Two block of mass m1, m2 are connected with two springs(spring constant k1,k2) on a frictionless surface.
The motion of the system can be analysis with x1,x2 and v1,v2.
However, it can be analysis with xa=(x1+x2)/2, xb=(x1-x2)/2 amd va=(v1+v2)/2, vb=(v1-v2)/2.

Which one is better?
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Two blocks and two springs connected to a wall on a frictionless surface
Designed by Fu-Kwun Hwang (http://www.phy.ntnu.edu.tw/ntnujava/)
Deptartment of Physics, National Taiwan Normal University, Taiwan