There’s something I’ve left out so far. Here on Earth, moving objects want to travel forever at the same speed, but there is one force that always slows them down.

And that’s FRICTION.

Rub your hands together. That’s all friction is: it’s two things rubbing against each other. There is friction almost everywhere, even in the air. When you feel wind, that’s the friction of the air rubbing up against you.

Now rub your hands more quickly. Notice anything? Right, they get HOT. Friction creates heat.