So what is kinetic energy? There's motion everywhere in our world. What if we could harness energy that would otherwise be wasted to power our gadgets and generate clean electricity? Is it too good to be true? We've written manyarticles about various things that do that, from small gadgets to big infrastructure, but we've never really looked at the field as a whole, with an explanation of how it works and an overview of the pros and cons of trying to harness kinetic energy.
So first thing first: Kinetic energy is the energy of motion. Accelerating an object from a rest position to a certain velocity takes energy, and the object maintains that energy as long as its speed doesn't change. When the object decelerates, that energy from its motion can be transferred in various ways.
If we're talking about a brake pad on a bicycle wheel, the movement of the wheels is progressively stopped using friction and the kinetic energy is transformed into heat, which in this case doesn't do anything useful. But there are ways to harness kinetic energy to either generate useful mechanical work or electricity. This is what many have tried to do to make use of energy that would be otherwise wasted.