Defining Simple Harmonic Motion

Most periodic motion is not simple harmonic motion. SHM is the special case where the restoring force grows linearly with displacement from equilibrium and always points back. One equation does the work: $F = -kx$. This topic covers what that equation forces to be true about the motion, and three places students reliably get it wrong.

Three traps catch students on this topic. First: treating the restoring force as a constant set when the block was released, rather than something that changes with $x$. Second: thinking acceleration is zero at the turning point because the block is briefly at rest, when actually $|a|$ is at its peak there. Third: thinking the force is largest at equilibrium because that is where the block moves fastest, when really the force there is exactly zero. The lesson works through each one; the diagnostic checks whether you have stopped falling for them.