Friction heats up the brake pads of a car as it stops what is this an example of?

This an example of the law of Conservation of Energy.

The car has quite a bit of kinetic energy while it's rolling.  If you want to stop it, you have to take that kinetic energy away from the car, AND you have to do something with that energy.

If it's an electric car or hybrid, you can turn the kinetic energy into electrical energy, put it back into the batteries, and use it again later.

If it's just an ordinary gas guzzler, there's no way to save the kinetic energy.  You use the car's kinetic energy to scrape two rough surfaces together, that turns it into heat, and the air blows the heat away.

Next time you want the car to roll again, you have to make more, new, kinetic energy.  So you take chemical energy out of more gas, and you use the motor to turn the chemical energy into kinetic energy.

It's all the  law of Conservation of Energy ... in action.