# Objects displacement after applying force doesn't match calculations

Hello, I’m trying to test just a simple setting using AddForce and I have difference between results from unity and results from math equation, Couldn’t find out what’s wrong any tips would be appreciated…
m = 1;
gravity and drag are turned off.
Starting position is (0; 0)
Objects final position becomes (202; 0). However with my calculations I should get (200; 0).
Since F=ma, (F = 100, m = 1) => a = 100.
S = 1/2 * a * t2 (t = 0.02 * 100, since deltatime for fixed update is 0.02 and I add force 100 times)
so I get S = 200, But unity says 202, I get similar results In case I use impulse force mode. What do I miss?

``````int count = 0;
float x = 100;
float y = 0;

Vector2 force;
Rigidbody2D rb2d;

void Start ()
{
rb2d = GetComponent<Rigidbody2D>();

force = new Vector2(x, y);
}

void FixedUpdate()
{
if (count < 100)
{
}
else
{
rb2d.velocity = Vector2.zero;
rb2d.angularVelocity = 0;
}

count++;
}
``````

If I set x = 10 final position is (2,2 ; 0). If I set x = 1, than - (0.04 ; 0). Neither of these match my calculations.

The Physics simulation is an approximation. In your example you are using the 2D Physics engine, but there’s a very good explanation of the approximations in the 3D Engine PhysX manual . Check out the Physics vs. PhysX section:

http://docs.nvidia.com/gameworks/content/gameworkslibrary/physx/guide/3.3.4/Manual/Introduction.html#physics-vs-physx