When the Truck moves in a straight line, it is in the State of Motion only along the straight line. However, When the Truck turns and changes it's direction, due to it's Inertia Of Direction, it resists any change in it's direction along the straight line, it had been travelling before.
As we know that, Inertia is directly proportional to the mass of the object.
When the Truck is empty, there is no load on it's carriers. Hence, As the Net Mass is the mass of the Truck itself, the Inertia Of Direction too would be less.
On the other hand, when the Truck transports additional loads on it's carriers, it's Total Mass now increases due to the additional mass of the load as well as pre-existing mass of the Truck itself. As the mass is greater in this case, the Inertia Of Directionis greater too.
Due to an Higher Inertia in the Second Case, the Truck finds it difficult to turn itself to the desired direction.
