The "damping force" is proportional to the velocity and always opposes it. The crucial point is NOT that R1 is "i" or "-i" but that it is "-x' i". It is horizontal, so "i" and it is opposite to the velocity, x', so -x' i. Similarly, R2 is opposite to the speed relative to position "B" which is x'- y'. "H" is the spring force. If the spring is compressed, that is if x is less than , the force tends to "uncompress" and if the spring is stretched, that is if x is larger than [tex]l_0[tex] the force tends to "unstretch". In either case it isoppositeto so proportional to