The amount of energy that object gain by using the following formula force times, distance travel times, cosine of the angle between them. Now, if we do not neglect friction, if we actually include friction, for example, when this box is traveling along this horizontal surface, there is friction in a real world situation. So if we do not neglect friction, we find our overall work by following the following formula the work done is equal to change in our potential energy of the object plus change in the kinetic energy of the object plus the change in the internal energy of our object due to friction. Now, for the most part, whenever friction acts on an object, it changes the object's internal energy. And internal energy is simply the energy of the individual molecules found in that object. If you sum up all the different types of energies on those individual molecules, you will get the internal energy of that system.