Work and Potential Energy (Conclusion): Feynman's Physics Explained
The power of potential energy is that it lets us use conservation of energy directly. For a conservative force — gravity, or an ideal spring — the sum of kinetic and potential energy is a constant, a result that follows straight from Newton's laws. For nonconservative forces like friction, energy is not conserved in the same simple way; it leaks away as heat. The clean test for a conservative force: the work done around any closed loop is zero, so you can never get free energy by going around in a circle.
The big idea
You can't get free energy by going in a loop.
Think about it
Why is a machine that claims to run forever with no energy input (perpetual motion) impossible?
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