# How complicated is Rocket Science?

And now a complete side track from you...

Yeah, well I don't like feeling stuck, you know?

So do you understand why the apparent zero-velocity you have, when you stand still, is really due to the surface and both are accelerating, which is why you experience weight - the only way to overcome the force that makes you weigh something is to free fall towards the center.
??

So do you understand why the apparent zero-velocity you have, when you stand still, is really due to the surface and both are accelerating
So it's only an apparent zero velocity with respect to the surface?
And which way is the surface accelerating?

When people say rocket science, I think they meant Solid Rockets, because Russia could not master that technology during the cold war. I had to go through special security class on this when working at a rocket facility. It is really simple - if you know what you are doing but very hard if you do not - like everything else in life.

If you make a mistake, those giant rockets could go sideways rather than straight up like it happens to cheap bottle rockets - that is why it is so hard. The mixture is simple but the density has to be absolutely perfect because once you ignite, there is no control, unlike liquid rockets.

Dywyddyr said:
So it's only an apparent zero velocity with respect to the surface?
And which way is the surface accelerating?

Yes of course when you are at the same velocity as the planet, the planet isn't moving away from you, or towards you, because you're stuck to it.

A rocket is stuck too, but accelerate the rocket away from the surface - if it runs out of fuel it slows and stops. It stops again after accelerating under free fall back to the surface.

How are the two zero-velocity points related? Why does the moon accelerate towards the earth?

The inescapable conclusion is "anything that sticks to the surface of earth is accelerating".

The inescapable conclusion is that some other force is counterbalancing the force due to gravity. This other force is the normal force. It is what keeps you from sinking into the floor.

Yes of course when you are at the same velocity as the planet, the planet isn't moving away from you, or towards you, because you're stuck to it.
So the relative velocity is ZERO.
I.e. no acceleration.

No, when you have zero velocity with respect to the surface, you weigh something don't you?
Why, when you have zero relative velocity, is there a force called gravity that "weighs" you, (you know, "down" as we say)?

And the surface "keeps" you where you are, it feels solid right? But the earth has only a shallow crust, relative to its radius. The surface moves around and vibrates up and down. You don't notice because the movements take years or centuries. The reason this happens is because the surface is accelerating too, straight "down", and various sideways (tectonic) and up and down forces exist as well as gravity (Ice sheets, volcanic uplifting, etc).

Go back to hanging a weight (yours) from a spring. The spring "keeps" you suspended above the surface => the spring balances the downward acceleration that gives you weight F = mg; therefore the spring applies an equal and opposite force -F.
When you stand on the surface instead, the surface acts in the same way a spring does that suspends you above the surface.

Try experimenting with springs and known weights, see if you can determine G, the constant. (Hint, you use the fact that the springs stretch by different lengths, to "eliminate F = mg").

Last edited:
No, when you have zero velocity with respect to the surface, you weigh something don't you?
Which has what to do with accelerating or not?
If you aren't moving you aren't accelerating.

Why, when you have zero relative velocity, is there a force called gravity that "weighs" you, (you know, "down" as we say)?
How do you conflate having zero velocity with there being gravity? The "why" is misplaced.

According to The Telegraph, "it's not a rocket science" is among the top 10 of irritating expressions which has been compiled by researchers at Oxford University. The complete list:

1 - At the end of the day

2 - Fairly unique

3 - I personally

4 - At this moment in time

5 - With all due respect

6 - Absolutely

7 - It's a nightmare

8 - Shouldn't of

10 - It's not rocket science

Dywyddr: you keep forgetting that when you aren't moving with respect to the surface, you weigh the same as when you are moving.

How do you explain the constant weight, when your velocity changes? You know that when you swing on a rope, your weight "vanishes" briefly at the end of each swing. Why does this occur? Why does a spring stop you from moving, if you are always accelerating under g?

Dywyddr: you keep forgetting that when you aren't moving with respect to the surface, you weigh the same as when you are moving.
And?
You are subject to a force: but as DH pointed out you are also subject to another force (from the ground upwards). No acceleration because there's no movement.

No, when you have zero velocity with respect to the surface, you weigh something don't you?
Why, when you have zero relative velocity, is there a force called gravity that "weighs" you, (you know, "down" as we say)?
The ground is pushing you up at the same time gravity is pulling you down. The net force is nearly zero. There is a residual downward force, just enough to make you rotate with the Earth. That is easily calculable. For someone at the equator, it is $$6378.137\,\text{km} * (2\pi/\text{sidereal day} )^2$$, or 3.39 centimeters/second[sup]2[/sup], or about 1/290 g. This decreases with latitude and becomes zero at the poles.

That is from the perspective of Newtonian mechanics. Since you earlier said the Moon is accelerating toward the Earth I assume that you are thinking in terms of Newtonian mechanics. General relativity has a different perspective. An object sitting still on the surface of the Earth is accelerating with respect to a local inertial frame. It is accelerating up, not down. (This is exactly what an accelerometers sitting on the surface of the Earth reads: 1 g up.)

RIght, but Newton's constant is generally the first stop, not Einstein, when you "teach" the subject.

Newton is elegant because you can easily use the local frame, various weights and springs and derive a value for G, which is independent of weights and springs. Now you know that, anywhere you are in space, a mass attached to a spring will accelerate and the spring will deform.
You can use this knowledge to tell if you're in orbit around a large mass, or traveling freely through space (inside a windowless space station or a capsule, or in an elevator). Conclusion: the density of matter is an induced 'property of gravity', Gravity accelerates matter to itself.

Conclusion: the density of matter is an induced 'property of gravity'
What?
No.
If that's the case why are some metals (for example) denser than others, yet 1 kg of each has the same gravitational pull?

Newton is elegant because you can easily use the local frame, various weights and springs and derive a value for G, which is independent of weights and springs. Now you know that, anywhere you are in space, a mass attached to a spring will accelerate and the spring will deform.
You can use this knowledge to tell if you're in orbit around a large mass, or traveling freely through space (inside a windowless space station or a capsule, or in an elevator).
You cannot use any local experiment such as a mass attached to a spring to distinguish whether you are in orbit around a large mass or traveling freely through space. Similarly, you cannot use such a local experiment to distinguish between being at rest on the surface of a planet versus being in an spacecraft that is accelerating in empty space. Einstein's thought experiment on this matter formed the basis of the equivalence principle. You have it 100% wrong. Try again.

as DH pointed out you are also subject to another force (from the ground upwards). No acceleration because there's no movement.
As I pointed out, when you hang from a spring you are subject to the force of the spring and the force of gravity; when these balance (the spring stops stretching) mg = -kx.
That is to say, the spring stores energy at position x; if you know the constant k, and you know g, you just calculated your weight as a function of x.

DH said:
You cannot use any local experiment such as a mass attached to a spring to distinguish whether you are in orbit around a large mass or traveling freely through space.
Yes you can. Because in an orbit you don't travel with constant velocity.
Similarly, you cannot use such a local experiment to distinguish between being at rest on the surface of a planet versus being in an spacecraft that is accelerating in empty space.
I didn't say you can use springs and different masses to tell that. I said you can use springs and weights to tell if you are in orbit or traveling freely. And, you can.
Einstein's thought experiment on this matter formed the basis of the equivalence principle. You have it 100% wrong. Try again.

I believe I shall. But first I'll wait for your proof that springs and weights are no use to an observer in orbit around the earth, in a windowless capsule. You claim that the observer could not tell any difference between orbital and free velocity, using simple accelerated masses at the end of springs. I would like you to demonstrate that this is true...

As I pointed out, when you hang from a spring you are subject to the force of the spring and the force of gravity; when these balance (the spring stops stretching) mg = -kx.
And when the forces balance there's no movement: no acceleration.