Tractive effort is the force that is required to propel the vehicle against all resistance to motion. The resistance to motion is known as the tractive resistance and it consists of several elements that vary according to operating conditions. In order to proceed at uniform speed up a gradient the vehicle must have sufficient power to overcome all resistances opposing motion. When a vehicle accelerates, the force causing acceleration is opposed by a force of equal magnitude acting horizontally through the centre of gravity of the vehicle. The opposing force is known as the inertia force and because it acts at height of h metres above the datum it exerts a turning moment that tends to lift the front of the vehicle. There is an apparent transfer of load from the front to the rear axle. The principle of moments can be used to determine the amount of load transfer.