# How complicated is Rocket Science?

This reminds me of a joke about a mathematician and a physicist. The physicist goes to the staff room to get a coffee, and sees the mathematician beaming with joy. "What's the smile for?" asks the physicist.

'I've just perfected a formula that will determine the winner of any horse race using some basic, discoverable data.'

"Wow", says the physicist, "shall we go to the race track and test it out? We could be rich by this evening!"

'Ah' says the mathematician, 'at the moment it only works for a spherical racehorse running on a Euclidean plane in a vacuum.'

Bzzzt! No, when there is no movement, there is zero relative velocity.
And acceleration induces movement.

If there was no acceleration you would be weightless.
Why do you attribute lack of acceleration to weightlessness?
Is a falling body weightless?
Is it accelerating?

Acceleration "induces" movement if an external force is applied to make something move, yes.
Why do you think you aren't accelerating under the external force of gravity, when "motionless" on the surface of earth?

Is a body in free-fall weightless? Yes, because if you accelerate towards g, the force of "weight" is zeroed by the accelerative force of g. You continue to accelerate until something slows or stops you motion (the atmosphere, the surface).

So that means motion is a resultant of forces; and the force of gravity is constant, continuous and always positive (or negative, if you prefer to think gravity attracts, but that's a convention).
However acceleration due to a constant force is constant so, you are constantly accelerating under the external force of gravity, when "motionless" wrt the surface.

Last edited:
But tell you what, I'll figure out a rough trajectory for a "rocket" of some kind and post it alongside your proof that two bodies in uniform relative motion, which are however in elliptic orbits and accelerating towards g, remain at the same distance from each other.

That's a strawman, noodler. This isn't just about math; it is about physical reality. Suppose you are in a windowless elevator car that is floating in empty space, far from any massive body. You release an object. Hard as you try, you cannot give it a velocity of exactly zero relative to the car (or to yourself). The object will drift and eventually hit a wall. Now suppose you instead are in a windowless elevator car that is orbiting a planet well above the atmosphere. Same thing happens here.

Einstein referred to local experiments. How well you can control and measure things is what defines locality. You are assuming perfect control, perfect measurements. In such a case the local inertial frame shrinks to a point. In the real world of imperfect control and imperfect measurements, a local inertial frame has physical extent.

Acceleration "induces" movement if an external force is applied to make something move, yes.
Er, acceleration occurs because of a force - Newton.

Why do you think you aren't accelerating under the external force of gravity, when "motionless" on the surface of earth?
Because the surface of the Earth provides a counter-force. And "motionless" needn't be in inverted commas.

Is a body in free-fall weightless? Yes, because if you accelerate towards g, the force of "weight" is zeroed by the accelerative force of g. You continue to accelerate until something slows or stops you motion (the atmosphere, the surface).
So a weightless body can be accelerating - contrary to your previous assertion.

However acceleration due to a constant force is constant so, you are constantly accelerating under the external force of gravity, when "motionless" wrt the surface.
Nope, you don't move so you aren't accelerating.

Dywyddyr said:
acceleration occurs because of a force
yes, an external force, as I stated.
If the surface of the earth "provides" a counter force, why do you feel weight? Shouldn't the counter force make your weight vanish?
Or, wait. maybe because applying a force that accelerates you away from the surface means applying the counter force that cancels gravitational acceleration...?
Then, if you don't accelerate "UP" I guess that leaves the other direction to accelerate in.

Because you are most definitely accelerating, if you WEREN"T you wouldn't be on the surface, you would be sliding along it, possibly, or flying horizontally through the air. You realize you have an angular velocity?

"If you don't move you aren't accelerating" is a misconception. That is the last thing I have to say to someone who it seems "just doesn't get it".

Bye..

DH said:
Suppose you are in a windowless elevator car that is floating in empty space, far from any massive body. You release an object. Hard as you try, you cannot give it a velocity of exactly zero relative to the car (or to yourself).
. Ok, but suppose instead you release a number of objects, very slowly and carefully so they have a minimum relative motion.
How long will it take for the objects to be attracted to a wall by the gravitational potentials? What if the objects are in space instead?
The object will drift and eventually hit a wall. Now suppose you instead are in a windowless elevator car that is orbiting a planet well above the atmosphere. Same thing happens here.
So you aren't aware of the precession of objects in orbit? They always precess independently, and, you can release an object, or several objects that tell you they're in orbit along with you. If you don't use objects because you object to the unwanted drift factor, you attach a spring to the side of the elevator, and attach an object with mass to the end of the spring. The spring will oscillate slowly as the attached mass precesses independently of the orbiting elevator.

Perhaps you have heard about the inversion of geometry, close to a black hole and the event horizon? You can use the same basic accelerometer setup to accelerate away from the BH, so which way should you go, towards the hole, or away from the hole?

Last edited:
"If you don't move you aren't accelerating" is a misconception. That is the last thing I have to say to someone who it seems "just doesn't get it".
Bye..
Quite...
I've come across clueless people before but you're in a class of your own.

What Dywyddyrs problem is appears to be related to the inescapable fact that you stop moving wrt the surface of a planet when you get to it.

But he's missing the connection between why you accelerate towards a planet (in the first place) and why, when you get to the surface of one, you experience weight.
Weight is a force -> you have a force acting on you at the surface, because you have a force acting above the surface.
What keeps the surface where it is? The surface beneath it does, on down to the center.

If people want to believe that zero relative velocity means they have no acceleration - because that is only possible if they move ACROSS the surface, and you point out they have a force called weight acting on them so they are accelerating towards the center, and they tell you you are clueless, there it is...

I like to think my cluelessness means I understand that gravity is a force that accelerates me toward the center of massive bodies (after all, it does and you can prove it in a lab, hand in your results and, another clueless person will actually give you marks!!)

But he's missing the connection between why you accelerate towards a planet (in the first place) and why, when you get to the surface of one, you experience weight.
So if I fall toward a planet I accelerate toward the surface... ~10 m/sec[sup]2[/sup]
When I hit the surface what happens to that acceleration? It keeps building?
What happens to the velocity I accumulated?
No movement = no acceleration.

What Dywyddyrs problem is appears to be related to the inescapable fact that you stop moving wrt the surface of a planet when you get to it.
Dywyddyr's problem is apparently the same as mine: He has a hard time suffering fools gladly.

But he's missing the connection between why you accelerate towards a planet (in the first place) and why, when you get to the surface of one, you experience weight.
You accelerate toward a planet because the only force acting on you is gravity. You no longer accelerate when you are at rest on the surface of a planet because some other force, namely, the normal force is keeping you from falling into the planet.

Suppose you are standing the surface of a non-rotating planet far from any star. What is your velocity with respect to an inertial observer co-moving with the planet? What is your acceleration with respect to this observer? Suppose you are standing on a scale. That the scale registers 700 newtons is irrelevant to the discussion of acceleration. The scale measures the normal force, not the net force. Acceleration is the ratio the net force acting to you to your mass.

No movement = no acceleration.

Seems noodler skipped his calculus classes, and doesn't quite understand that acceleration is derived from velocity, and is the rate of change of velocity.

Perhaps phlogistician has forgotten that acceleration is derived from forces applied to a mass.

Deriving velocity means 1) you need a velocity. 2) you need a distance 3) you need a time. Then you need to do the first three again so you have two velocities.
The last two things make up the first. That's 4 things you need to find to calculate acceleration, if you have two velocities.

Since a force acts tangentially the acceleration a force "gives to" a mass is linear by default.

What the fools here have missed altogether, I think, is that zero RELATIVE velocity does NOT mean zero ACCELERATION. Try reading Einstein's theory more closely. Then, when you think you have got it, come back and explain why an astronaut experiences an inertial force when they accelerate in empty space, in a gravitational field G.

That's far away from any massive body with gravitational potential. Where does the INERTIA come from, if there's no acceleration towards any massive body, in fact the astronaut accelerates towards "nothing"?

Geez, what a pack of ignoramuses.

Last edited:
Deriving velocity means 1) you need a velocity. 2) you need a distance 3) you need a time. Then you need to do the first three again so you have two velocities.
The last two things make up the first. That's 4 things you need to find to calculate acceleration, if you have two velocities.
Initial velocity, end velocity, acceleration, distance and time are all related by the SUVAT equations, assuming constant acceleration. If you know three then you can compute the other two.

I believe this is covered around age 14 in schools noodler.

Since a force acts tangentially the acceleration a force "gives to" a mass is linear by default.
This is rife with poorly worded phrases. Not all forces act tangentially. An object in a circular orbit has a force (and thus acceleration) at right angles to its motion. as for 'linear', linear in what? In the force? Well F=ma implies a = F/m and so acceleration is linear in the net force yes. That says nothing about the direction of the force relative to velocity, such an expression would involve $$\mathbf{F}\cdot \mathbf{v}$$, the scalar product of force and velocity.

Since a force acts tangentially the acceleration a force "gives to" a mass is linear by default.is that zero RELATIVE velocity does NOT mean zero ACCELERATION
Constant relative velocity means they experience the same forces so they must have zero relative acceleration. It's a tautology.

Try reading Einstein's theory more closely
I am absolutely certain DH and I know more about it than you.

explain why an astronaut experiences an inertial force when they accelerate in empty space, in a gravitational field G.
What has that got to do with relative velocities? What's he relative to? Two astronauts in a constant gravitational field initially at relative rest will remain at relative rest.

Where does the INERTIA come from, if there's no acceleration towards any massive body, in fact the astronaut accelerates towards "nothing"?
Acceleration is not relative. Read some relativity.

Geez, what a pack of ignoramuses.
Funny how no matter how many people correct you in how ever many threads, its always someone else's fault people don't accept what you say. Weird eh?

Initial velocity, end velocity, acceleration, distance and time are all related by the SUVAT equations, assuming constant acceleration. If you know three then you can compute the other two.
Check out the problem here. IF you have constant ACCELERATION, you have a changing velocity. If there is NO FORCE to give an object acceleration, you have "NOTHING". SUVAT will tell you this.

Please can someone who knows more about it than me explain why an astronaut accelerating in empty space, far away from any gravitational bodies, experiences a force (like Einstein claims). WHERE does this force come from if there is NO g in the vicinity?

Perhaps I actually know the answer; perhaps NONE of you do (that is a direct challenge to DH, and the rest of the fools here).

P.S. DH where is that proof I asked you about? The one about divergence of orbits, in particles initially having zero relative velocity, in independent orbits that precess, um, independently??
Can you perhaps admit YOU WERE WRONG about the divergence thing, and about being able to use it to tell you're in a closed orbit too?

"Acceleration is not relative" UNLESS two bodies are accelerating with, um respect to each other? If you happen to accelerate wrt empty space, is that not relative either?

Last edited:
Check out the problem here. IF you have constant ACCELERATION, you have a changing velocity. If there is NO FORCE to give an object acceleration, you have "NOTHING". SUVAT will tell you this.
No acceleration, that is. The key questions:
• What acceleration yields a constant velocity?
• If an object has a constant velocity with respect to some observer, what is that object's acceleration relative to that observer?
• Is an identically zero velocity a special case of a constant velocity?

Please can someone who knows more about it than me explain why an astronaut accelerating in empty space, far away from any gravitational bodies, experiences a force (like Einstein claims). WHERE does this force come from if there is NO g in the vicinity?
A rocket.

In general relativity, gravitation is *not* a real force. It is a pseudo force, akin to the centrifugal and coriolis forces.

P.S. DH where is that proof I asked you about? The one about divergence of orbits, in particles initially having zero relative velocity, in independent orbits that precess, um, independently??
Stop talking about about mathworld. This is a question about the real world, where neither sensors nor effectors are perfect. How do you place two particles in space such that the relative velocity between them is exactly zero? How do you measure that this velocity is exactly zero (or exactly any particular value)?

" a rocket" is why an acceleration in free space means a resultant force. OK well, I guess someone should have told Einstein, before he went to all that trouble.

"Hey Al, it's a rocket mate". "Oh ok, thanks".

How do you place two particles in free space so their relative velocities are close to zero? How do you tell they maintain their relative positions? How do you KNOW that their motions can tell you about your own motion, if you are in their vicinity? For instance, if you happen to be orbiting a black hole?

DH said:
You accelerate toward a planet because the only force acting on you is gravity. You no longer accelerate when you are at rest on the surface of a planet because some other force, namely, the normal force is keeping you from falling into the planet.
Why is there a normal force at the surface? Why do you "no longer accelerate" when at rest on this surface.
Where does the weight I can feel come from? (I can definitely feel weight).
The surface is "only" restoring my position AT the surface, in a continuous way according to Einstein - would you like me to quote the parts where he says this?

Is a normal force because of a "norm" in some symmetry group? Is the group "all accelerations, in G", or some other group - the "rocket group" perhaps?
:shrug:

Last edited:
Another question I think could be addressed more closely:

If you can have relative velocity with a fixed surface (let's use the standard example), including zero relative velocity, why is this? Why is the surface fixed, what "fixed" or continues to "fix" the surface?

It's because the surface is curved, and the curvature term acts continuously against a determinable limit (pressure). When the limit is reached although curvature continues to "act" on it, you have a static surface, which is curved into a sphere, a sphere being the most probable shape.

So what is this curvature? What part of the overall energy of a planet, is due to curvature? What about G, which also acts continuously, is the curvature because of R, is R dependent on G, or WHAT?

Stop trolling.

WHAT??
It's a perfectly valid question. Or is it something you prefer to treat like the first one.
You recall, you disagreed with the proposition that you can use either a test ball of particles or springs and masses to tell if you are in orbit (without looking out a window)?

Do you have at least a back-of-the-envelope description of H = G + R ?

Houston, we have a moderator problem.

DH I think you may have to admit you don't know everything. You haven't accepted that your "solution" has a problem (it's too naive). But, at the time you seemed convinced, and now you've decided I'm "trolling". I think you're a problem at this forum, and I think you should consider giving up (or learning a bit more, or maybe accepting that you can't just accuse people of talking "rubbish"). I've seen the pattern and so can anyone else who looks over this thread.

You remind me of someone who prefers not to think about certain things, one of which is "I might be wrong".

Last edited:
Houston, we have a moderator problem.
I absolutely agree. You have completely derailed this thread with complete and utter nonsense.