The dumb questions thread

[barbos]

Can somebody summarize gravity assist slingshot for me?

A ship heads for the moon and gains energy from gravity. It circles round and leaves. It seems like the energy gained by gravitational acceleration should be lost leaving against the moon's gravity.

It does not circle. it passes it by changing its velocity in the process.

 

[bigfield]

Can somebody summarize gravity assist slingshot for me?

A ship heads for the moon and gains energy from gravity. It circles round and leaves. It seems like the energy gained by gravitational acceleration should be lost leaving against the moon's gravity.

A gravity assist transfers some of the moon's orbital momentum to the ship.

When you fly a ship past a moon but behind it in its orbit, gravity pulls the ship and moon towards each other; this slows the moon down slightly as it is pulled backwards toward the ship, while the ship will speed up relative to the planet as it is pulled forwards towards the moon.

Since the moon is massive relative to the ship, the moon's deceleration is infinitesimally small while the ship's acceleration is substantial, which is why it seems like the ship is getting kinetic energy for free.

 

[Jokodo]

Can somebody summarize gravity assist slingshot for me?

A ship heads for the moon and gains energy from gravity. It circles round and leaves. It seems like the energy gained by gravitational acceleration should be lost leaving against the moon's gravity.

Its incoming and outgoing accelaration, and thus the incoming and outgoing velocity at given distances is the same relative to the moon. So what you say about it loosing the kinetic energy gained during the approach again when it leaves is true in a moon-based reference frame (and only for so long as neither the moon or the ship are acted upon by other forces). In any other reference frame, it can gain up to twice the moon's velocity, depending on the angle of the ship's and the moon's original trajectories.

So a probe that encounters the moon almost head on (as seen from earth) with an earth-based velocity of 500m/s will have a velocity of 1500m/s relative to the moon, whose own velocity is about 1km/s. As it is slung around by the moon's gravity and now flies in the same direction of the moon, it's velocity relative to the moon will still be 1500m/s, but as seen from earth, those numbers now add up, thus we see the probe as running away ahead of the moon at 2.5km/s

 

[steve_bank]

Can somebody summarize gravity assist slingshot for me?

A ship heads for the moon and gains energy from gravity. It circles round and leaves. It seems like the energy gained by gravitational acceleration should be lost leaving against the moon's gravity.

A gravity assist transfers some of the moon's orbital momentum to the ship.

When you fly a ship past a moon but behind it in its orbit, gravity pulls the ship and moon towards each other; this slows the moon down slightly as it is pulled backwards toward the ship, while the ship will speed up relative to the planet as it is pulled forwards towards the moon.

Since the moon is massive relative to the ship, the moon's deceleration is infinitesimally small while the ship's acceleration is substantial, which is why it seems like the ship is getting kinetic energy for free.

That makes sense. The moon or planet is not standing stiil hence the term slingshot.

 

[Loren Pechtel]

Its incoming and outgoing accelaration, and thus the incoming and outgoing velocity at given distances is the same relative to the moon. So what you say about it loosing the kinetic energy gained during the approach again when it leaves is true in a moon-based reference frame (and only for so long as neither the moon or the ship are acted upon by other forces). In any other reference frame, it can gain up to twice the moon's velocity, depending on the angle of the ship's and the moon's original trajectories.

So a probe that encounters the moon almost head on (as seen from earth) with an earth-based velocity of 500m/s will have a velocity of 1500m/s relative to the moon, whose own velocity is about 1km/s. As it is slung around by the moon's gravity and now flies in the same direction of the moon, it's velocity relative to the moon will still be 1500m/s, but as seen from earth, those numbers now add up, thus we see the probe as running away ahead of the moon at 2.5km/s

Second this. This is the explanation of the gravitational slingshot maneuver.

The ship retains it's velocity relative to the object that it did the slingshot maneuver with. It's not the same with respect to an outside reference frame, though.

Note that it does not have to speed up--we do slingshots for speed when the destination is the outer solar system but we do them in reverse when the destination is inwards. It all comes down to how much time you're willing to trade for the boost, the more slingshots you do the more time but less fuel. It's actually more productive going inwards because the orbits are shorter and thus you drift less between encounters. AFIAK the record holder is Messenger with 6 slingshots in the 5 years after launch. A hell of a lot cheaper than the 8650 m/s needed to reach it's orbit on a tail of fire. That's more than any other planet.

Edit: Found a simple explanation with a train:

http://www.schoolphysics.co.uk/age14-16/Astronomy/text/Slingshot_/index.html

 

[Jokodo]

Its incoming and outgoing accelaration, and thus the incoming and outgoing velocity at given distances is the same relative to the moon. So what you say about it loosing the kinetic energy gained during the approach again when it leaves is true in a moon-based reference frame (and only for so long as neither the moon or the ship are acted upon by other forces). In any other reference frame, it can gain up to twice the moon's velocity, depending on the angle of the ship's and the moon's original trajectories.

So a probe that encounters the moon almost head on (as seen from earth) with an earth-based velocity of 500m/s will have a velocity of 1500m/s relative to the moon, whose own velocity is about 1km/s. As it is slung around by the moon's gravity and now flies in the same direction of the moon, it's velocity relative to the moon will still be 1500m/s, but as seen from earth, those numbers now add up, thus we see the probe as running away ahead of the moon at 2.5km/s

Second this. This is the explanation of the gravitational slingshot maneuver.

The ship retains it's velocity relative to the object that it did the slingshot maneuver with. It's not the same with respect to an outside reference frame, though.

Note that it does not have to speed up--we do slingshots for speed when the destination is the outer solar system but we do them in reverse when the destination is inwards. It all comes down to how much time you're willing to trade for the boost, the more slingshots you do the more time but less fuel. It's actually more productive going inwards because the orbits are shorter and thus you drift less between encounters. AFIAK the record holder is Messenger with 6 slingshots in the 5 years after launch. A hell of a lot cheaper than the 8650 m/s needed to reach it's orbit on a tail of fire. That's more than any other planet.

Edit: Found a simple explanation with a train:

http://www.schoolphysics.co.uk/age14-16/Astronomy/text/Slingshot_/index.html

Another interesting consequence of the fact that the velocity of the two objects relative to each other is the same before and after the encounter is that it's well nigh impossible for a solitary object without atmosphere to capture another solitary object (that is, without the help of a third object close enough to differentially effect them). For solar system moons which are hypothesised to be captured asteroids or KBOs in origin, the sun can be that third object but only for long and eccentric resulting orbits, or atmospheric breaking can be to blame, but that'll only work with a small periapsis.

Another possibility is that the captured moon was originally part of a binary asteroid system that was torn apart in the encounter: The original binary would keep it's total momentum relative to the planet, but it would be split unevenly between the two objects, with one leaving the planet's sphere of influence at a higher velocity than it entered, and the other one dropping below escape velocity.

 

[Wiploc]

Can somebody summarize gravity assist slingshot for me?

A ship heads for the moon and gains energy from gravity. It circles round and leaves. It seems like the energy gained by gravitational acceleration should be lost leaving against the moon's gravity.

I have an idea. Suppose the ship is under power, accelerating by throwing reaction mass, and also because the moon is pulling it. So, as the ship goes away from the moon, it won't be nearby for as long as it was on its approach. So the moon will have longer to speed it up than it had to slow it down. So there's a net gain in velocity.

If the ship is just coasting toward the moon while the moon speeds it up, and then still just coasting away while the moon slows it down, then I don't see how there would be any long-term gain in speed. The ship should lose as much speed while departing as it gained while approaching.

I'm not saying that I'm right. I'm just saying that either I'm right or I'm confused.

 

[Juma]

IIRC the largest effect on humans is the case of the three astronauts in the Apollo program who traveled to the moon twice; they gained (at most) in the order of between 0.01 and 0.05 milliseconds (depending on the exact flight paths and acceleration profiles of their missions), while the other twenty one who made it to lunar orbit only once would have gained about half as much.

Nobody else has come close to that; Low Earth orbit missions entail only a minuscule fraction of the acceleration experienced on a lunar mission.

Acceleration doesn't matter, velocity & time do. I would think long-staying ISS astronauts would be way ahead of the Apollo astronauts....

Yup: http://www.huffingtonpost.co.uk/2013/10/23/sergei-krikalev-time-travel_n_4147793.html

More than 2 years in orbit, .02 seconds of time dilation.

Ahem... acceleration do matter.. that is what solves the twin paradox: it isnt symmetric wrt acceleration.

 

[Jokodo]

Can somebody summarize gravity assist slingshot for me?

A ship heads for the moon and gains energy from gravity. It circles round and leaves. It seems like the energy gained by gravitational acceleration should be lost leaving against the moon's gravity.

I have an idea. Suppose the ship is under power, accelerating by throwing reaction mass, and also because the moon is pulling it. So, as the ship goes away from the moon, it won't be nearby for as long as it was on its approach. So the moon will have longer to speed it up than it had to slow it down. So there's a net gain in velocity.

If the ship is just coasting toward the moon while the moon speeds it up, and then still just coasting away while the moon slows it down, then I don't see how there would be any long-term gain in speed. The ship should lose as much speed while departing as it gained while approaching.

I'm not saying that I'm right. I'm just saying that either I'm right or I'm confused.

Sounds about right if your goal is to gain speed relative to the Moon. If all you want is to gain speed relative to some third object, such as Earth or the Sun, you can do so without firing by exploiting the Moon's own motion relative to that object.

This latter is what we talk about when we say slingshot or geavity assist.

 

[spikepipsqueak]

Can anyone explain to me why brick and concrete behave like thermal mass for storage but not if they are covered by tiles?

I is confused.

 

[bigfield]

Can anyone explain to me why brick and concrete behave like thermal mass for storage but not if they are covered by tiles?

I is confused.

Tiles tend to have a more reflective surface than brick or unpolished concrete; the structure absorbs less thermal radiation when covered by a reflective surface.

It's the same principle as white and black garments.

 

[beero1000]

Can anyone explain to me why brick and concrete behave like thermal mass for storage but not if they are covered by tiles?

I is confused.

Tiles would add to the thermal mass. However, textured/matte surfaces and dark surfaces are better at absorbing and radiating heat than smooth/shiny and light-colored ones. Depending on the tile used, that can have an effect on how much heat can be stored and made available in a day. Kinda like having a giant jug of water with only a little spout.

 

[Loren Pechtel]

Can somebody summarize gravity assist slingshot for me?

A ship heads for the moon and gains energy from gravity. It circles round and leaves. It seems like the energy gained by gravitational acceleration should be lost leaving against the moon's gravity.

I have an idea. Suppose the ship is under power, accelerating by throwing reaction mass, and also because the moon is pulling it. So, as the ship goes away from the moon, it won't be nearby for as long as it was on its approach. So the moon will have longer to speed it up than it had to slow it down. So there's a net gain in velocity.

Right and wrong.

What you are describing is correct but it's called the Oberth effect, it is not a gravitational slingshot.

Orbital mechanics 101 says that in almost all situations you burn as deep in a gravity well as possible. This often applies even if you're going to undo some of that later. (Do your capture burn at the edge of the atmosphere then raise your periapsis at the next apoapsis.)

If the ship is just coasting toward the moon while the moon speeds it up, and then still just coasting away while the moon slows it down, then I don't see how there would be any long-term gain in speed. The ship should lose as much speed while departing as it gained while approaching.

I'm not saying that I'm right. I'm just saying that either I'm right or I'm confused.

Your speed does not change relative to the moon. The point of a slingshot maneuver is that your speed can change as seen in the context of the parent body.

- - - Updated - - -

IIRC the largest effect on humans is the case of the three astronauts in the Apollo program who traveled to the moon twice; they gained (at most) in the order of between 0.01 and 0.05 milliseconds (depending on the exact flight paths and acceleration profiles of their missions), while the other twenty one who made it to lunar orbit only once would have gained about half as much.

Nobody else has come close to that; Low Earth orbit missions entail only a minuscule fraction of the acceleration experienced on a lunar mission.

Acceleration doesn't matter, velocity & time do. I would think long-staying ISS astronauts would be way ahead of the Apollo astronauts....

Yup: http://www.huffingtonpost.co.uk/2013/10/23/sergei-krikalev-time-travel_n_4147793.html

More than 2 years in orbit, .02 seconds of time dilation.

Ahem... acceleration do matter.. that is what solves the twin paradox: it isnt symmetric wrt acceleration.

Acceleration doesn't matter for figuring how much time dilation there is. Acceleration matters for solving the twin paradox because one twin is in an accelerated reference frame.

 

[Kharakov]

My dumb question of the day:

What is it called when overlapping standing (oscillating vertically, but not moving horizontally relative to one another) waves with different horizontal offsets (and maybe wavelengths) look like they are moving relative to one another?

 

[fast]

My dumb question of the day:

What is it called when overlapping standing (oscillating vertically, but not moving horizontally relative to one another) waves with different horizontal offsets (and maybe wavelengths) look like they are moving relative to one another?

That is not a dumb question. That is a smart question.

Please use the appropriate thread.

 

[Kharakov]

My dumb question of the day:

What is it called when overlapping standing (oscillating vertically, but not moving horizontally relative to one another) waves with different horizontal offsets (and maybe wavelengths) look like they are moving relative to one another?

That is not a dumb question. That is a smart question.

Please use the appropriate thread.

Phrasing though... I forgot a horizontally.

What is it called when overlapping standing (oscillating vertically, but not moving horizontally relative to one another) waves with different horizontal offsets (and maybe wavelengths) look like they are moving horizontally relative to one another?

 

[fast]

My dumb question of the day:

What is it called when overlapping standing (oscillating vertically, but not moving horizontally relative to one another) waves with different horizontal offsets (and maybe wavelengths) look like they are moving relative to one another?

That is not a dumb question. That is a smart question.

Please use the appropriate thread.

Phrasing though... I forgot a horizontally.

What is it called when overlapping standing (oscillating vertically, but not moving horizontally relative to one another) waves with different horizontal offsets (and maybe wavelengths) look like they are moving horizontally relative to one another?

Amplitude?

Give me multiple choice. Fill in the blank is hard.

He he

 

[steve_bank]

The addition of linear waves is called Superposition. More generally it applies to linear systems. Standing or not, at any point is the linear sum of all the waves.

If two sine waves of different frequencies add the result will be an ac wave. The Fourier Transform of new time signal will show the two original frequencies. Linear superposition.

In a non linear process outputs contain frequencies not present in the input signals.

 

[Kharakov]

What's the illusion of the waves moving forwards called:

They are offset in space and time, same wavelength and amplitude.

OQ: What is it called when overlapping standing (oscillating vertically, but not moving horizontally relative to one another) waves with different horizontal space offsets and maybe wavelengths and time offsets look like they are moving horizontally relative to one another?

 

[steve_bank]

https://en.wikipedia.org/wiki/John_Harrison
https://en.wikipedia.org/wiki/Marine_chronometer

As we are talking about clocks, I watched a show on the development of a British Navy mechanical chronometer resistant to shipboard motions and temperature..