The tractor is able to move by the power of the internal engine through the transmission system, which gives the drive wheels a driving torque Mk.
The drive wheels, which have obtained the driving torque, then exert a small, backward horizontal force (tangential force) on the ground through the tire tread and tire surface.
The ground exerts a horizontal force Pk on the same driving force but in the opposite direction. this Pk force is the driving force (also called propulsion force) that propels the tractor forward.
The tractor moves forward when the driving force Pk is sufficient to overcome the rolling resistance of the front and rear wheels and the traction resistance of the attached implements. If the drive wheels are lifted off the ground, i.e., the driving force Pk is zero, the drive wheels will only spin in place, and the tractor will not move.
Similarly, if the sum of the rolling resistance and the traction resistance is greater than the driving force Pk, the tractor will also not move. Therefore, it is evident that the movement of a wheeled tractor is achieved through the interaction between the driving torque and the ground, and the driving force must be greater than the sum of the rolling resistance and the traction resistance.
