Hi there!
When the acceleration of a block in SHM is 0 m/s², the NET FORCE must equal 0 N.
0 m/s² acceleration is obtained when the spring is at its equilibrium point, meaning that:
∑F = -kx (x = displacement from equilibrium point in meters)
If the block is at the equilibrium point, displacement = 0m, so:
∑F = -k(0) = 0 N
With Newton's Second Law, ∑F = ma. If the net force is 0 N, then there is no acceleration experienced by the block.
Thus, the other quantity that must be 0 is the block's NET FORCE.
