Here A and B represent the two extreme position of a S.H.M. For velocity, the direction from A to B is taken to be positive. For acceleration and the force, the direction is taken positive if directed along AP and negative if directed along BP.
(a) At the end A, the particle executing S.H.M. is momentarily at rest being its extreme position of motion, hence its velocity is zero. Acceleration is +ve because directed along AP. Force is also +ve since the force is directed towards AP i.e., -ve direction.
(b) At the end B, velocity is zero. Hence, acceleration and force are negative as they are directed along BP. i.e., along the -ve direction.
(c) At the mid-point of AB going towards A, the particle is at its mean positive P, with a tendency to move along PA, i.e., -ve direction. Hence, velocity is -ve. Both acceleration and force are zero.
(d) At 2 cm away from B, going towards A, the particle is at Q, with a tendency to move along QP, which is negative direction. Here, velocity, acceleration and force are all -ve.
(e) At 3 cm away from A going towards B, the particle is at R, with a tendency to move along RP, which is a positive direction. Here, velocity acceleration and force, all are +ve.
(f) At 4 cm away from A going towards A, the particle is at S, with a tendency to move along SA, which is negative direction for velocity.
Therefore, velocity is negative but acceleration is directed towards mean position, i.e., along SP, hence +ve. Similarly, force is also +ve.
