Representation of Acceleration in the Inertia Force

Hello Friends

I want to make a simulink model to calculate the traction force required from the tractor

I know the equation is summition of the forces which are

Ftr= Frr + Faero + Fgrade + Fdrawbar + m * a

I want to know how to represent the a of the inertia force in the model ?

If I derivative velocity to get acceleration, I will get a=0 because the velocity will be constant as it the maximum vehicle speed

Thanks in advanced helpful guys in that amazing community