Hello everyone! :)(adsbygoogle = window.adsbygoogle || []).push({});

This is my first post in this forum! :) I am a computer programmer but the last weeks I am trying to combine my field with astronomy and astrophysics. I decided to create my own nBody simulator but I have problems understanding the calculation of the velocity and speed during the integration. My skills in physics are not that good so please bear with me. Let's start with a simple example. Assume that we have a set of bodies with some initial velocity. I thought that the position of each body is calculated like this:

for each integration frame (or whatever you can call it) do the following:

X = Xo + Uo*dt + 0.5 * A * dt * dt;

U = Uo + A * dt;

Uo = U;

X is the position at the end of the interval. Xo is the position at the start of the interval.

Uo is the velocity at the start of the interval. dt is the time interval. A is the acceleration at the start of the interval. What I do above is to calculate the position at the end of the interval using the position, velocity and acceleration at the start of the interval. Then I calculate the velocity at the end of the interval in order to use it for the next physics-frame/integration. Isn't this right?

After checking some NBody simulation examples from nVidia and Microsoft SDK I realized that they use the following to calculate the position of each particle:

U = Uo + A*dt; // This will Calculate the velocity at the end of the interval. Right?

X = Xo + U*dt;

Uo = U;

From my point of view this is wrong because here the position at the end of the interval is calculated using the velocity at the end of the interval and not the velocity at the start of the interval. Apparently, using the second method, the nbody system looks "nice" in nVidia's and Microsoft's demos. But if I replace these 3 lines of code with my solution, the nbody won't look "nice". Using the second solution I get a "galaxy-looking" result. With a lot of particles at the center of the "galaxy". With my solution, the "galaxy" still have more particles on its center than on the edges but the difference is not very noticeable like it is when using the 2nd solution.

What is the difference between these two solutions? And why my solution is wrong?

Thanks a lot for your time.

Cheers,

Bekos

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Problem understanding NBody simulation physics

Tags:

Loading...

Similar Threads for Problem understanding NBody |
---|

I Colonies on Mars -- fundamental problem |

A Trouble visualizing and understanding the celestial sphere |

I Can we experimentally understand the interior of a star? |

I Trying to understand gravity |

B Does Modern Astronomy have a problem? |

**Physics Forums | Science Articles, Homework Help, Discussion**