If an object has an average velocity of zero but has a positive instantaneous velocity for every instant during a time interval, it means the object is moving in opposite directions at different times, such that the displacements cancel each other out over the entire time interval.
Here's how we can describe the motion of such an object:
The object undergoes symmetrical back-and-forth motion: The object moves in one direction for a certain period of time and then reverses its direction, repeating this pattern throughout the time interval.
The object covers equal distances in opposite directions: For every positive displacement the object makes in one direction, it covers an equal distance in the opposite direction during another part of the interval. This ensures that the net displacement over the entire interval is zero, resulting in an average velocity of zero.
The object's speed remains constant: Although the object changes direction, its instantaneous velocity remains positive at all times. The magnitude of the velocity remains the same throughout the motion, indicating that the object moves at a constant speed.
A common example of this kind of motion is an object oscillating back and forth around its equilibrium position, such as a pendulum swinging or a mass attached to a spring undergoing simple harmonic motion. In both cases, the object experiences displacement in one direction followed by an equal displacement in the opposite direction, resulting in an average velocity of zero.