# Equation of motion/ static equilibrium

1. Sep 12, 2009

### sara_87

1. The problem statement, all variables and given/known data

I have notes on mechanics, and under the titile equation of motion/ static equilibrium there is the equation:

$$\rho$$$$\frac{\partial^2(u_i)}{\partial(t)^2}$$-$$\frac{\partial(\sigma_{ij})}{\partial(x_j)}$$=fi(x,t)

2. Relevant equations

3. The attempt at a solution
I know that rho is the density, t is time, sigma is stress but what does the 'u' and 'f' represent?

thank you