Consistent mass matrix

Mass matrix computed by considering mass to be distributed uniform throughout the element is known as consistent mass matrix.

Consistent matrix for bar element is given by

$[M] = \frac{\rho Ah_e}{2} \begin{bmatrix} 2 & 1 \\ 1& 2 \end{bmatrix}$

Lumped mass matrix

Mass matrix computed by considering mass to be distributed equally at the two nodes of the element is known as lumped mass matrix.

$[M] = \frac{\rho Ah_e}{2} \begin{bmatrix} 1 & 0 \\ 0& 1 \end{bmatrix}$