Newton's first law is sometimes known as the law of inertia. It is the law that states that an object at rest will stay at rest and an object in motion will stay in motion unless a force acts upon it. For example, if I was working with a wrench in space an it slipped, it would keep on going in one direction with a constant speed unless it hits something. Hope this helps!
The nucleus contains the majority of an atom's mass because protons and neutrons are much heavier than electrons, whereas electrons occupy almost all of an atom's volume. I hope this helps you :D
