I am attempting to use AddForce for 2d movement on two axes (x,y). This part of my script is not working and I can't put my finger on why.

Questions & Answers
1

if (Input.GetKeyDown(KeyCode.LeftArrow))
rb.AddForce(movementForce, 0f, 0f, ForceMode.Impulse);

    }
2

Input.GetKeyDown happens only in one frame, you should probably use Input.GetKey instead.