basically i have a door with a knob and i want to when the player comes near the door, hes hand like point to the knob.
like that:
Try animations. That should work!
yeah but im trying to do like dont matter the position of the player, his arm ll point to the knob, and when it get like 2 stds of distance, the door opens
You could use inverse kinematics 
Someone made a similar system that you are trying to do with inverse kinematics using this module
Hello! CCDIK is a great module, though it’s a lot easier to work with IKConstraints that Roblox has released officially. I heavily recommend using them instead.
What you could do is like make a seperate model just for the arm and the actual arm of the humanoid be a transparency with 1 whenever its running. Have like an attachment that is near the shoulder. Use CFrame.lookat(AttachmentPosition,DoorKnobPosition) on the arm and then what you could do is Arm.CFrame:ToWorldSpace(CFrame.new(0,0,-2)). Have this running on the client with RenderStepped. You also check for distance with (AttachmentPosition-DoorKnobPosition).magnitude.
This is all just a concept idea, and I dont know if this would even work as I thought out in my head.