Energy
The capacity to do work. Mechanical energy has two forms: kinetic energy (KE = ½mv², energy of motion) and potential energy (PE = mgh for gravitational, ½kx² for elastic, energy of position or deformation). Total mechanical energy is conserved in the absence of non-conservative forces (friction, air resistance).
Formula
KE = ½mv² | GPE = mgh
Real World
A skier at the top of a 50 m slope at Val Thorens has maximum gravitational potential energy; by the bottom, most of this has converted to kinetic energy (ignoring friction), so they reach high speed.
Exam Focus
When energy is 'lost' to friction, explicitly state it converts to thermal energy — exam mark schemes require this, not just 'energy is lost'.
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