Description
mgy assumes g is constant — but it isn’t, and it gets weaker the farther you travel from Earth. This longer video covers two big ideas: building a potential energy formula that works anywhere in the universe, then flipping everything around to read force and equilibrium straight off a U(x) graph, no formula required.
✅ Derive U_g = −Gm₁m₂/R from the universal law of gravitation, and explain its sign convention
✅ Calculate the gravitational potential energy of a system with more than two objects
✅ Find force from a U(x) graph, and classify equilibrium points as stable or unstable
⏱️ Timestamps
0:00 – Hook: why mgy breaks down once you leave Earth’s surface
1:11 – Why we need a new Ug: the universal gravitation force law
1:40 – The sign convention: Ug = 0 at infinite separation
2:34 – Deriving universal Ug = −Gm₁m₂/R
5:24 – Worked example: a boulder falling from R = 2Rₑ — sketching Ug(y)
6:25 – Does Ug increase or decrease as the boulder falls?
7:10 – Solving for the boulder’s landing speed
9:00 – Ug with more than two objects — summing over every pair
10:47 – Worked example: finding F(x) from U(x) = x − x²
12:00 – Graphing F(x) and U(x) together
13:31 – Reading the sign of F(x) directly from a U(x) graph
14:44 – Defining equilibrium: where F = 0
15:31 – Stable vs. unstable equilibrium
16:39 – The big picture: systems favor lower potential energy
This video is part of the MasteryFlip approach to AP Physics C — watch, take notes, then come to class ready to work problems together. Curious about the MasteryFlip Framework? Check out the ReachEveryStudent Podcast, new episodes every Monday at reacheverystudent.com.