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# Here is a dumb physics question for you smart people

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You're in a futuristic vehicle flying at a given speed.

You need to dock with the mothership through its trap door on the underside.

You match the speed of the flying mothership so you are under the trap door.

Trap door opens and you use your vertical thrusts to enter into the hanger bay.

Trap door closes.

At what point of the sequence does your horizontal speed become zero?

When is your speed relative to the inside of the mothership? When the trap door closes or when you enter inside the ship?

You match the speed of the flying mothership so you are under the trap door.

Whenever you are at the same speed, regardless of distance, you have the same relative speed to the inside of the ship.

I suppose in this example you'd always give your speed relative to the mothership, giving it relative to anything else is useless to you in your calculations... So even approaching the ship, you'd say you were moving at 5m/s or whatever, but always relative to the ship, not 500m/s relative to earth back in the distance.

Regards,

Ró.

Whenever you are at the same speed, regardless of distance, you have the same relative speed to the inside of the ship.

How about you 'hover' inside the ship and then the trap door closes. You are still maintaining your speed relative to the mothership's speed. Will you slam into the end of the ship?

How about you 'hover' inside the ship and then the trap door closes. You are still maintaining your speed relative to the mothership's speed. Will you slam into the end of the ship?

No you won't slam as your speed all along would have been relative to the ship, not just when the trap door closes or you enter the ship. I understand why you could find that confusing though, it does require quite the bit of logical thinking through...

Regards,

Ró.

I imagine your indicated airspeed would reduce to zero the moment the pitot tube

no longer received the intake of air from the earth's atmosphere.

ANAMIV

You really need to specify, if you are still in the earths atmosphere (RIP ! ), or in outer space.

( Much easier in Outer Space ! )

It's all relative ...

"God does not care about our mathematical difficulties. He integrates empirically."

Albert Einstein ( 14 March 1879 – 18 April 1955)

Lets say the mothership is travelling at 100 knots and your vehicle enters the hanger at a hundred knots.

When the trap door closes you are still going at a hundred knots but now relative to the inside of the ship which is at zero knots. So wont you have to cut your throttles and speed to zero the minute the trap door closes to 'land' inside?

That is like jumping in an aircraft and flying to the back at 500 mph because you are not touching the aircraft anymore.

Lets say the mothership is travelling at 100 knots and your vehicle enters the hanger at a hundred knots.

When the trap door closes you are still going at a hundred knots but now relative to the inside of the ship which is at zero knots. So wont you have to cut your throttles and speed to zero the minute the trap door closes to 'land' inside?

Okay, but you're not getting the relativity bit here. The Mothership is travelling 100kts, but relative to what?

So let's just say it's relative to earth, and your ship is also travelling 100kts relative to earth, so when they dock, both will still be going 100nts relative to earth, but your ship will be going 0kts relative to the mothership. Making any sense?

That is like jumping in an aircraft and flying to the back at 500 mph because you are not touching the aircraft anymore.

Perfect example.

Regards,

Ró.

You really need to specify, if you are still in the earths atmosphere (RIP ! ), or in outer

"God does not care about our mathematical difficulties. He integrates empirically."

Albert Einstein ( 14 March 1879 – 18 April 1955)

Earth!

That is like jumping in an aircraft and flying to the back at 500 mph because you are not touching the aircraft anymore.

So in this example when the trap door is open but you are inside the mothership, are you still relative to earth or the ship?

Actually, I have "sim" experience with this. Anybody here a fan of the space-sim "Orbiter"?

Well, it turns out that if you're, say, 5 meters below the mothership and your relative speed is 0 m/s then that (obviously) going to have (basically) identical orbital parameters. If you thrust "up" into the mothership you've changed your orbital parameters. You're generally going to continue in that direction until you reach your new apoasis (highest point in orbit) and begin the other part of the ellipse and start heading back downhill towards your periapsis. If you're in a perfectly circular orbit to begin with (which really never stays circular due to orbital perturbations/solar wind/etc), then by thrusting in any direction you will change your orbital into an ellipse by some degree. It will not change it very much, but it is without a doubt changing the shape of your orbit.

If (for example's sake) you entered in infinitely large room, then you would not always just drift towards the ceiling. At some point you'd start drifting down and around the inside of the ship. Unless you thrust "down" and null your relative velocity again, you will hit the ceiling.

This is basically why astronauts strap themselves down to sleep. Over time they will drift around the cabin. Which is not good since the walls/ceilings are generally filled with switches and buttons.

EDIT: Docking is a different story. Even during the docking process the ships are moving towards/away from each other. It isn't until they make contact, connect and attach to each other that they have zero relative velocity. If you even watch videos of spacecraft docking (older Apollo videos of the CSM docking to the LM show this the best) you can see a definate "rocking" of the spacecraft as they connect and the docking ship gets "grabbed" by the larger, more massive target vehicle.

As you entered the hangar bay, you would no longer be subject to the resistance (and aerodynamic lift) imparted by a mass of air moving towards you at your initial speed. The air in the hangar bay would be moving at zero horizontal velocity relative to your ship, so you would lose all of that lift!!!

EDIT: This is assuming that you are in the atmosphere. If you are in space, then you would not have any forward thrust at all once you are directly underneath the "trap door", otherwise you would accelerate beyond it. All you would need to do is use a small amount of "upward" thrust to get into the hangar bay, and then a small amount of "downward" thrust to land inside the mothership once the "trap door" is closed.

Okay, but you're not getting the relativity bit here. The Mothership is travelling 100kts, but relative to what?

So let's just say it's relative to earth, and your ship is also travelling 100kts relative to earth, so when they dock, both will still be going 100nts relative to earth, but your ship will be going 0kts relative to the mothership. Making any sense?

Yes that makes sense. So if the trap door is open, and you are inside the ship hovering, you are still relative to earth and going 100 knots?

The minute the trap door closes you become relative to the inside of the ship and you should cease all speed?

Yes that makes sense. So if the trap door is open, and you are inside the ship hovering, you are still relative to earth and going 100 knots?

The minute the trap door closes you become relative to the inside of the ship and you should cease all speed?

Not quite cease all speed, you'd still be moving away from earth at 100kts if you think about it right? That speed hasn't gone anywhere, if that makes sense.

Basically think of your ship having many different speeds if that helps, so:

100kts relative to earth,

0kts relative to the mothership

70kts relative to jupiter

90kts relative to mars

And so on, these speeds are all happening at once, it just depends on which one you want to use, and which are relevant, in this case the speeds relative to Jupiter and Mars are of no concern to us, just the one relative to earth and to the Mothership.

I've probably just made this clear as mud haven't I...  :mellow:

Regards,

Ró.

Yes that makes sense. So if the trap door is open, and you are inside the ship hovering, you are still relative to earth and going 100 knots?

The minute the trap door closes you become relative to the inside of the ship and you should cease all speed

Not sure if my above post helped at all. You're relative velocity depends on what you're looking at. Your "inertial frame of reference".

Are you referring to spaceflight, or atmospheric flight?

Basically think of your ship having many different speeds if that helps, so:
100kts relative to earth,
0kts relative to the mothership
70kts relative to jupiter
90kts relative to mars

And so on, these speeds are all happening at once, it just depends on which one you want to use, and which are relevant, in this case the speeds relative to Jupiter and Mars are of no concern to us, just the one relative to earth and to the Mothership.

Exactly.

Are you referring to spaceflight, or atmospheric flight?

I was thinking spaceflight. Now I'm not sure of the OP's original intentions. I'm at work and have to keep jumping back and forth haha

Are you referring to spaceflight, or atmospheric flight?

Space I'd hope, otherwise we'd have all aerodynamic and gravitational forces to take into effect...  :mellow:

When is your speed relative to the inside of the mothership? When the trap door closes or when you enter inside the ship?

It may help to think of yourself and your ship as being satellites. Both entirely with entirely unique orbital properties. Just because you move inside something, doesn't mean that your relative velocity is just going to drop to zero.

If a car drove by at 85 mph (miles. The rest of the world, please feel free to convert to meters haha) with the door open, and timed yourself perfectly and jumped in as it went by, you're not going to have a good day.

Actually, I have "sim" experience with this. Anybody here a fan of the space-sim "Orbiter"?

Well, it turns out that if you're, say, 5 meters below the mothership and your relative speed is 0 m/s then that (obviously) going to have (basically) identical orbital parameters. If you thrust "up" into the mothership you've changed your orbital parameters. You're generally going to continue in that direction until you reach your new apoasis (highest point in orbit) and begin the other part of the ellipse and start heading back downhill towards your periapsis. If you're in a perfectly circular orbit to begin with (which really never stays circular due to orbital perturbations/solar wind/etc), then by thrusting in any direction you will change your orbital into an ellipse by some degree. It will not change it very much, but it is without a doubt changing the shape of your orbit.

That kind of makes sense if I think of it as orbits; as you move into the hanger bay you have to maintain your orbit speed concurrent to the mothership speed, however of course you will have to take into consideration wind resistance, gravity, etc.

I can see how it would be a completely different proposition in space.

Basically think of your ship having many different speeds if that helps, so:

100kts relative to earth,

0kts relative to the mothership

70kts relative to jupiter

90kts relative to mars

And so on, these speeds are all happening at once, it just depends on which one you want to use, and which are relevant, in this case the speeds relative to Jupiter and Mars are of no concern to us, just the one relative to earth and to the Mothership.

I've probably just made this clear as mud haven't I...  :mellow:

Regards,

Ró.

I just remember the physics experiment we did in high school with the bouncing ball on the moving train

Why not both?

If the earth is moving 250000 mph. absolute in the universe, and the mothership is moving 500 mph. relative to earth in that same direction, it is moving 250500 mph absolute. If it moves 500 mph. relative to earth in the opposite direction, it moves 249500 absolute.

Now lets regard the earth as stationary for the sake of argument. The mothership moves 500 mph. relative to the earth. You are moving 0 mph relative to the mothership. Because if A=B and B=C, A=C, you are therefore moving at 500 mph. relative to the earth AND 0 mph relative to the mothership AT THE SAME TIME. Therefore it does not matter when the trap-door closes or indeed if it ever does. You will still be moving relative both to the earth and to the mothership. Simply closing the door does not make the earth cease to exist.

In practice, when doing a docking scenario like this, the only speeds that matter are the speed of the dock and the speed of the thing that's docking, so if you're landing on a mothership, it does not matter how fast you're going relative to the earth, only how fast you're going relative to the mothership.

Say it takes X% throttle to maintain your speed relative to the earth. It will continue to require that much throttle regardless of your position relative to the mothership. In order to turn off your engines and continue to move at that speed - or rather to continue to move at the speed relative to the earth AND remain motionless relative to the mothership, you want to become physically attached to the ship somehow and move as a single unit. You would want to use some sort of docking grapple, methinks.

Thanks for the explanation, but what if you don't dock? Do you maintain your current speed (adjusting for resistance) inside the mothership so you are moving consistently relative to earth?

Thanks for the explanation, but what if you don't dock? Do you maintain your current speed (adjusting for resistance) inside the mothership so you are moving consistently relative to earth?

Assuming the mothership is flying at a constant velocity, and you are below it, flying at the same velocity, your speed relative to the mothership is zero. Relative to the earth it is x m/s

If you move up into the mothership(without docking), you are still doing x m/s relative to the earth and 0 m/s relative to the mothership.

If you dock into the mothership, you are still doing x m/s relative to the earth and 0 m/s relative to the mothership.

ie) you will always be travelling at a constant speed relative to the mothership(0 m/s) and earth (x m/s)

All of this is of course assuming that there is no acceleration taking place.

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