Description
A block slides down a ramp with noticeable friction, flies off the edge, and lands on the ground. Is mechanical energy conserved during this? The honest answer: it depends entirely on one choice you make before you even start the problem. This video is about that choice — how selecting your system determines whether mechanical energy stays constant, and why energy overall is always conserved even when mechanical energy isn’t.
✅ Describe the energy(ies) present in a system
✅ Define mechanical energy as the sum of a system’s kinetic and potential energy
✅ Explain how selecting a system determines whether its total mechanical energy is constant
✅ State that energy is always conserved overall, even when mechanical energy within a chosen system is not
⏱️ Timestamps
0:00 – Hook: a block, a ramp, friction — is mechanical energy conserved?
1:19 – A single object can only have kinetic energy
1:50 – Mechanical energy = K + U
2:02 – The big idea: you choose the system
2:22 – Total energy (K + U + thermal) vs. mechanical energy (K + U only)
2:48 – When is mechanical energy constant? Zero external work, no nonconservative forces inside
3:47 – Worked example: choosing your system wisely (block → ramp → projectile)
4:54 – Block + Earth system: still not conserved, but more useful
5:17 – Which system choice isolates friction cleanly?
6:29 – Worked example: two paths, same height
7:55 – Result: v = √(2gh) — the bump never mattered
8:24 – Extending the idea: initial speed, and springs
9:47 – Big picture: energy is always conserved, even when the bookkeeping looks messy
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.