Newbie questions

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Newbie questions

Postby d3x0r » Sat Mar 09, 2019 4:35 pm

(well new to newton)

1) does newton handle like a heavy body on a small light body stacked on a static body?

and I've been trying to dig through the code to find how gravity works...
2) Is gravity applied (or really all forces) as a full step to velocity or is it perhaps applied as the average of initial and resulting accelerations? such as... if I make a simple simulation of a ball drop 100m and run the simulation at 1fps, 5fps, 10fps, 30fps, 90fps the results are all pretty close; rather than all other physics engines that result entirely differently?
I realize v += ( ia + fa )/2 is like twice the works of v += fa
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Re: Newbie questions

Postby Julio Jerez » Sat Mar 09, 2019 5:22 pm

For what I can read, I think you are asking if Newton uses a first order Euler or a mid point integrator.

The answer is neither of the two.
It is true that when acceleration is constant like gravity is, a mid point integrator make the integration exact independent of the time step, it is also true that this is the one and only situation where this is true.

Physics libraries can't be coded to deal with the special case, they need to deal with the general case where the acceleration is not constant.
For that reason Newton uses a semi implicit integration, that is it assume velocity at the end of the time step, as if it was the velocity at the begining to integrate the position.
This is actually very stable, but does not yield time independent results.

If this is part of your requirements, then you will better served by some other library.
Julio Jerez
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