Why Rigidbody.AddForce is not acting like the correct physical force?

I have created a rocket which had to have a force and an acceleration as a result of the force. I used the Rigidbody.AddForce function but it acts like a constant initial velocity.
Is there any way to achieve real physical force with acceleration?

here is the code for the rocket’s force:

// rocket is a cloned projectile
clone.rigidbody.AddRelativeForce (Vector3.forward * power);

I found the problem. I had Put theclone.rigidbody.AddRelativeForce (Vector3.forward * power); line inside of a condition! it was inside the FixedUpdate but it could not be updated every frame! kind of silly…