Tweening a curved line to a straight line while keeping aspect ration

How do I animate  a curved line(like hook shape) to straighten out like with shape tween while keeping the length of the  line (aspect ratio)the same? I am doing a demonstration of how curvatures effects lengths of lines etc so the be true I need to keep the length the same while straightening out, I can easily do it if I didn't need to keep the aspect ration the same but I want the hook end to drop down. Please help me, thank you.