Mini Physics Quizzes Mega Thread

[Rtyh-12]

The idea came to me a while ago, when I read a mini-exam on a "Welcome" thread. The person (I can't recall the name of the thread, sorry) posted a question with 4 possible answers. Then someone else said that the idea was great, and that "we should have more such questions", "to keep the community up to date with their physics". So, read the following example (the one that was posted on the thread I mentioned before):

A DeltaGlider fires its front left RCS thruster only. What happens?

1.It starts to rotate to the right.​

2.It translates right.​

3.Both of the above.​

4.Nothing happens.​

(or something like that, I can't remember exactly)

RULES:

1.Don't post a new mini-quiz until the last one posted has been correctly answered (exception: if the last mini-quiz has been posted more than 5 days ago).​

2.Only post 1 mini-quiz per post unless you post an answer to another mini-quiz.​

3.The same person can't post quizzes twice in a row. So A posts a quiz, B answers, B posts a quiz, A answers and posts another quiz etc.
​

4.Respect the rules above.​

 

[Urwumpe]

3.

A classic for confusing engineering students.

 

[Polaris]

I pick answer 3. (But with all due respect, it depends on whether the RCS is set on rotation or translation)

 

[Ripley]

Well I'd say number 1.

If it fires "front left RCS thruster only", it means it IS already set on rotational.

 

[RisingFury]

I pick answer 3. (But with all due respect, it depends on whether the RCS is set on rotation or translation)

No it doesn't.

---------- Post added at 16:49 ---------- Previous post was at 16:48 ----------

Well I'd say number 1.

If it fires "front left RCS thruster only", it means it IS already set on rotational.

You get both, rotation and translation in this case.

 

[orb]

(But with all due respect, it depends on whether the RCS is set on rotation or translation)

RCS mode is set neither on rotation nor translation. It's a single thruster firing, and that's why the craft is rotating and translating at the same time, because there's torque and there's no another thruster firing at the opposite side of the center of mass to prevent translation.

 

[Urwumpe]

No it doesn't.

You get both, a rotation and translation in this case.

Exactly. The force that produces the torque that produces the rotation, is the same force that also accelerates the CoG of your spacecraft. You can only have no rotation, if the single thruster is directly pointing at the CoG, but translation can only be countered by firing thruster pairs, so the translating forces neutralize each other.

 

[Ripley]

...It's a single thruster firing, and that's why the craft is rotating and translating at the same time, because there's torque and there's no another thruster firing at the opposite side of the center of mass to prevent translation.

:facepalm:

 

[Rtyh-12]

Ok, since nobody seems to want to post another one, I'll give you the correct answer and another mini-quiz. The answer was number 3. Well done to all who said three!

And now, let me think of another question.

I'm still thinking...

I've got it! A "sequel" to the other one. The same DeltaGlider is floating around in space (after killing the rotation). It fires rotational RCS to the left. After it gets a nice rotation speed, it fires rotational RCS up. What happens?

1.It starts to spin upwards and to the left.
2.It starts to spin around two different axes at the same time.
3.Depends on how much the thrusters were kept on.
4.All hail the fuzzy diicez!!!!111!1!

P.S.:I've tweaked the quiz rules and now they prevent the same person from posting quizzes twice in a row. I know that I've just violated this rule, but if nobody is willing to post another quiz...

 

[Urwumpe]

2. This should usually result in nutation, and actually affect all three principal axes.

It is not that I didn't know a new quiz question, it was just that I thought we resolve the first one before we get to the next one.

 

[ghostrunner01]

I say 4!

Just kidding my actual answer is 2. I agree with Urwumpe.

 

[n72.75]

2.

Assuming the Deltaglider has an angular frequency of much less then the angular state propagator frequency.

 

[Unstung]

2
If the thruster that fires up is located at the center of the spacecraft, would it be 1?

 

[Grover]

2:
it would rotate constantly left and also upwards as well (im not sure about the conditions for the vertical rotation to oscilate, although when ive nudged the stick in time acceleration it has done that to me)

 

[Rtyh-12]

Wrong! Number 4 was the right one!

Kidding, 2 was correct. We could actually test it in Orbiter.

 

[Grover]

ok, i have another question, one taken almost directly from a physics mock i had a while back:

if a small space probe is travelling in a straight line through deep space at 500m/s, weighing 500kg, and it fires a small capsule weighing 200kg directly backwards at a relative velocity of 250 m/s, what is the resultant kinetic energy of the probe?

bit of calculating to do, but itll wake up the brain. no complex equations either, just common sense and physics knowledge

good luck!
-=Grover=-

 

[Urwumpe]

ok, i have another question, one taken almost directly from a physics mock i had a while back:

if a small space probe is travelling in a straight line through deep space at 500m/s, weighing 500kg, and it fires a small capsule weighing 200kg directly backwards at a relative velocity of 250 m/s, what is the resultant kinetic energy of the probe?

bit of calculating to do, but itll wake up the brain. no complex equations either, just common sense and physics knowledge

Since all is relative to a frame of reference and the velocities very low - just use newton...

[math]E_{kin} = \frac{1}{2} \cdot m \cdot v^2[/math]

Now, you have two ways getting the change in energy - by energy and by impulse, both should have the same results.

At the beginning of the scenario:

[math]E_{kin, probe}^0 = \frac{1}{2} \cdot (500 kg + 200 kg) \cdot (500 m/s) ^2 = 87.5 MJ[/math]

The energy for releasing the small capsule is:

[math]E_{kin, capsule}^1 = \frac{1}{2} \cdot 200 kg \cdot (250 m/s)^2 = 6.25 MJ[/math]

So, the energy of the probe after releasing the capsule is 6.25 MJ higher (since the capsule has a lower kinetic energy) than it was before, and it also lost 200 kg mass: 93.57 MJ

Releasing the capsule increased the speed to 612.37 m/s.

---------- Post added at 01:04 PM ---------- Previous post was at 12:52 PM ----------

One spaceflight quiz question from my side:

What is the maximum chamber pressure in a Kerosene/ oxygen rocket engine, that you can reach with a gas generator cycle/open cycle?

A 2.5 MPa
B 6.9 MPa
C 11.5 MPa
D 35.2 MPa

 

[Grover]

actually, i found 150Mj total energy of the probe, using momentum conservation to work out its velocity, then quaring and multiplying by 150 (half the mass)

and low velocities, doesnt matter, it was a question on a physics paper

 

[Rtyh-12]

Simple answer: the probe isn't moving relative to itself, so its kinetic energy is zero

More complicated answer...whatever...uhh...whatever you said there...

 

[T.Neo]

A 2.5 MPa
B 6.9 MPa
C 11.5 MPa
D 35.2 MPa

C?

:shifty: