Question

i do have 2 points on a 2d plane. one has already an vector that does determine in which direction it will move.

now i want to add a vector to this existing vector. so he accelerates in the direction of the other point.

to be a bit more clear, it is about 2 asteroids flying in space (only 2d) and gravitation should move them a bit closer to each other.

what i did build till now is this:

c = body.position - body2.position;
dist = c.Length();

acc = (body.masse * body2.masse) / (dist * dist);

xDist = body2.position.X - body.position.X;
yDist = body2.position.Y - body.position.Y;

direction = MathHelper.ToDegrees((float)(Math.Atan2((double)yDist, (double)xDist)));

body.velocity.Y = body.velocity.Y + (float)(Math.Sin(direction) * acc);
body.velocity.X = body.velocity.X + (float)(Math.Cos(direction) * acc);

in the moment the direction calculated is completly off. surely i am making just a stupid mistake, but i have no idea.

Answer 1

You need to pass your direction angle in in radians to Math.sin and Math.Cos (rather then in degree as you do in your smaple code).

see also:
http://msdn.microsoft.com/en-us/library/system.math.sin.aspx

The angle, a, must be in radians. Multiply by Math.PI/180 to convert degrees to radians.