There are 5 forces acting:
 The external force, F_{e}, to the right.
 The force due to k_{1}.
 If x_{1} increases, k_{1} elongates
which causes a force at x_{1} to the left.
 The resulting force is k_{1}·x_{1} to the left.
 The force due to b_{1}.
 If x_{1} moves to the right (i.e., the positive
direction), the friction b_{1} causes a force (at x_{1}) that is
to the left.
 The resulting force is b_{1}·v_{1} to the left.
 The force due to k_{2}.
 If x_{1} increases, k2 compresses which causes a
force at x1
to the left.
 If x_{2} increases, k2 compresses which causes a
force at x1
to the left.
 The resulting force is k_{2}·(x_{1}+x_{2})
to the left (or k_{2}·(x_{2}+x_{1})
to the right)
 The force due to m1 (don't forget this  the inertial
force!).
 The resulting force is m_{1}·a_{1} to
the left (the inertial force is always in the opposite
direction from the define positive direction).


There are 3 forces acting:
 The force due to k_{2}.
 If x_{1} increases (to the right), this
compresses the spring and causes a force to the right at x_{2}.
 If x_{2} increases (to the left), this
compresses the spring and causes a force pulling x_{2} to the
right.
 The resulting force is k_{2}·(x_{1}+x_{2})
to the right.
 The force due to k_{3}.
 If x_{2} increases, this elongates the spring and causes a
force to the right.
 The resulting force is k_{3}·x_{2} to the
right.
 The force due to b_{2}.
 If x_{2} moves to the left (i.e., the positive
direction), this elongates the dashpot and causes a
force to the right.
 The resulting force is b_{2}·v_{2} to the left.
