Chapter 5

Weighty Subjects

“Gravity is a habit that is hard to shake off.” ~ Terry Pratchett

Inertia Is Mass, but Mass Is Not Weight

Chapter 5 is all about the difference between mass and weight. They are definitely connected — but one of them is constant and the other one is variable.

And then there’s inertia and orbits and weightlessness and all that. How do satellites (like the International Space Station) just keep circling the Earth? Why don’t they fall down? Or go winging off into space?

Sneak peek: here’s a little illustration I did to show how satellites at different distances from the Earth use speed to balance the gravitational pull of the Earth.

The closer a satellite is to the Earth, the stronger the gravity. The stronger the gravity, the faster the satellite must be going if its inertia is going to keep it from falling to the ground.

The you-can-do-this-at-home activity at the end of the chapter lets you play with this balance between gravity and inertia. Have fun!

Activity Video for Chapter 5

Here are the links from the text in Chapter 5:

21.  Physics-Speak “Mass | physics | Britannica.”

22.  Deeper Dive “What Is the International Space Station? – NASA.” 30 Oct. 2020,

23.  Deeper Dive “File:Comparison satellite navigation orbits.svg – Wikimedia ….”
Be sure to click the illustration to get the animation going.

24.  Deeper Dive “Inertia and Mass – The Physics Classroom.” 31 Jan. 2001,

25.  Great Video “It’s Not What you Think. Where does mass come from? YouTube.” 21 Sep. 2018, Warning: Starts with ads.

26.  Great Video “PBS Space Time – The True Nature of Matter and MassYouTube.” 6 Jan. 2016,