Fundamentals of Physics Extended (10th Edition)

In the previous part of the problem we found that $v=20−15t^2$ To find the acceleration equation we need to find the derivative of the velocity equation or $\frac{d}{dt} (20 - 15t^2) = -30t$ Therefore $a = -30t$ To find when acceleration is equal to 0 we set a = 0 and solve for t 0 = -30t and therefore, acceleration is 0 when t =0