Force, acceleration and speed

[PeterRoss]

I have always had troubles understanding even basics of physics, and most part I know about it I learned from Orbiter, so you can figure how much I don't know. Could you guys please help me to comprehend the matters specified in the thread title?

From what I remember from school and wikipedia, the force applied to a body provides acceleration to it, i.e. body speed is increasing as long as force is applied.

Now the 'real' situation: I have a DGIV landed on the Moon, force vectors are shown, I can see the numerical amount of gravitational force applied to the DG. I have LoadMFD to monitor acceleration too.
Then I start hover autopilot and set vertical speed to +1 m/s. DG is starting to ascend, acceleration is jumping for a moment and falls back to a free fall value, I can understand this.
Then I look at force vectors. I can see that hover thrust force value is 22 N bigger than the gravity force value.
I increase the ascent speed up to +40 m/s and I see the same difference in thrust and gravity forces that I've seen before: 22 N.
Then I decrease the vertical speed to -1 m/s or any other value, actually - as long as the vertical speed is constant the difference between thrust and gravity is 22 N.
I can't see the difference for VS 0 m/s becaue autopilot can't stabilize thrust, but I guess it is the same 22 N.
Then I teleport DG to Earth and set VS to +1 m/s. I see that hover thrust force value is actually lower than the gravity force by more than 500 N and there's 139 N of drag force too, but the DG keeps ascending anyway.
When I increase VS to +40 m/s the thrust force in increasing to compensate drag, but the difference between (thrust-drag) and gravity is still the same.

Now the questions:
1) Are the forces' values indicated in Orbiter correct for such situation in real life?
If it is, I don't understand a thing at all. If it's not, then:
2) If a body is ascending in a gravity field with a constant vertical speed, does it mean that the body is moving with zero acceleration?
3) If it is, are the lifting and gravity forces equal in this case?

 

[Quick_Nick]

If you have your units correct, 22 N is basically imperceptible. Less than milliGs of acceleration on a DGIV.

 

[PeterRoss]

If you have your units correct, 22 N is basically imperceptible. Less than milliGs of acceleration on a DGIV.

So it could fall under some margin of error of force value calculation? I guess it means 'Yes' for 2) and 3) questions?

 

[Quick_Nick]

So it could fall under some margin of error of force value calculation? I guess it means 'Yes' for 2) and 3) questions?

Correct.

 

[jedidia]

1) Are the forces' values indicated in Orbiter correct for such situation in real life?
If it is, I don't understand a thing at all. If it's not, then:

Within reasonable accuracy limit and excluding any not simulated factors. I.E. in a vacuum, the values should be pretty close.

2) If a body is ascending in a gravity field with a constant vertical speed, does it mean that the body is moving with zero acceleration?

Velocity doesn't matter to acceleration, at least in a vacuum and at non-relativistic speeds, and therefore also has no influence on how much force must be applied to reach that acceleration.

So to answer your question, acceleration is defined as change in velocity. If there is no change in velocity, by definition there can be no acceleration. Which does not at all mean that there are no forces acting on a body.

3) If it is, are the lifting and gravity forces equal in this case?

Yup. Because anything else would result in acceleration, either downward or upward, depending on what force was stronger

 

[Phoenix]

I am the author of LoadMFD. I don't have the source code to it anymore, but from what I remember, it calculated G-force by only taking one G-body into account and assumed the gravity from it was perfectly spherical.

In the preTimeStep function, my MFD made a prediction (using RK4 integration) of how fast the vessel should be moving after Orbiter updates the vessel, assuming gravity alone was acting upon the vessel. In the postTimeStep function, after Orbiter had updated, the actual vessel velocity was compared with the prediction. This difference is G-force when scaled appropriately for time step and G.

Orbiter itself may take more than one G-body into account when updating a vessel's position and velocity, and may use non-spherical gravity calculations. This is the main reason why my LoadMFD only measures G-force to two decimal places. If HoverMFD is calculating gravity the same simplified way as my MFD, this may account for the differences you are seeing.

I could be wrong, though.

 

[PeterRoss]

Orbiter itself may take more than one G-body into account when updating a vessel's position and velocity, and may use non-spherical gravity calculations. This is the main reason why my LoadMFD only measures G-force to two decimal places. If HoverMFD is calculating gravity the same simplified way as my MFD, this may account for the differences you are seeing.

I wasn't using HoverMFD, but Orbiter genuine visual helpers. I guess it can use simplified calculation as well.

And thank you for your great Load MFD. It helps me greatly in understanding how things are working and what should I feel if I were a pilot. :tiphat:

 

[kamaz]

I am the author of LoadMFD. I don't have the source code to it anymore,

That's going in tangent, but this is why I put source code for my addons on OH...