Question: How to omit the undesired term?

Sorry for boring you my friends. I am haunted by a problem of how to omit the undesired term.

For example, in the following equation, the a(t) , b(t), c(t), u(t), v(t), w(t), psi(t), phi(t), theta(t), varsigma(t), tau(t) and upsilon(t) and their first and second direvative to time t are considered as first order small variables. How could I omit the term greater than second order of small variables?

If we omit the undesired by hand, the omitted equation takes the form of:

R^2*rho*h*(diff(w(t), t, t))*Pi+R^2*rho*h*(diff(c(t), t, t))*Pi = 0;

The original equation is given as: 

-R^2*rho*h*cos(Omega*t)*(diff(tau(t), t, t))*a(t)*Pi+tau(t)*R^2*rho*h*(diff(tau(t), t))^2*a(t)*cos(Omega*t)*Pi-tau(t)*R^2*rho*h*(diff(tau(t), t))^2*b(t)*sin(Omega*t)*Pi+tau(t)*a(t)*Pi*cos(Omega*t)*(diff(varsigma(t), t))^2*R^2*h*rho+tau(t)*R^2*rho*h*a(t)*Omega^2*cos(Omega*t)*Pi-tau(t)*sin(Omega*t)*Pi*(diff(varsigma(t), t))^2*b(t)*R^2*h*rho-tau(t)*R^2*rho*h*b(t)*Omega^2*sin(Omega*t)*Pi+2*tau(t)*R^2*rho*h*(diff(a(t), t))*Omega*sin(Omega*t)*Pi+2*tau(t)*R^2*rho*h*(diff(b(t), t))*Omega*cos(Omega*t)*Pi-varsigma(t)*a(t)*sin(Omega*t)*Pi*(diff(varsigma(t), t))^2*R^2*h*rho-varsigma(t)*a(t)*sin(Omega*t)*Pi*Omega^2*R^2*h*rho-varsigma(t)*Pi*cos(Omega*t)*(diff(varsigma(t), t))^2*b(t)*R^2*h*rho-varsigma(t)*Pi*cos(Omega*t)*b(t)*Omega^2*R^2*h*rho-2*varsigma(t)*sin(Omega*t)*Pi*(diff(b(t), t))*Omega*R^2*h*rho+2*varsigma(t)*Pi*cos(Omega*t)*(diff(a(t), t))*Omega*R^2*h*rho+2*a(t)*Pi*cos(Omega*t)*(diff(varsigma(t), t))*Omega*R^2*h*rho-2*sin(Omega*t)*Pi*(diff(varsigma(t), t))*b(t)*Omega*R^2*h*rho+R^2*rho*h*(diff(tau(t), t, t))*sin(Omega*t)*b(t)*Pi+R^2*rho*h*(diff(w(t), t, t))*Pi+R^2*rho*h*(diff(c(t), t, t))*Pi-R^2*rho*h*(diff(tau(t), t))^2*c(t)*Pi+2*varsigma(t)*(diff(tau(t), t))*a(t)*Pi*cos(Omega*t)*(diff(varsigma(t), t))*R^2*h*rho-2*varsigma(t)*(diff(tau(t), t))*sin(Omega*t)*Pi*(diff(varsigma(t), t))*b(t)*R^2*h*rho+a(t)*sin(Omega*t)*Pi*(diff(varsigma(t), t, t))*R^2*h*rho+Pi*cos(Omega*t)*b(t)*(diff(varsigma(t), t, t))*R^2*h*rho-varsigma(t)*Pi*c(t)*(diff(varsigma(t), t, t))*R^2*h*rho-2*tau(t)*R^2*rho*h*(diff(tau(t), t))*(diff(c(t), t))*Pi-2*varsigma(t)*Pi*(diff(varsigma(t), t))*(diff(c(t), t))*R^2*h*rho+2*sin(Omega*t)*Pi*(diff(varsigma(t), t))*(diff(a(t), t))*R^2*h*rho+2*Pi*cos(Omega*t)*(diff(varsigma(t), t))*(diff(b(t), t))*R^2*h*rho-Pi*(diff(varsigma(t), t))^2*c(t)*R^2*h*rho-2*R^2*rho*h*cos(Omega*t)*(diff(tau(t), t))*(diff(a(t), t))*Pi+2*R^2*rho*h*(diff(tau(t), t))*sin(Omega*t)*(diff(b(t), t))*Pi+tau(t)*R^2*rho*h*(diff(b(t), t, t))*sin(Omega*t)*Pi-tau(t)*R^2*rho*h*(diff(a(t), t, t))*cos(Omega*t)*Pi-tau(t)*R^2*rho*h*(diff(tau(t), t, t))*c(t)*Pi+2*R^2*rho*h*Omega*sin(Omega*t)*(diff(tau(t), t))*a(t)*Pi+2*R^2*rho*h*(diff(tau(t), t))*Omega*cos(Omega*t)*b(t)*Pi+varsigma(t)*sin(Omega*t)*Pi*(diff(a(t), t, t))*R^2*h*rho+varsigma(t)*Pi*cos(Omega*t)*(diff(b(t), t, t))*R^2*h*rho = 0;

Thank you in advance for taking a look ;)