Final answer:
The maximum instantaneous velocity of 20 m/s occurs at t = 2 s, at which point the acceleration is zero. To determine when the velocity is zero or negative, the velocity function must be analyzed; the acceleration being zero indicates no change in the velocity.
Step-by-step explanation:
The subject of this question is instantaneous velocity in the context of mathematics, specifically in the application of calculus. Instantaneous velocity is defined as the rate of change of position with respect to time at any particular moment. This can typically be found by taking the derivative of the position function with respect to time.
Given the information, at t = 2 s, the velocity reaches its maximum value of 20 m/s. This suggests that there is no change in velocity at this point, which means that the acceleration, which is the derivative of the velocity, is zero. Therefore, the time at which the instantaneous velocity is greatest is 2 seconds, and the acceleration is zero at this time as well.
To find when the velocity is zero or negative, one would typically set the velocity function equal to zero or investigate when the velocity function takes negative values. This can involve solving equations or analyzing graphs depending on the complexity of the velocity function provided.