Question 11-2

For simple harmonic motion, acceleration a depends on displacement x (more accurately, proportional to negative displacement):

Hence, acceleration is zero at zero displacement. Note that the velocity at this point is not zero. As a matter of fact, it is moving the fastest at zero displacement.

For example, consider a pendulum. The bob has zero acceleration at the lowest point. At this point, the string is vertical and the tension from the string cancel out the weight of the bob. Net force acting on the bob is zero, therefore acceleration is also zero. However, at the lowest point, the bob is moving the fastest in the horizontal direction (conservation of mechanical energy).