fuel is a source of mechanical energy for cars to move & the car runs out of fuel ->  the car does not have a source of mechanical energy to move  a car is moving on a level road & if one surface is moving on another surface then those surfaces are touching ->  the moving car and the road are touching on their surfaces  if an object is not moving it does not have any motion to counter & friction acts to counter the motion of two objects when their surfaces are touching ->  friction acts to counter the motion of only moving objects when the surfaces of two objects are touching  the moving car and the road are touching on their surfaces & friction acts to counter the motion of only moving objects when the surfaces of two objects are touching ->  friction will counter the motion of the moving car  friction will counter the motion of the moving car & countering the motion of a moving object means decreasing the mechanical energy of the moving object ->  friction will decrease the mechanical energy of the moving car  the car does not have a source of mechanical energy to move & friction will decrease the mechanical energy of the moving car ->  the car does not have a source of mechanical energy and friction will decrease the mechanical energy of the moving car  the car does not have a source of mechanical energy and friction will decrease the mechanical energy of the moving car & if something is decreased and there is no source of that something to provide more of that something, then that something will decrease to 0 ->  the mechanical energy of the moving car will be decreased by friction to 0  the mechanical energy of the moving car will be decreased by friction to 0 & if an object has 0 mechanical energy, then the object will stop moving -> the car will stop when the mechanical energy in it is decreased by friction to 0