C# XNA Simulate Gravity

Question

I have a question regarding using primitive shapes and simulating a circle to be free falling. How would I go about this? Do I use the formula for gravity?

score 13 · Answer 1 · edited May 23 '17 at 12:07

It depends how detailed you want to be. Newtonian gravity laws dictate an force which has inverse square relationship between two bodies and their distance.

alt text

F is the magnitude of the gravitational force between the two point masses,
G is the gravitational constant,
m₁ is the mass of the first point mass,
m₂ is the mass of the second point mass, and
r is the distance between the two point masses.

So if you are simulating large bodies (say a planetary system) you need to apply gravity between each pair of objects.

Obviously, once you know the force you can apply Newton's second law (force=mass*acceleration) to calculate the acceleration you should apply to each body.

On the other end of the simplicity scale, if you have a fixed reference frame with respect to a single very large body (the ground) and multiple small objects, all you really need to do is apply a constant acceleration towards the ground. In this case, (in a vacuum) gravity on earth applies a constant acceleration of 9.81m/s² to all objects. CodeInChaos gives a good code example of this.

_{(Of course we now know that Newtonian gravity isn't quite right and it breaks down at very large and very small scales. But that's a bit beyond this answer)}

CodesInChaos · Accepted Answer · 2010-10-19T07:48:12.743

8

Gravity is just constant acceleration downwards.

Pseudocode:

const Vector3D Gravity=(0, 0, -9.8 m/s^2);
Vector3D Acceleration=Gravity;//insert other forces here
Vector3D Position+=Speed*DeltaT+0.5*Acceleration*DeltaT*DeltaT.
Vector3D Speed+=Acceleration*DeltaT;

edited Oct 19 '10 at 07:48

answered Oct 19 '10 at 07:42

CodesInChaos

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score 2 · Answer 3 · answered Oct 19 '10 at 08:01

2

You can use a physics engine, like Farseer:

http://roy-t.nl/index.php/2010/09/10/xna-farseer-platformer-physics-tutorial/

answered Oct 19 '10 at 08:01

Frederik Gheysels

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Wow, cool that you linked my tutorial. The Farseer homepage is located at http://farseerphysics.codeplex.com/ btw. – Roy T. May 20 '11 at 15:11
@Roy T.: I've seen it on GoT once. :) – Frederik Gheysels May 20 '11 at 16:15

score 0 · Answer 4 · answered Oct 19 '10 at 09:06

Simple version: if your circle has direction and speed then gravity is just a additional direction and speed pointing at the direction you want your gravity to pull.

Even simpler: just move the location of the circle downwards.

After this just fine tune it to fit your purpose with acceleration and setting the speed.

C# XNA Simulate Gravity

4 Answers4