# The dynamic equation of motion of a structure contains M, C and K as mass, damping and stiffness matrices of the structure, respectively. If F is an external load vector, then which option is correct about the equation?

The dynamic equation of motion of a structure contains M, C and K as mass, damping and stiffness matrices of the structure, respectively. If F is an external load vector, then which option is correct about the equation?

(a) M$\ddot{x}$ + K$\dot{x}$ + Cx = F

(b) M$\ddot{x}$ is time-dependent

(c) All the forces are time-independent

(d) The equation is of 3^rd order

I got this question in a national level competition.

My enquiry is from Eigen Value and Time Dependent Problems in section Single Variable Problems of Finite Element Method

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The correct option is (b) M$\ddot{x}$ is time-dependent

To elaborate: The dynamic equation of motion of a structure is a 2^nd order equation. It is written as M$\ddot{x}$ + C$\dot{x}$ + Kx = F, where M, C and K are the mass, damping and stiffness matrices of structure, respectively. All the forces in the equation are time-dependent. M$\ddot{x}$ is inertia force, Kx is spring force, and C$\ddot{x}$ is damping force.

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