (m/s) induced behind the shock wave" it is all an indication that the frame of reference is stationary to the ground and not the shock wave. What velocity is induced on stagnant air right after the shock wave passes through? Our calculation shows that the air experiences an induced velocity in the same direction as the propagation of the shock wave. You have to momentarily use a frame of reference fixed to the wave because the equations are derived for such a frame of reference, but then you have to make the correction. This "correction" of course only applies to velocity. Thermodynamic properties (pressure, temperature, density) do not need to be changed. 

[COLOR= rgb(34, 34, 34)]In case of problem 25-6 in PPI's Practice Problems, they say "the Mach number before a shockwave is 2" -- here they have placed the frame of reference on the wave. The wave is stationary and the air in [/COLOR]front of it flows with a velocity such that M=2. 

 

[cfm03]

When our problem states that "A normal shock wave travels ... through stagnant ...air." and then we say "the velocity (m/s) induced behind the shock wave" it is all an indication that the frame of reference is stationary to the ground and not the shock wave. What velocity is induced on stagnant air right after the shock wave passes through? Our calculation shows that the air experiences an induced velocity in the same direction as the propagation of the shock wave. You have to momentarily use a frame of reference fixed to the wave because the equations are derived for such a frame of reference, but then you have to make the correction. This "correction" of course only applies to velocity. Thermodynamic properties (pressure, temperature, density) do not need to be changed. 
In case of problem 25-6 in PPI's Practice Problems, they say "the Mach number before a shockwave is 2" -- here they have placed the frame of reference on the wave. The wave is stationary and the air in front of it flows with a velocity such that M=2.   

Slay, you are correct. The problem's last sentence does say "induced velocity behind shock wave". The clarity has been there all along. Please disregard my earlier recommendation.
My apologies...again I misseed important piece of information.

 

[Vel2018]

When our problem states that "A normal shock wave travels ... through stagnant ...air." and then we say "the velocity (m/s) induced behind the shock wave" it is all an indication that the frame of reference is stationary to the ground and not the shock wave. What velocity is induced on stagnant air right after the shock wave passes through? Our calculation shows that the air experiences an induced velocity in the same direction as the propagation of the shock wave. You have to momentarily use a frame of reference fixed to the wave because the equations are derived for such a frame of reference, but then you have to make the correction. This "correction" of course only applies to velocity. Thermodynamic properties (pressure, temperature, density) do not need to be changed. 

In case of problem 25-6 in PPI's Practice Problems, they say "the Mach number before a shockwave is 2" -- here they have placed the frame of reference on the wave. The wave is stationary and the air in front of it flows with a velocity such that M=2. 

"What velocity is induced on stagnant air right after the shock wave passes through?" - I don't know if I am looking at it right, but by your explanation it seems that the difference is where PPI problem the air is moving and shockwave is stagnant, and in your problem air is stagnant and wave is moving, so by just changing the frame reference makes the velocity behind the shock different? 

The thing that confuses me is both are still behind the shockwave, so no matter the air or the shockwave is moving, behind the shock is behind the shock?

 

[Slay the P.E.]

"What velocity is induced on stagnant air right after the shock wave passes through?" - I don't know if I am looking at it right, but by your explanation it seems that the difference is where PPI problem the air is moving and shockwave is stagnant, and in your problem air is stagnant and wave is moving, so by just changing the frame reference makes the velocity behind the shock different? 

The thing that confuses me is both are still behind the shockwave, so no matter the air or the shockwave is moving, behind the shock is behind the shock?

Yes. It’s the same velocity but its direction and magnitude change depending on the frame of reference.

Imagine a blast wave passing through a littered street. A bunch of debris and stuff will be “picked up” and fly in the direction of the wave, right behind it, but not as fast as the wave is traveling. This is what someone watching the thing go by will observe. They are “seeing” the induced velocity. But the imaginary dude riding on that wave will see the debris and junk actually traveling away from the rear side of the wave, but not as fast as he sees the air approaching the wave towards the front side.

 

[Vel2018]

Yes. It’s the same velocity but its direction and magnitude change depending on the frame of reference.

Imagine a blast wave passing through a littered street. A bunch of debris and stuff will be “picked up” and fly in the direction of the wave, right behind it, but not as fast as the wave is traveling. This is what someone watching the thing go by will observe. They are “seeing” the induced velocity. But the imaginary dude riding on that wave will see the debris and junk actually traveling away from the rear side of the wave, but not as fast as he sees the air approaching the wave towards the front side.

Exactly they are the same. The sample problem in the PPI air is moving, this is like the imaginary guy riding the shock wave, but the question is the same, there is a velocity induced behind the shock, the reference is the shock wave, same in you problem although the wave moves and air is stagnant; assuming both samples are travelling at same speed. Should we get a different magnitude of velocity behind the shock? 

The way I see it is only the observer changed its velocity but technically not the velocity behind the wave? 

 

[Slay the P.E.]

 Should we get a different magnitude of velocity behind the shock? 

Yes, we should. Our problem asks for velocity induced which is the velocity behind the wave measured from a stationary reference frame. Their problem asks for velocity behind the wave measured from a moving frame of reference. 

 but the question is the same,

No it’s not. We ask for the induced velocity. They don’t.

The way I see it is only the observer changed its velocity but technically not the velocity behind the wave? 

Correct. 

 

[Vel2018]

Yes, we should. Our problem asks for velocity induced which is the velocity behind the wave measured from a stationary reference frame. Their problem asks for velocity behind the wave measured from a moving frame of reference. 

No it’s not. We ask for the induced velocity. They don’t.

Correct. 

PPI problem ask same, it is induced velocity behind shock, only difference there is the air was moving. There won't be any velocity induced if it weren't from the shock wave, no matter how you look at it air moving or wave moving shock wave is shock wave, and behind it was a velocity induced by the shock wave.

And with your question, although it did say that air is stagnant and wave is moving, since the standard of tables and graphs are made from the air is moving, I guess it just needed to add some clarity there what the problem is really looking for.

Because I don't think its right to assume that we have to measure the velocity while standing from a far looking at the wave unless specifically stated with "respect to observer, looking at the wave with X distance" etc again its because of the standard tables and graphs are not made this way.

It is like looking at the moon, and asking its size, size of it does not change no matter you go near or far, its size has already been measured its a fact. (same with velocities behind shock)

Unless you specifically ask with respect to your location what is its size, then there I can say its an inch. Its the same with this problem #10. If you ask whats the velocity induced by a 600m/s shock wave, it would be the same with air or wave moving, but if you ask to measure it with respect from a stationary observer then ok you are right. 

 

[Slay the P.E.]

The wording of both problems make it very clear which frame of reference each one is using. 

Why would you report a velocity behind the wave measured from the wave when it is your understanding from the problem statement that you are working with a reference frame on the ground?

For example: 

A man on a bus stop sees a car traveling east at 50 mph and a train next to the car also traveling east 20 mph faster than the car. What is the velocity of the train?

would you answer 0 mph because that is the velocity of the train with respect to an observer on the train?

 

[Vel2018]

The wording of both problems make it very clear which frame of reference each one is using. 

Why would you report a velocity behind the wave measured from the wave when it is your understanding from the problem statement that you are working with a reference frame on the ground?

For example: 

A man on a bus stop sees a car traveling east at 50 mph and a train next to the car also traveling east 20 mph faster than the car. What is the velocity of the train?

would you answer 0 mph because that is the velocity of the train with respect to an observer on the train?

would you answer 0 mph because that is the velocity of the train with respect to an observer on the train? ---> Well of course not, the question is not the speed of the train but the speed that was left behind caused by the shock, which would be the same irrespective if the shock moves or the air moves. The only function of that velocity behind the shock is the actual velocity of the shock itself, it doesn't matter if you are on it or you are watching it from afar, the velocity behind the shock is the same. 

This is what it did on the problem as I understand it, the velocity behind the wave had a negative value(this is the velocity induced by the shock wave) then it subtracted it to the actual speed of the shock, what do you get is the speed of the "shock wave" relative to the velocity induced by it (not the velocity induced behind it). But this wasn't the question being asked. 

 

[Slay the P.E.]

... it doesn't matter if you are on it or you are watching it from afar, the velocity behind the shock is the same. 

This is not correct. The magnitude and direction of any velocity depends on the frame of reference; i.e. literally if you are on an object or are watching said object from afar. If you’re on the wave you “see” the air downstream going away from the wave. If you’re on the ground you see the air downstream of the wave going in the same direction of the wave. Both magnitude and direction are different depending on where the observer is. Do you agree?

Once you establish the frame of reference you use your calculation tools (in this case the tables) and then you report your velocities in the frame of reference being used in the problem. Right?