Modeling Vehicle Acceleration - Simulink

Hello I am modeling a vehicle that has a seperately excited DC motor as the power plant utilizing the constant-torque and constant-power regions for traction. The vehicles acceleration is governed by the following equation

The problem with my model is that the component, the rolling resistance, is a constant value and causes the velocity to go negative for the first 0.7 seconds when the model is executed . Any suggestion on how to fix this issue?

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