Conservation of energy
The principle of conservation of energy basically states that:
Energy cannot be created or destroyed, it can only be transferred from one form to another.
What that really means is that the total amount of energy in a system is always the same, even if the energy is transferred from one form to another.
Why does it seem like things lose and gain energy?
Section titled “Why does it seem like things lose and gain energy?”When we see things losing and gaining energy, it’s because the energy is being transferred from one form to another. For example, when a car slows down, it seems like it’s losing energy, but really the kinetic energy of the car is being transferred to other forms of energy, such as heat energy in the brakes.
A car driving through space
Section titled “A car driving through space”Theoretically, in space, there are no resistive forces such as friction or air resistance, so a car that starts moving will keep moving at that same speed forever - even without accelerating!
Similarly, when braking, we’re adding friction to the system in order to transfer some of the kinetic energy of the car to heat energy in the brakes, which is why the car slows down.