Estimate the equivalent spring constant and equivalent damping coefficient of the excavator, assuming that the base AC is fixed.

Step 1: Calculate the cross-sectional area and moment of inertia for each arm.
The arms are approximated as hollow steel tubes. We will use Young's modulus for steel, $E = 29 \times 10^6 \text{ psi}$.

For arm AD:
Outer diameter $D_1 = 10 \text{ in.}$
Inner diameter $d_1 = 9.5 \text{ in.}$
Length $L_1 = 100 \text{ in.}$
Damping coefficient $c_1 = 0.4 \text{ lb-s/in.}$ (assuming standard units for damping)

Cross-sectional area $A_1$:

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