How does propulsion work?

What is inertia?

Inertia is a word we use when we talk about matter and movement. Basically, our idea of inertia goes back to Sir Issac Newton's first two laws of physics:

1. An object at rest tends to stay at rest.

2. An object in motion tends to stay in motion.

Inertia is the quality in matter (matter is anything you can touch) that lets it stay still if it is still, or keeps it moving if it is moving.

If you want to overcome inertia, you have to apply a force. A force will make something that is still start to move, like flicking a wad of paper with a pencil will make it move. Also force, due to resistance, will slow or stop something that is already moving. The wad of paper will be slowed by resistance made by rubbing up against the air it is passing through.

How do conventional rockets work?
How do objects in space travel?

What is resistance?
Why is mass important?
What is thrust?
Every action has an equal and opposite reaction?

What is specific impulse?