system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the difference between momentum and inertia?

My 5-year-old really likes vehicles, and we've been talking about rockets and rocket cars and why they're not as agile as regular cars when it comes to turning and maneuverability. They are designed to just go straight, in the case of rocket cars. 

I think this is because they have a great deal of forward momentum. But as we talked, I realized I no longer remember much about momentum -- or inertia -- and what the difference is between them. 

And one thing I have learned in answering these questions from my son is that you need a rock solid understanding to be able to explain things in a way that promotes good comprehension for a young child. And when you have gaps in your own knowledge of a subject, you will realize it right away.

Anyway, thanks for helping a mom out!
gpt
Inertia is an objects resistance to changes in motion. For instance, a heavier object is harder to push around so it has more inertia. Inertia does not change with speed (unless you consider einstein's theories, which can be ignored in daily life) so a certain object always has the same amount of inertia, which means it will always take the same amount of force to accelerate. 

Momentum is the total amount of motion of all the parts of the object. Specifically it's the velocity of an object times its mass, in other words, the amount of motion times the amount of stuff. An object with a lot of momentum can push a lot of objects out of the way before it stops, while an object with less momentum can only push a few things out of the way before it stops. (an object with a lot of mass counts as many objects with smaller mass, because it can be thought of as a lot of lighter objects glued together). 

A faster object can push more on things (more momentum), but it still takes the same force to change its speed (inertia does not depend on speed).