Energy is conserved when the net work done by non-conservative forces is zero. Non-conservative forces are things like friction, air resistance, applied forces, anything that adds or removes energy from the system. If any of those are present and doing work, energy is NOT conserved and you need the full work-energy theorem instead. Before every energy problem ask yourself: is there friction? Is there an applied force doing work? If yes, conservation of energy alone won't get you there. Quick check: a ball rolls down a ramp with friction. Can you use conservation of energy? Reply below.

Three questions every student should answer before sitting down to study: 1. What topic is my exam covering? 2. Where exactly did I lose points last time? 3. What type of problem do I consistently get wrong? If you can't answer all three — you're not ready to study yet. You're ready to diagnose. That's what we do here. Find the block first. Then fix it.