Question

the image shows one complete cycle of a mass on a spring in simple harmonic motion. which describes the system at point d?
the velocity has the maximum upward value because the acceleration upward is at a maximum.
the net force is at a maximum in the downward direction, so the velocity begins to decrease.
the velocity is zero because the net force is zero.
the net force is zero, so the acceleration is zero.

Explanation:

In simple harmonic motion (SHM) of a mass - spring system, at the equilibrium position (where the net force is zero), the acceleration \(a=\frac{F_{net}}{m}\) (by Newton's second law \(F = ma\)). When \(F_{net} = 0\), \(a = 0\). Point D is at the equilibrium position (similar to the middle position between the extreme points like A, E and the lowest point). At extreme points (A, E), the net force is maximum (spring force is maximum as displacement from equilibrium is maximum) and velocity is zero. At equilibrium (D), the spring force balances the weight of the mass, so \(F_{net}=0\), hence \(a = 0\), and velocity is maximum (not zero as in the third option). The first option is wrong because at equilibrium, acceleration is zero (not maximum upward). The second option is wrong as at D, net force is zero (not maximum downward) and velocity is maximum (not beginning to decrease).

Answer:

The net force is zero, so the acceleration is zero.