In motion problems you can use the truth, but you can also sometimes use the principle of conservation of energy, and it is often simpler.
In a closed system, energy will not be created or destroyed, but it can change forms, ie from potential energy to kinetic energy.
In the carnival game where you try to roll a ball over a hill and up an incline so that it doesn’t return over the hill on the way back, it would be impossible if there were no friction, but since there is friction- it is very difficult to be precise enough.
The energy lost due to friction makes it possible to go over the first incline and still not quite have enough energy to make it back over.
The presence of friction may make a system seem like it loses energy, but the energy in friction becomes heat and is not really “lost”.
When manipulating equations for initial and final energy, it is not really necessary to memorize a negative sign to put in a certain place, but one should know that the initial will always equal the final and the sign, for something like friction, should be changed accordingly.