Physics
Forgetting Energy Lost to Friction
Students set mechanical energy before equal to mechanical energy after even when friction is doing work.
Quick check
Why students make it
They remember that energy is conserved and treat it like mechanical energy is always conserved. Friction converts some mechanical energy into thermal energy, so the object has less mechanical energy later.
The rule done right
Use , where is negative. If you use energy lost as a positive amount, write .
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