Rigidbody sphere slows down very fast

I have a plane without any physic material, and a sphere (Physic material also set to None)

Code (CSharp):

1. Vector3 moveVector = clickedPosition - rigidbody.position;
2. rigidbody.AddForce (moveVector, ForceMode.Impulse);

Sphere starts to move on mouse click and it works.
But i can't understand why it slows down so quickly.

Video: https://drive.google.com/file/d/0B1fx5ANOw2asaVl3clZLMUFmaUU/view?usp=sharing

After AddForce sphere starts to move fast but some seconds later it slows down very quickly but
continues to move at very low speed.

I've already tried:

Change mass, Drag, Angular Drag, change physic material (both sphere and plane), Physics Manager (Sleep Threshold) but with no luck.

I expected that after applying a force sphere's slowing should be more smooth, or "linear", but now i see a sharp slowdown at the end of the moving. I can't explain it correctly, it is shown in video.

I think i am doing something wrong.

 

if the collider has no physics material, it will use a default material that has a friction of .6 I believe. Try adding a physics material to the ground or ball, but set all values to 0, and set combine friction to minimum. Now the ball would move forever. Add friction to your desired amount.

In the project settings -> physics, you can set the default material to use when no material is given.